UNIVERSITY OF CALIFORNIA
COLLEGE OF AGRICULTURE
AGRICULTURAL EXPERIMENT STATION
BERKELEY, CALIFORNIA
ADOBE CONSTRUCTION
J. D. LONG
BULLETIN 472
September, 1929
UNIVERSITY OF CALIFORNIA PRINTING OFFICE
BERKELEY, CALIFORNIA
1929
ADOBE CONSTRUCTION
J. D. LONGi
Natural earth, or soil, has been used as a building material in
almost all countries and by almost all civilizations. 2
Archaeologists report the use of unburned brick and "green
bricks" in the excavated areas in Mesopotamia and Egypt. What is
thought to be the biblical "Tower of Babel" in the former country is
of this construction ; sundried bricks are said to form the foundation
courses for the pyramids. Succeeding civilizations in India, Mon-
golia, Morocco, Italy, Spain, France, Germany, Kussia, Scandinavia,
England, and the Americas have made use of the material.
In some of the French provinces that formed the seat of operations
of the World War, the rammed earth method of construction had a
relatively large part in all structural work. In England, the material
has been used as far back as the Romano-British era, but the tradi-
tional methods had become almost obsolete within the last half -century
until a revival of interest in the rammed earth method immediately
after the World War. 3
In the Americas, also, earth construction dates back to antiquity. 4
At "Chan-Chan," Peru, and at Casa Grande, Arizona, are elaborate
structures of earth. The former is thought by some archaeologists to
be of an age comparable with that of the biblical civilization excavated
in Mesopotamia; 5 the Casa Grande was so old as to be an object of
superstitious awe to the natives when the first white explorers entered
the region in the seventeenth century. 6 What is said to be the first
building erected by the white immigrants in what is now the United
States is the "Palace of the Governors" built in Sante Fe, New
Mexico, in 1609 of sun-dried, or "adobe" brick. Originally used as a
seat of the territorial government, it is now a state museum.
1 Junior Agricultural Engineer in the Experiment Station.
2 Williams-Ellis, Clough. Cottage building in cob, pise, chalk, and clay.
125 p. Country Life Press, London, 1919.
s Ellington, Karl J. Modern pise building, p. 76-96. K. J. Ellington, Port
Angeles, Wash., 1924.
4 Hodge, Fredrick W. Aboriginal use of adobe. Archaeologist 3(8):265.
1895.
s Lehman, Walter. The art of old Peru. p. 52. Longmans, New York, 1924.
6 Pinkley, Edna T. Casa Grande, the greatest valley pueblo of Arizona.
23 p. Arizona Archaeological and Historical Society. Tucson, Ariz., 1926.
4 University of California — Experiment Station
Earth-walled houses were erected by some of the early settlers on
the Atlantic seaboard. 7 ' 8 Several scattered examples of such structures
have been found, extending from the New England States to South
Carolina. One two-story rammed earth residence now in use in
Washington, D. C, is said to have been erected in 1773. 9 Plantation
buildings and a church of rammed earth erected in South Carolina in
the period 1820-52 are still in use. A two-story sun-dried brick
farmhouse in Illinois, erected during the Civil War, is still occupied by
descendants of the builder. 10 Modern residences of earth-wall con-
struction have been built since 1920 in Washington, D. C, in Michigan,
North Dakota, Arkansas, Colorado, and in all the southwestern states. 11
In the early history of California adobe was extensively used as a
structural material, and for certain purposes and in certain areas, at
least, ,it was considered very satisfactory. 12 Most of the string of
Spanish Missions were so built, and several are still in use, although
many have fallen into decay through misuse and neglect. The first
custom-house and the first theatre in the State, still in use in Monterey,
are of this construction. The buildings and fortifications erected in the
early Spanish presidio at San Francisco were of adobe. One small
building of this regime, its exact erection date unknown, is now used
as the Officers' Club of the U. S. Presidio which now occupies the
site. Several adobe haciendas of the original Spanish settlers are
still extant, as are a number of similarly constructed buildings in the
mining camps of the goldrush days. With the influx of commercial
materials, however, the use of adobe became almost a lost art until the
revival of interest in the subject during the past five years.
The modern construction of this nature in California has been
relatively inconsequential in amount. Since commercially produced
materials have cheapened in cost, have been popularized by sales-
appeal advertising, and have gained the preference of most profes-
sional builders, the advantages of a native material have often been
overlooked. No material being entirely free of objectionable features,
certain disadvantages of adobe have without question also contributed
to its lack of popularity. Because of certain structural weaknesses of
7 Johnson, S. W. Rural economy. William Elliot, New York. 1806.
8 Report of the Commissioner of Patents, United States Department of
Agriculture, p. 450-455. 1844.
9 Coffin, Edward W. and H. B. Humphrey. Lower cost buildings, p. 42. The
Publicity Corporation, New York. 1924.
io Information given by Chas. W. Russell, Virginia, 111., March 26, 1928.
ii Long, J. D. Progress in earth wall construction. Agr. Engr. 9:183-185. 1928.
12 Tyson, Philip T. Geology and industrial resources of California, p. 41-42.
Wm. Minifee & Co., Baltimore. 1851.
Bul. 472] Adobe Construction 5
adobe, its use can not be recommended for large structures. The fact
that some of the old Spanish missions of adobe have stood through the
century with inadequate foundations, insecure roof fastenings, and
high walls unsupported through long lengths is interesting ; but these
should not be taken as examples of safe design.
Nor are the houses erected a century ago to meet the requirements
of that day indicative of what can be done with the material now.
There can be no doubt that earth structures, when properly built to
meet our modern requirements, can be relatively comfortable, durable,
and healthful shelters. They appeal especially to builders who desire
to follow the methods of tradition, to those with a sentiment for the
use of local materials produced on the land where the structure is to be
built, to those distant from a point of supply for commercially pro-
duced materials, to those desiring to secure particularly comfortable
structures, and to those who desire to utilize their own labor or that of
unskilled workmen to decrease the capital expenditure required. The
use of the material is frequently advantageous for rural structures. 13
Attention should be directed to the fact that adobe soils are not
necessary for adobe construction. In fact, the characteristic tendencies
of adobe soils to crack during drying make their use for earth wall
construction difficult or even impossible. Soils ranging from sandy
loams to clay loams are preferable.
METHODS OF EARTH WALL CONSTRUCTION
Several methods of constructing earth walls have been developed. 14
These may be divided into two general classifications : those in which
the soil is used merely as a filler and is dependent upon some other
material for its structural strength, and those in which the soil itself
attains sufficient strength to carry both its own weight and that of the
roof or other structural members resting on the wall. Five structural
systems are used in modern earth-wall construction. These are the
so-called "cajon" method coming under the first classification, and
the ''poured adobe," "cob," "adobe brick," and "rammed earth"
construction, all coming under the second classification.
All five methods are commonly classed as adobe construction. Each
method requires a manipulation of moist or wet soil to "puddle" it.
In the puddled state the soil grains are brought close together so that
■13 Report of the earth wall building committee, American Society of Agri-
cultural Engineers. Trans. Amer. Soc. Agr. Engr. 21:S31-41. 1927.
]4 Long, J. D., The use of earth for wall construction. Trans. Amer. Soc. Agr.
Engr. 19:205-220. 1925.
6
University of California — Experiment Station
there is a mechanical locking between the angular soil particles, and so
that the surfaces in contact can be cemented by the very fine clay
particles, or colloids, in the soil.
Organic material in soil is usually of a spongy nature and appar-
ently opposes both the mechanical locking and the colloidal cementa-
tion of the grains. For this reason it is usually advised that the top
six to twelve inches of the surface soil be discarded in adobe con-
struction.
Cajon or Wall Filling Material. — This method is one in which the
soil is used merely as a filler and is dependent on other materials for
Fig. 1. — A Mexican's home erected of 2 X -4 wood studs lathed on both
surfaces and with the space between tilled with mud. An example of cajon
building. The appearance could be greatly improved by applying stucco.
structural support. The structural framework of the wall is made up
of wood timbers, and the earth is placed between these to form the
solid wall.
The exact details of the work depend upon the attendant conditions
and upon the preference of the builder. Boards nailed to the exposed
sides of the structural framework form cavities between the timbers
into which the well-mixed mud is poured (see fig. 1). The mud may
be carried in lumps and dropped snugly into place between the
timbers, or sun-dried bricks may be formed and laid in a wall between
the members of the framework.
Because there is no bond between the earth and wood and the mud
tends to pull away from the wood in drying, the combination of the
two different materials as in this method cannot be considered best
Bul. 472]
Adobe Construction
practice. Some very attractive and apparently durable structures
have been erected by this method, however, and there is the advantage
that much thinner walls can be used than in the other methods.
The wall surfaces may be finished by weather boarding nailed to
the structural timbers or by stucco or plaster applied over a wire mesh
reinforcing.
The term "cajon" coming from the Spanish, means in this usage
"wall filling material." The method has been used to a very limited
extent in California.
Fig. 2. — A ranch house under construction by the poured adobe method.
At the left is a view looking into the forms on top of the wall, and at the
right are shown the two farm laborers who hauled the soil from a selected site,
mixed it to a stiff mud in the wagon box, and spread it in the forms. The
forms are 1 X 10 inch boards which clamp against the completed wall and are
wired together, leaving a net depth inside of about six inches. Wood spacers
hold the form to the desired width at the top. The vertical cracks which
usually appear more or less regularly in the completed courses are visible in the
picture on the left. The simplicity of the required equipment and of the
construction work are factors of importance to this method. Careful work-
manship is required only in properly setting the forms to keep the wall
plumb. (Method devised by J. M. Howells. Pictures by courtesy of L. W.
Taylor.)
Poured Adobe or Mud Concrete. — The poured adobe method of
construction modifies the cajon method to the extent that no wood
studs are used, but thoroughly mixed mud is poured between forms
directly in place in the wall and allowed to dry. The forms are
removed, and the mud wall alone supports the roof load.
One method of forming such walls is to handle the mud much as
monolithic Portland cement concrete is used in common practice.
Water is added to the soil, and the mass is thoroughly mixed to a
mushy consistency. Straw or other vegetable fiber may be added. The
mixture is then shoveled into the wall forms. As with concrete, the
water content must be adjusted to give the most workable mass ; too
dry a mixture will prove difficult to work into place to fill the form,
while too wet a mass will flow through cracks in the forms, will shrink
more in drying and will take much longer to dry out thoroughly.
8 University of California — Experiment Station
The form may be handled in various ways. For simplicity and
economy, forms made of 1 X 10-inch boards may be used. These are
placed one board high around the foundation of the structure to be
erected. One board is placed on either side of the foundation; the
two are allowed to cover down four inches on the foundation and are
separated at the top edge with spacers to the width of the foundation.
They are then drawn tight with tie wires run through holes at con-
venient points. As four inches of the width of the board clamps to
the foundation, the net depth of the inside of the form is six inches.
The mud layer poured into this form is allowed to dry sufficiently to
retain its shape, and the forms are removed, lifted, and clamped to the
Fig. 3. — Interior of poured adobe incubator house of Sutter City Hatchery.
Erected in 1928. A 3% -inch layer of loose soil was spread over the ceiling for
insulation. The soil for walls and ceiling was excavated from the interior of
the house, making the floor approximately two feet below grade. In preparing
the mud for the walls, trenches were spaded the length of the structure, the
loosened soil flooded with water and covered with straw, and the mass mixed by
hand.
section previously poured. This procedure is continued until the wall
is raised to the desired height 15 (see figs. 2 and 3).
Another method of forming is to erect 2 by 4-inch studs every two
feet on either side of the foundation, nail 1-inch boards horizontally
to their inner surfaces to the height desired for the wall, and draw
the two sides of the form tightly together at the desired width with
tie wires. The wall is then poured as rapidly as the soil can be mixed
and carried into place, pourings being kept at an approximately even
height to equalize the pressure on the forms. When the mixture has
dried sufficiently to retain its shape, the wires are cut and left in the
isGray, R. M. A mud ranch house designed by a California engineer. Agr.
Engr. 7:276. 1926.
Buu 472]
Adobe Construction
wall, the forms removed, and the remainder of the structure completed.
The walls of the house shown in figure 4 were so built.
In either variation of this method it is advisable to put the rough
frames for the doors and windows in place before the mud is poured
into the form. With many soils, as the walls dry out vertical cracks
will develop through each of the horizontal layers representing the
various pourings. Unless these are continuous through a considerable
height of the wall or of a width greater than %6-hi c lij they do not
appear to weaken the wall materially. The cracks are sometimes so
regularly spaced as to give the wall an appearance of sun-dried brick
construction. Mixing straw with the soil lessens the tendency to
cracking. Plastering and stuccoing should be delayed until the wall
is thoroughly dry.
Fig. 4. — Farm home of Mr. and Mrs. J. W. Murphy, erected by the poured
adobe method near Farmersville, Tulare County, California, in 1927.
Since the soil is handled but once, and the processes are all rela-
tively simple, this method is probably the quickest and cheapest adobe
construction method. As some soils crack so much that they are not
suitable for this method, however, it is considered advisable first to
make small test specimens or erect short sections of wall before
starting the construction of any elaborate structure. The cohesion
between the soil grains in these preliminary test structures, the amount
of cracking occurring as the mud dries, and the apparent strength of
the test pieces may be taken as a basis for judging the fitness of a
soil for construction use by this method.
The name "poured adobe" is a logical definition of the procedure
of the method. Another name which has sometimes been applied is
"mud concrete." While relatively few structures have been erected
10 University of California — Experiment Station
by this method in California, it apparently is second in popularity
only to the sun-dried brick method.
The English Cob. — Cob is a stiff mud piled in relatively thick
layers directly in the wall without using forms. The mud must be
mixed to a stiff consistency so that it will have little tendency to
slump.
Straw or some other organic fibre is usually mixed with the mud
and the mixture then laid in place along the length of the wall in
layers as high as they are thick, the thickness frequently being 2 or
2% feet. After being solidly compacted and shaped approximately
to the width desired with the forks on which it is handled, the mud is
allowed to dry. Before it has become entirely dry, a board is placed on
top of the layer to serve as a straight-edge, and the sides of the layer
are trimmed plumb with a hay knife or similar tool. Another layer
is then placed on top of the completed layer, and the process continued
until the wall reaches the desired height. 16
Cob is the traditional name applied to this form of building in
England, where it was used to a considerable extent up to fifty years
ago. 17 It is supposed to derive its name from the use of cobble stones,
which in certain localities were dropped into place in the mud layers
as the work progressed. The method has not been used to any great
extent in this country.
Sun-dried or Adobe Brick. — Bricks may be molded from a stiff
mud, dried in the sun, and laid up in a wall by methods similar to the
laying of the standard burned brick. Straw is usually added to the
mud mixture from which the bricks are molded.
A common procedure of the Mexican workmen who do most of this
work in the southwestern part of the United States is to spade a six-
inch layer of the area intended for the basement. A layer of straw
or grass about one and a half inches thick is then spread over the
spaded soil ; horse manure is frequently preferred, apparently because
the straw in it is usually broken into relatively short lengths which
work more readily into the mud.
After the mass has been thoroughly wetted, the laborers remove
their shoes, roll up their trousers, and proceed to tramp the straw
well into the mud. Short-handled hoes are used to help turn and
cut the mass. When it is thoroughly mixed to a stiff mud, it is
shoveled on to a wheel barrow or litter and carried to a smooth, level
16 Adams, J. W. Adobe as a building material for the plains. Colorado Agr.
Exp. Sta. Bui. 174:1-8. 1910.
] 7 Weller, H. O. Building in cob and pise de terre. Building Eesearch Board,
Dept. of Scientific and Industrial Research. (London.) Special Report 5:3-14.
Bul. 472]
Adobe Construction
11
area nearby, which is free of vegetation and trash. Here a bottomless
wood form is laid flat on the ground and filled with mud. Care is
taken to ram the mud into all corners of the form, and the top is then
struck off level. The form is then carefully withdrawn, leaving the
brick lying flat on the ground (see fig. 5). The inner faces of the
form are hurriedly washed to free them of any material which might
have adhered to spoil the shape of succeeding bricks, and the molding
process is repeated. The bricks are allowed to lie flat for a day or so
until they are sufficiently strong to hold their shape when handled.
They are then set on edge so that the air may circulate freely on both
sides. This precaution is necessary in order that the bricks may dry
Fig. 5. Molding the "dobies" flat on the ground. The form is removed
immediately, and the mud allowed to dry one or two days, after which the
brick is stood on edge so that it will dry evenly from both sides.
evenly on both faces; otherwise they will tend to warp and are more
liable to crack. After three days to a week or more of drying in this
position, the bricks are stacked in loose piles adjacent to the building
site. With this equipment, 250 to 300 bricks are a day's output for
two men.
The forms in common use for molding the brick are constructed of
surfaced lumber, fastened with wood screws, and are usually of a
size to form bricks 4 by 12 by 18 inches. A brick of this size will
weigh between 40 and 50 pounds, about the maximum weight for
convenience of handling. It has a volume of one-half cubic foot, and
its shape and dimensions are such that it will work readily into walls
of 12, 18, 24, and 30-inch thicknesses. Other sizes, both larger and
smaller, are used, the maximum being 6 by 12 by 24 inches, forming
a brick containing one cubic foot volume and weighing about 100
pounds.
12
University of California — Experiment Station
Occasionally a builder will line his form with sheet metal to secure
more perfectly formed bricks and to secure greater ease of form
removal. Frequently the molds have partition boards and are large
enough to form two or three bricks with one placing. Setting the
form on a concrete or wood platform rather than on the ground results
in more perfectly formed bricks.
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Fig. 6. — Forms used in making sun-dried brick. At the top is shown a
form for a brick 6 X 12 X24 inches. At the bottom are three views of a form
for molding three bricks, each 4 X 12 X 18 inches. Either form may be lined
with sheet metal to make its removal from the bricks easier.
An improvement on the simple rectangular brick form is to place
shaped 1-inch strips on the inner faces of the forms so that when the
bricks are molded they will have keystone-shaped grooves running
vertically on either edge (see fig. 6). This groove provides an excel-
lent plaster and stucco key for the wall formed of the brick. 18 Other
shapes of blocks may be nailed inside the brick molds to form specially
shaped bricks which may be required around door and window
openings.
is Suggested by C. V. Maddux, labor commissioner, Great Western Sugar
Co., Denver, Colorado. Report of Earth Wall Building Committee, American
Society of Agricultural Engineers. Amer. Soc. Agr. Engr. Trans. 21:1927.
Bul. 472]
Adobe Construction
13
Various devices and practices have been used to lessen the labor of
making adobe brick. Colorado farmers frequently select a patch of
prairie sod where the grass is thick and tall, then plow this sod about
four inches deep, and work this with a disc harrow or cultivator, add-
ing water until the soil is too wet to be worked further with the
machine. Further mixing is accomplished by leading horses back
and forth over the plowed area, and by adding water until the mass
can be handled with a 6-tined manure fork, when it is carried out to
one side and molded into bricks. 19
Fig. 7. — The Farm Bureau of Tulare County, California, used a concrete
mixer to pour ' ' flats, ' ' which were then cut into brick size with a knife drawn
along a straight-edge. (Courtesy of W. E. Gilfillan.)
One of the earliest power mixing devices in use in California was
the horse-power mixer. One horse on a 10-foot sweep was the moti-
vator. The sweep was attached to the upper end of a vertical spindle
which revolved in a wood casing two feet square. Soil and straw were
inserted at the top of the casing, the mass was well wetted, and the
horse was started on his circular path. Four or six horizontal arms on
the spindle served to mix the mass, which was then removed at the
bottom. Similar mixers using a horizontal spindle in a semi-cylin-
drical vat and powered with a gas engine have been used recently.
Standard concrete mixers have also been employed.
Where mixing machines with a large capacity are used the molding
of the individual bricks becomes the most laborious and slowest part
is Sjogren, J. W., and J.W. Adams. Adobe brick for farm buildings. Colorado
Agr. Exp. Sta. Bul. 308:4. 1926.
14
University of California — Experiment Station
of the process. To overcome this, some builders with machine mixers
use large flats about six feet wide and of any desired length, into
which the mud is poured to the desired thickness for the bricks.
These flats are then cut into the individual brick sizes by using a
straight-edge and knife (see fig. 7). After the mud has dried
sufficiently to retain its shape the forms are removed and the bricks
"broken out" and placed on edge to complete the drying. Unless
exceptional care is taken the bricks formed in this way are not so
uniform nor so perfectly shaped as those molded individually.
After enough bricks have been molded and dried, the wall is laid
up, the bricks being laid by methods similar to those used with
Fig. 8. — Laying the sun-dried bricks. Mud is spread in half-inch layers to
serve as mortar between bricks.
common brick (see fig. 8). A mortar made of mud, wetter, but other-
wise similar to that used in the brick, is usually laid in about half-inch
thicknesses between the brick courses. Lime and cement mortars may
be used, and when squeezed from the joint provide a good stucco
bond. Broken bits of stone, tile, or concrete are sometimes incor-
porated in the edge of the mortar joint to provide a plaster bond.
Door and window frames should be built into the wall as the work
progresses (see fig. 9). The bricks are readily cut to fit by strokes of
the edge of the trowel, and in the better-class work shaped bricks to
fit against door and window jambs may be molded in specially fitted
molds. In the fitting of door and window frames, account must be
taken of the fact that some settling will occur as the mud mortar dries.
This is discussed in detail in the door and window openings section
under structural design.
Bul. 472]
Adobe Construction
15
" Sun-dried brick" is probabty the best descriptive English term
to apply to this form of building, but the name ' ' clay lump ' ' which is
applied to such construction in England is also good. 20 Completed
buildings of sun-dried bricks are shown in figures 10 and 11.
Fig. 9. — The door and window frames are put in place as the adobe brick
walls are erected. The frames should be firmly braced to prevent distortion.
(Courtesy of M. A. Meador.)
In the southwestern United States the method is commonly known
as " adobe brick" or "adobe" construction. The word "adobe"
comes from the Spanish and means "brick," the Spanish verb
"adobear" meaning "to knead." In the United States this method
is the best known and most widely practiced of the forms of earth
construction.
Fig. 10. — Sun-dried brick cottage on the U. S. Department of Agriculture
Cotton Experiment Station at Shafter, Kern County, California. Erected in
1924. The walls are covered with a cement stucco.
Rammed Earth or Pise de Terre. — This method consists of tamping
moist earth in place in the wall between forms. The soil should be
just moist enough to hold together in a ball when it is squeezed in
the hand, and yet dry enough to fall apart when dropped to the
20 Williams-Ellis, Clough. Cottage building in cob, pise, chalk, and clay.
p. 121-125. Country Life Press, London. 1919.
16
University of California — Experiment Station
ground from waist height. A farmer will recognize this as being
about the right moisture content for plowing. For the best work and
the Smoothest wall finish, screening the earth through a half -inch mesh
screen to remove the stones, clods, and vegetation and to pulverize the
soil so that it will consolidate evenly is an advantage. 21
If the soil as it comes from the ground is considered to have the
proper moisture content, it is screened and is ready for the wall. If
it does not have sufficient moisture as it comes from the ground, or if
it has stood in piles and dried out, it will require moistening. This is
best done with a garden sprinkling can or a hose throwing a fine spray
Fig. 11. — The Farm Bureau office building erected of sun-dried brick in
Visalia, Tulare County, California, in 1928. Nearly 100 members of the
county organization volunteered their efforts for various periods during the
brick making and wall erection. Skilled labor was secured for the floor and
roof construction, and for other details in the building. The exterior walls
were given one coating of a cement wash, applied with a brush.
of water. The soil while being wetted should be thoroughly mixed to
obtain a uniform moisture distribution.
Forms of various designs have been used in this work. The main
requirements are that they shall be sufficiently rigid to withstand the
high pressure exerted on them as the earth is tamped into place within,
and that they shall be economical of construction and convenient to
use. In almost all designs these requirements have resulted in the
use of sectional forms, the forms being removed and reset after the
completion of each small panel of wall.
A convenient practice has been to use 10-foot lengths of V/? or
2-inch plank. These are cleated together to form two panels, each
21 Betts, M. C, and T. A. H. Miller. Rammed earth walls for buildings.
U. S. Dept. Agr. Farmers' Bui. 1500:1-25. 1926.
Bul. 472]
Adobe Construction
17
about 30 inches high and 10 feet long. One panel is placed on either
side of the foundation, and the two are bolted together with long bolts
running through the panels near both top and bottom edges and across
the wall space. To avoid springing of the panels because of the
internal pressure, the bolts are used in conjunction with removable 2
Fig. 12. — Forms for rammed earth construction made according to design
shown in figures 13, 14, and 15.
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Fig. 13. — Set-up of forms for rammed earth wall, showing arrangement for
forming corner and partition walls and blocking out openings. This form is
adjustable to all wall thicknesses which are divisible by three inches.
18
University of California — Experiment Station
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K— /Q
r&r% hole
Z."*4"*34 brae 9
•
o
-%' » v5" machine bolts
% hofes
**~ countersink heads * /vmjCt °
» e
O O O o I
OOOOOOOOO 000000000*000 oo o o o o
3 OOOOO
°
^ ho/e& , -J" centers <S^ « °
Fig. 14.— Side and edge views of 7 and 10-foot panels for rammed earth
forms. To construct, use six 1% X 6-inch, tongued and grooved, full length
planks for each panel. Make two panels of each length. The row of holes
spaced three inches apart along the bottom of each panel is necessary to
permit adjustment for various wall thicknesses (varying by three-inch dif-
ferences) in setting up to form corners and partitions. Down a straight length
of wall, cross bolts and stiffeners are spaced approximately three feet apart.
All woodwork is to be surfaced and oiled or painted.
Fig. 15. — Inside and outside corner members for the rammed earth form.
Two filler members for each allow a choice of making curved or square corners.
The filler members are held in place by friction.
Bul. 472]
Adobe Construction
19
by 4-inch stiffeners. These stiffeners should be spaced not more than
three feet apart down the length of the panels (see figs. 12, 13, 14, 15,
and 16).
The panels are allowed to overhang or clamp to the top six inches
of the foundation, about 24 inches net depth being left inside the form.
Vertical stops are placed at either end and held in place by cleats
nailed to the panels. These end stops, such rough frames as may be
required to frame out door and window openings, and spacing
Jf » 3 carriage, bolt
nith thumb nut.
'6 chamfered
to s hope
End Stop
to block off end of form
Jt/ for a /Z~ rra/l. R amort
Z''o~ nhen blockin? ouf «
door or *v/r>dorv opening.
%J
Cross Bolt
Use h jfock /Z" Zonae,
* na/l desired. Equip
A
..o
<STiFrzncri
Uoe Z'-*' stool.,
forge end
grind
M
T
T/riL fiuT-3
Use ft •■!" stock
i
«"» far
y^
<r.
Brace Member
— /» stiffen Joint nhen
form* are buffed foyerfter
/• farm Jong Jengfh of r*af/.
HfJfiD TAMPERS
-made from 4'» 6' * ISL'
wwrf blocks, hardnooJ
preferred.
'?<*'
%'?>£
angle
Fig. 16. — Hand tamping tools and accessories for the rammed earth forms.
members placed adjacent to the top row of bolts hold the two panels to
the desired wall width. The nuts on the cross bolts are then tightened.
Care must be exercised to keep the forms plumb and in the true line of
the wall during this tightening process. Special forms are required
for corners and intersecting walls in order that these planes of possible
fracture may be molded in one piece.
The damp soil is placed in the form and leveled to a uniform 4-
inch layer. Much variation in the level of the layer leads to later
difficulties in finishing the wall section. A 4-inch depth appears to be
the practical limit to which it is possible to compact loose soils under
a hand tool, and hence it is the most economical depth. In experi-
mental work by the Agricultural Engineering Division of the Cali-
20
University of California — Experiment Station
fornia Agricultural Experiment Station it has been found that
layers greater than 4 inches show evidences of loose structure in the
lower horizons of each layer when the forms are removed, and accord-
ingly do not develop a wall of uniform strength or texture. Layers
of soil less than 2 inches in depth usually break away under the tamper
and fail to consolidate in a uniform layer. 22
Hand tamping has necessarily been the common procedure in the
past (see fig. 17). The tamper itself is of hard wood, soft wood faced
with iron, cast iron, or large pipe or boiler plate welded into shape.
The bottom face should not be a flat surface; a wedge-shaped base
will shift and compact the soil grains more thoroughly in the tamping
process. The results of an investigation of these two tool types con-
ducted at the University Farm, Davis, are given in table 1.
A tamper weighing from 8 to 15 pounds is usually preferred by the
workman.
TABLE 1
The Effect of the Tamping Tool on the Compressive Strength of
Eammed Earth Specimens
Soil series
Method of puddling
Initial
moisture,
per cent
Age,
days
Volume
weight,
pounds
per cu. ft.
Compressive
strength,
pounds,
per sq. in.
Yolo loam
Tamped with flat-faced
tamper
12 9
191
96.7
91.8
Yolo loam
Tamped with wedge-faced
tamper
12.9
191
104.5
103.3
Tamping should be directed first along the faces of the form and
then through the interior of the mass, care being taken to compact all
parts uniformly and to the same level. The workmen will usually find
it more convenient to stand inside the form; with their feet they can
aid materially in compacting and leveling the soil surface. The blows
should be quick and sharp rather than heavy ; and where two or more
men are working on the same wall section, the tamper blows should not
be in unison, for the vibrations so set up may weaken the structure.
The four-inch layer of loose soil will usually be compacted to about a
two-and-a-half -inch depth by the third or fourth time any given area
is tamped over.
22 Some experimenters advise the use of thinner layers, stating that layers
of loose soil 1 and 2 inches deep consolidate to a more uniform wall than
where thicker layers are used. The shape of the tamping tool face is probably
the determining feature, a narrow faced tamper penetrating deeper and com-
pacting more solidly in the deep soil layers than a flat tamper. See: Miller,
R. C, Eammed earth. North Dakota Agr. Exp. Sta. Bui. 217:66. 1928.
Bul. 472]
Adobe Construction
21
When sufficiently compacted, the soil layer will give a resonant
thud under each blow of the tamper and will allow but one-eighth to
one-fourth-inch impression. Another four-inch layer of loose soil
should then be placed in the form, and the process repeated until the
form is filled to near the top with compacted soil. The form bolts may
then be loosened and drawn from the wall, the form removed and set
up at the end of the wall section just completed, and the operation
repeated until the first course of the wall extends entirely around the
structure (see fig. 17). The forms can, however, be removed from a
section of newly tamped wall, raised immediately, and set, and another
Fig. 17. — Tamping the second course on a rammed earth poultry house.
Experimental work at the Branch of the College of Agriculture, Davis, Cali-
fornia, in 1926.
layer of wall tamped on top of the first course. In any case the work
is continued until the wall extends to the desired height, entirely
enclosing the structure.
Since the hand-tamping method is necessarily laborious and slow,
attempts have been made recently to adapt machinery to the task. 23
Of the labor-saving equipment which has been suggested or tried, the
compressed air tamper appears to be the most logical. Both in its
flexibility and in its relative ease of transportation and use on the wall,
it shows many marked advantages. The rhythm of the tamper imparts
a vibration to the wall, but it appears to have no destructive action on
the strength. Pneumatic tamping equipment in experimental use on
University Farm is shown in Fig. 18.
23 Vivian, C. H. Homes of rammed earth. Compressed Air Mag. 32:1945-48.
1927.
22
University of California — Experiment Station
Publications describing this type of construction have almost uni-
formly urged that no straw or other vegetable matter be added or
permitted to remain in the soil composing the wall. This recommenda-
tion would not always seem desirable in the light of recent experimen-
tation at the California Experiment Station where, with an alluvial
loam soil, an admixture of approximately % part of oat straw by loose
volume gave an increased strength amounting to 80 per cent in small
specimens (see table 3). It has also been observed in the work at this
institution that rocks and moist clods up to the size of a walnut may
be compacted in the soil layer without difficulty or without any
noticeable weakening of the completed wall. In some instances,
Fig. 18. — A compressed ail tamper may be used successfully in the rammed
earth method. (Pneumatic equipment lent by the Pacific Telephone and Tele-
graph Company for experimental work at the Branch of the College of
Agriculture, Davis, California, in 1927.)
therefore, if the soil is well broken up in removing from the borrow
pit and does not require additional moisture, putting it through a
screen may be an unnecessary refinement.
The rammed earth method is frequently spoken of by its French
name, pise de terre. It has been used successfully in several parts of
the United States. Structures of this make are to be found in this
state in gold mining towns now deserted and in a few modern
residences.
Rammed Earth Blocks. — A limited amount of work has been done
with rammed earth pre-cast into blocks of convenient size and shape,
which can be laid up in a wall as adobe brick are laid. Such a
procedure obviates the use of the heavier, more expensive forms
Buu 472] Adobe Construction 23
required by the standard process and gives to the rammed earth
method the same adaptability to working conditions as the sun-dried
brick. The rammed brick are usually less dense, however, and more
liable to break than the sun-dried brick.
In laying the rammed earth bricks the structural strata or
laminations of the brick should be placed horizontally in the wall, so
that the vertical thrust of the wall may be perpendicular to them. A
convenient shape and size for the block is 9 inches deep, 12 inches
wide and 18 inches long. One workman can hand tamp about four
such bricks an hour. For columns or posts it may sometimes be
advisable to form blocks considerably deeper than broad, as 9 X 9
inches in cross-section by 18 inches deep. Blocks of the latter size and
shape supported loads varying from 12,000 to 27,000 pounds maximum
in tests made at the University Farm.
The form used for this purpose should be sturdily built, and
secured with quick-acting clamps to speed up the work. The sides of
the form should be completely removable or hinged to drop below the
level of the bottom so that the brick may be securely grasped for
removal. Angle irons rabbeted into the inside top edges of the sides
will prevent damage to the form from mis-directed tamper blows.
CHOICE OF CONSTRUCTION METHOD
The choice between methods depends upon the soil to be used, the
climate, the size of the working crew, and the preference of the builder.
Coarse-grained soils tamp more readily and so appear to be more
suitable for the rammed earth method than the small, smooth-grained
soils. A soil composed of % clay, % sand, and % gravel approaches
the ideal for pise construction. Heavy soils which contract markedly
on drying and so would be unsuitable for a monolithic wall may
frequently be used in the precast, sun-dried units.
The damp rammed earth process appears to be less dependent on
good drying weather and may be preferable in the temperate zones to
the wet sun-dried brick, poured earth, or cob processes. Under most
California conditions, no difficulty will be experienced with the latter
methods in securing suitable drying.
Because of the weight of the type and size of forms generally
recommended for the rammed earth construction, a three-man crew is
required for greatest efficiency in construction. More men may be
used, but the extra equipment necessary is considerably more expen-
sive than that necessitated by the other methods. The sun-dried brick
method requires handling the same unit volume of soil from three to
24 University of California — Experiment Station
four times more than in the monolithic processes, but is very easily
adaptable to any sized working crew. With the poured adobe and
cob processes the size of the working crew must be adjusted to the
rate of drying of the completed work in order to secure labor efficiency.
Persons interested in earth construction frequently exhibit decided
preferences for a certain method. Proponents of the poured adobe or
sun-dried brick methods advance the simplicity of the former and
the flexibility of the latter as outstanding advantages. Preference
for the rammed earth method is usually based on the fact that this
is ' ' cleaner than the mussy, mud methods, ' ' or that it leaves a smooth
wall surface which makes the wall finishing easy and economical.
Accurate, comparative costs of the various methods are difficult to
secure. In one poured adobe farmhouse two laborers raised the
18-inch walls for the 27 X 33-foot structure an average of six inches a
day, or at the rate of 4.5 cubic feet of wall per man hour. In sun-
dried brick construction two laborers can make about 250 brick
4 X 12 X 18 inches in a day, and can lay in the wall approximately
the same number per day. This is at a rate of 3.5 cubic feet of com-
pleted wall per man-hour of labor. Reports of rammed earth con-
struction vary widely, largely, it would seem, because of soil differ-
ences; but the rate averages between 1 and 2 cubic feet of wall per
man-hour. On a small experimental rammed earth structure, at the
University Farm, where pneumatic tamping equipment was used, a
rate of 7 cubic feet per man-hour was attained.
SELECTION OF THE SOIL MATERIALS
One of the chief advantages of the soil as a building material is
that it is a native product. This is advantageous both from the prac-
tical viewpoint where the proximity decreases transportation costs and
the capital expenditure required ; and from the artistic which finds its
most satisfying expression in the use of natural materials and simple,
restful architecture.
For the sake of economy in cartage costs the soil to be used should
be taken from as near the building site as possible. The maximum
saving usually results if the soil excavated from the cellar is satis-
factory for use in the walls.
Since all soils are not suitable for use in walls it is necessary that
a study of the available soils be made as the preliminary to all con-
struction projects. Where no one soil is satisfactory it may still be
possible to secure successful results by mixing various soils types or by
applying various admixtures to an individual soil.
Bul. 472]
Adobe Construction
25
Soil Characteristics. — Tests which have been made at the California
Agricultural Experiment Station on small specimens of soil puddled
as it would be for use in walls have shown that the specimens molded
from the wetter mixes, such as would be used in making sun-dried
bricks or pouring a mud concrete wall, are stronger than the
damp mix tamped as for making a rammed earth wall (see table 2).
In the dried specimens from six soil series common to California,
which had been mixed to various moisture contents and tamped damp
into the molds or worked into them from a mud consistency, the mean
unit compressive strength ranged from 60 to 785 pounds per square
inch, averaging 473 pounds per square inch for the wet, puddled
mixes, and 203 for the damp, rammed specimens. In but one soil
series, a sandy loam low in colloidal material, was the strength de-
veloped by the rammed earth specimens greater than that of the
specimens molded from a mud consistency. In two clay loam series
the resulting strengths of the tamped specimens was less than one-half
that of those molded from mud. None of the specimens contained
straw or other admixture.
TABLE 2
Compressive Strengths of Puddled Soils
Soil series
Placentia loam
Yolo fine sandy loam
Yolo loam
f
Hanford sandy loam
Holtville silty clay loam
Yolo clay loam
Initial
moisture
content,
per cent
4.18
8 55
14 17
26.02
10.9
15.9
28.9
12 9
7.7
15.5
25
9.4
17.1
27.8
11 9
17.3
29.1
Method
of
puddling
Tamped
Tamped
Worked .
Stirred...
Tamped
Tamped
Worked
Tamped
Tamped
Tamped
Worked .
Tamped
Worked .
Worked .
Tamped
Tamped
Worked .
Moisture
when
tested,
per cent
Volume
weight,
pounds
per cu. ft.
2.18
100.5
2.38
125.5
2.6
134
2.2
125
3.4
104.5
3.1
107.5
3.6
110.5
4.3
106
0.9
115.5
0.6
110
1.1
103
1.6
94.2
3.5
114
4.2
106
5.1
96.5
5.5
102.5
5.7
93
Compressive
strength,
pounds
per sq. in,
72.
443
750
785
190
342
445
158
112
153
109
60
95
500
102
268
570
These experiments and observations led to the conclusion that
coarse-grained soils, such as loams and sandy loams, tamp more
readily, acquire a higher relative strength, and so appear to be more
suitable for use by the rammed earth method than the soils of small,
smooth grain such as the clay and clay loams.
26 University of California — Experiment Station
There are reports of certain soils which have a tendency to go
through a process called "slacking" after puddling; that is, the soil
crumbles as it dries or if it is subjected to heavy frosts. Such soils
would appear to be wholly unsuited for building purposes, but are not
common in California.
The results of the laboratory tests quoted here should not be
considered to represent accurately the conditions of actual construc-
tion. Monolithic mud walls of the mud-concrete method which have
cracked in drying, or sun-dried brick laid up in mud mortar of uncer-
tain strength, may have actual structural strengths less than those
represented in these small uniform specimens. All of the soils, used
gave compressive strengths sufficiently high to have a considerable
factor of safety in supporting any ordinary roof load if built in walls
twelve or more inches thick. Samples taken from a twenty-year-old
adobe brick building in Brawley, Imperial Co., averaged 109.5 pounds
per square inch, and others from the century old Mission San Antonio
de Padua at Jolon, Monterey Co., averaged 260.
Soil Admixtures. — It is not uncommon for builders to mix two or
more soil types in the hope of securing a better building material.
Usually these mixtures are of a clayey soil with one too sandy to be
used alone, or of sand or loam with a soil that is too clayey. In one
test of the series conducted at the California Experiment Station sand
and gravel were added to a clayey loam in the proportion of one part
of admixture to four parts of soil, giving compressive strengths
slightly above those of the natural soil (see table 3). A one to three
proportion of gravel lowered the strength about one-third from that of
the natural soil.
There appear to be two beneficial results from mixing straw with
the earth used in wall construction. In the wet mixes the straw
apparently serves as drainage channels, assisting in conducting the
moisture from the center of the mass as it dries. It has been observed
that some soils shrink and crack less in the poured adobe and sun-
dried brick methods if straw is added to the mix. And in one series
of tests using one soil type in the rammed earth method the straw
proved a reinforcement, increasing the compressive strength nearly
80 per cent (see table 3). The proportions used in practice vary, but
by loose volume are frequently one to five (one inch of loose straw
scattered over a five-inch depth of loose soil). Straw in lengths of
two to six inches is easier to incorporate in the mixture than the long
length.
An objection is frequently raised to putting straw in an earth
wall on the grounds that it will decompose. There would appear to be
Bul. 472]
Adobe Construction
27
little justification for this belief : adobe bricks removed from century-
old missions and haciendas show the straw to be well preserved, and in
some cases as bright in color as if it had but recently been removed
from the field. By washing away adobe bricks gathered from various
localities in this state and Lower California, Mexico, Prof. George W.
Hendry, of the Agronomy Division of the College of Agriculture,
University of California, identified numerous grasses, grains, and
weeds that had been used in the brick, and thereby added materially
to the historical knowledge of the agronomy practiced in these regions
a hundred years ago. 24
TABLE 3
The Effect of Admixtures on the Compressive Strength of Puddled
Specimens of Yolo Loam Soil
Admixture
None
Marys ville sand
Putah Creek gravel
Putah Creek gravel
Hydrated lime
Oat straw
Proportion of
admixture to
soil by loose
volume measure
Initial
moisture
content,
per cent
Method
of
puddling
12 9
Tamping
1 :4
13 2
Tamping
1 :4
13.2
Tamping
1 :3
14 3
Tamping
1 :4
114
Tamping
1 :5
12
Tamping
Volume
weight,
pounds
per cu. ft.
106
116
122
124
109.5
107.5
Compressive
strength,
pounds
158.5
167.5
164.5
108
198.0
284
Various chemical admixtures have been suggested usually based on
the theory of flocculating the soil colloidal material and so reducing
the tendency to shrinking. 25 Lime is one of the materials most
commonly suggested. In some rammed earth test specimens a 1 to 4
mix of hydrated lime and a clayey loam soil gave compressive strengths
approximately 25 per cent greater than those of the natural soil (see
table 3).
COMPUTING VOLUME OF EXCAVATION NECESSARY TO
PROVIDE THE WALL MATERIAL
It is sometimes advisable before starting actual construction to
compute the amount of excavation necessary to provide soil for the
erection of the walls. The volume of the walls can easily be determined
from the plans of the structure by multiplying the thickness by the
length, and the product by the height of the walls, and subtracting
the volume of the openings. The volume so determined, however, does'
24 Hendry, Geo. W., and Margaret P. Kelly. The plant content of adobe
brick. Quart. California Hist. Soc. 4:361-365. 1925.
25 Botkins, C. W. The influence of flocculation on the compression strength
of Gila clay loam. Agr. Engr. 8(9):252. 1927.
28 University of California — Experiment Station
not represent the volume necessary in the excavation, because the
earth when puddled for placing' in the wall is compacted more than
occurs naturally in the ground. The exact amount of the compaction,
or the per cent of the shrinkage which results, varies with different
soils, in the various methods of handling, and also with the amounts
of stones and organic matter thrown out; but it is advisable to
estimate the excavation as 25 per cent more than the volume of the
walls. Thus it can readily be seen that a relatively small basement
area will provide soil enough for the walls of a house (see fig. 3).
STRUCTURAL DESIGN OF THE WALL
In any building work where materials with certain minimum re-
quirements as to durability, strength, fire safeness, and the like are
used, the results obtained in the structure and the satisfaction it will
give in use depend not so much on the selection of the material as
on the way in which it is used. A knowledge of the material to be
used, of the forms of architecture to which it is best suited, and of
the general principles of architecture and construction is necessary for
the best results.
Because of the characteristics of the adobe material and the manner
of its use, certain modifications of standard construction practice are
necessary and new types of construction technique will have to be
developed in its use. It is advisable that these features be carefully
studied and planned for in advance.
Integral Reinforcement. — The great majority of earth wall struc-
tures are erected with no reinforcement except the straw or other
binding material which may be mixed intimately with the soil as it
is prepared. Some builders, however, believe it wise to place in the
walls as they are erected steel rods, barbed wire, woven wire fencing,
or rough wood battens laid horizontally or vertically. Others believe
that the puddled soil shrinks away from such materials and fails to
develop the bond necessary for securing increased rigidity.
The reinforcing value or the relative efficiency of these various
devices would be difficult of exact determination. Such reinforcements
appear, however, to have some value. One modern residence in El
Centro, Imperial County, so reinforced, has survived rather severe
earthquake shocks without damage. This structure is built with
12-inch walls of 4 X 12 X 18-inch sun-dried brick, with three strands
of barbed wire laid horizontally in every third mortar joint and with
%-inch iron rods 36 inches long spaced 3 feet apart and driven down
through every six courses of brick.
Bul. 472] Adobe Construction 29
A practice which would appear to have merit is that of using a
6-foot width of 1-inch mesh, 18-gauge chicken wire, laid in every sixth
mortar joint of sun-dried brick construction, the edges being allowed
to hang down on either side and stapled with the 2-inch staples to
the wall faces for a plaster and stucco reinforcement. For a 12-inch
wall, with 4-inch bricks and a %-inch mortar joint, a 6-foot wire width
placed in every sixth mortar joint will lap two or three inches on
either side of the wall over the wire below.
It is sometimes recommended by builders that barbed or woven
wire be placed between courses of poured adobe or rammed earth
constructions, particularly around the corners, in order to help
obviate vertical cracks. 26
Wall Thickness. — Where an earth mixture is used as a bearing
wall, a minimum thickness of twelve inches is advisable because of
the structural strength of the material. In the rammed earth method
walls less than 9 inches in thickness have been found to induce con-
struction difficulties, in that the closely spaced forms make tamping
difficult. For non-bearing walls, such as some partition walls, garden
walls, and the like, thicknesses of eight, six, and even four inches, have
been used.
For two-story construction it is recommended that the first-story
walls be not less than eighteen inches thick and the second-story walls
a minimum of twelve inches.
Wall Heights. — Although there are examples of earth wall struc-
tures which exceed two stories in height, this is believed to be a prac-
tical limit for modern construction. In regions subject to earthquakes
or severe wind, one-story construction is advised.
Where walls one foot thick rise more than ten feet between floor
and ceiling, cross walls or buttresses should be introduced to stiffen the
wall every ten feet of its length. Gable ends may be erected of earth
and should be well tied to the roof framing.
Wall Foundation Courses. — One of the necessities for a durable
earth wall is a strong, watertight foundation raised sufficiently above
the outside grade to protect the wall against moisture, particularly
the splash of water dripping from the roof. Where low foundations
have been used, no roof gutters provided, and no waterproofing placed
on the exterior of the wail, it is not uncommon to see the lower foot
or so of the wall eroded.
In several examples of earth-walled houses in this country, the
walls rise from the ground surface or from a shallow trench without
26 Ellington, Karl J. Modern pise building, p. 36. K. J. Ellington, Port
Angeles, Washington. 1924.
30
University of California — Experiment Station
a foundation of harder material for a waterproofing layer. Where
the site is well drained, and surface water is prevented from running
against the foot of the wall, this may prove a satisfactory construction
for temporary or semi-permanent structures. For the better class of
residence construction, however, it has been found that without damp-
proofed foundations the moisture conducted up through the wall
discolored the interior wall finish for some distance above the floor,
made a damp interior, and rotted out the floor timbers.
i^v^ W ( '
Fig. 19. — A type of concrete foundation which may prove economical for
some work. The raised portion of the footing immediately under the wall is
to provide a ledge to hold the wall form when starting the first wall course.
For adobe brick construction, where no wall form is required, the footing may
be a flat slab. The tar damp-proofing is applied to the face of the wall behind
the stucco and concrete skirting. The skirting is poured after the wall is
erected. The floor joists may be buried in the wall as shown, or supported on
the foundation as shown in figures 21 and 25. The triangular nailing block
sketched is suitable only for use in the rammed earth and poured adobe
methods. An adobe wall should not be carried below grade unless special
precautions are taken to protect it from external moisture.
Where a temporary structure without masonry foundations is
desired, a wide trench should be dug into the sub-soil and the trench
bottom solidly tamped. A damp-proofing such as a thick paint coat of
tar should be applied to the trench bottom, and the adobe wall erected
to a height of at least a foot above grade, where a layer of the damp-
proofing is carried across the top and down either side to the bottom.
This procedure should help to protect the earth wall against the
entrance of moisture and against the splash of water from the eaves
Bul. 472]
Adobe Construction
31
(see fig. 30). A concrete splash apron sloping away from the base
of the wall is a good feature to add to such footings.
Where a foundation is used, monolithic concrete is considered
preferable to any other material because of its durability and the
fact that it tends to span any small areas of sub-grade settlement,
Where there is any possibility of the sub-grade being insecure, the
concrete should be reinforced. Because the earth wall is heavy it is
advisable, if the bearing soil is soft, to make the width of the foun-
dation footing three times the thickness of the wall.
Tar
Grade
poirTt-^/";
\\v//^oV//^
^r'earfh or qrcivel fill < x
S > v > xx UL >to UL *ma g sa 'JU aJa a ■ - y,"
Fig. 20. — An economical foundation where a concrete floor is used. If the
wall is to be of sun-dried bricks, so that the ledge above the floor will not be
necessary to hold the wall-form for the first course of the wall, and if no water
will be used on the floor, the ledge may be dispensed with and the floor
continued straight out to the outside edge of the foundation. Wire mesh
concrete reinforcing, laid over the entire floor area as the concrete is poured,
will materially aid in preventing cracks.
Since a concrete foundation equal in thickness to the wall and
carried a foot above the grade line is a relatively expensive construc-
tion item, various attempts have been made to design a foundation
requiring a minimum amount of material. Where a concrete floor is
being used in the building, the foundation need, perhaps, not be
carried more than a few inches below the fill into the original ground
(see figs. 8 and 20). A damp-proofing paint course or a plaster dado
(see fig. 34), should be carried two feet above grade if stucco is not
used on the exterior wall of such designs. Another design, satisfactory
if the vertical joint between earth wall and concrete skirting is made
watertight, is shown in figure 19.
Waterproof Course. — To avoid the conduction of moisture from the
ground into the wall, a waterproof course of tar paint or asphalt
32
University of California — Experiment Station
roofing should be applied to the top of the foundation before the wall
is started. Where no foundation is used, the waterproofing should be
placed in the bottom of the trench excavated for the wall, up over
the sides of the wall to a height a foot or more above grade, and
through the thickness of the wall at that height as described in the
previous section on foundations.
Preventing Dry Rot and Termite Damage. — Precautions should be
taken to prevent dry rot and termites (" white ants") from attacking
the timbers used in conjunction with the earth walls.
Hud or lime mor far Joints.
Rgh. — continuous na>
chair rait or picture molding
1*4' Rgh." continuous nailing atrip for wood base,
ha'
//i
y v
Length and depth of an adobe brick.
Floor joisf with bevel -cut end to
minimize fire damage.
Cut or <ohim joist to level on foundation ledge,
Concrete foundation
and footing.
PLAN
;|7
Joists -^
w
Fill ledge
wilt? brick -bats
or adobe to
top ofjoisfe.
Fig. 21. — Alternate floor detail for adobe brick construction and a con-
tinuous nailing strip detail. (Courtesy E. D. Herreras.)
Dry rot is a fungous growth that attacks wood. Since moisture is
necessary to the growth of the fungus, adequate water-proofing at the
foot of the wall will largely overcome the possibility of attack. Wood
members lying on the concrete foundation or in the earth wall near
the ground level should be treated with a wood preservative before
they are put in place. No wood members should be permitted to rest
directly on the ground. Leaving a minimum of 12 inches air space
between the bottom of the floor joists and the ground, and ventilating
this space with a 6 by 18-inch screened opening on each side of the
building, are also advisable.
Termites, though they have never been known to tunnel through
earth walls, have been observed to erect their sheltering tunnels over
Bul. 472] Adobe Construction 33
the outside of the walls into roof members. Metal shields properly
placed along the tops of concrete foundations would prevent this
infestation. Where termites are known to exist, all scrap lumber
should be cleared from under the floor area, and all floor timbers
exposed on the under side should be treated with a wood preservative.
More complete information on precautionary measures is avail-
able in publications devoted to dry rot 27 and termites. 28
Openings for Windows and Doors. — It is usually considered that
the forming required to block out a window or door opening requires
more time and labor than if the wall were solid. Fastening the door
and window frames into the wall also presents, a problem because of
the low tensile strength of the material and hence the relatively slight
holding power of the wall for inserts and ties. For this reason and
also because of rough usage often given these parts of the structure,
particular care should be taken to secure tightly fitted frames.
The frames for openings are usually built in place in the sun-dried
brick walls, but with walls erected by the other methods it is fre-
quently preferable to block out the opening with a temporary frame
and make provisions for putting in the finish frame later. This
procedure involves burying in the wall, at the time of erection, nailing
blocks to which the frames can be fastened. Such nailing blocks
should be 2 by 4 inches or larger, should extend 12 inches or more
back from the door opening, and should lie as near the center of the
wall thickness as possible. A cross-piece nailed to the buried end of
the block will iix it more solidly in the wall. A vertical 2X4 timber,
buried in the wall at the side of the opening and secured with blocks
or metal ties extending back into the wall, is sometimes used (see fig.
22). Corrugated metal strips like those used in standard brick work
are satisfactory for this purpose.
Another method sometimes used to fasten the frames to the wall
is to bury long bolts with large washers as the wall is erected, leaving
enough of the end exposed to secure the rough frame. Nailing through
the frame with large spikes to reach into the earth wall, and nailing
bent metal ties which extend into the wall on the back of the frames,
are methods also in use.
Care should be exercised to make a tight joint between the frame
and wall in order to exclude drafts and moisture.
27 Hunt, George M. The preservative treatment of farm timbers. IT. S.
Dept. Agr. Farmers' Bul. 744:1-32. 1916.
28 Snyder, T. E. Preventing damage by termites or white ants. IT. S.
Dept. Agr. Farmers' Bul. 1472:1-22. 1926.
Light, S. F. Termites and termite damage. California Agr. Exp. Sta., Cir.,
314:1-29. 1929.
34
University of California— Experiment Station
A lintel is usually built into the wall to support its weight above
the opening (see fig. 23). This lintel should be the full width of the
wall and should have a minimum of nine inches of bearing on either
side of the opening. Because of the tendency of all earth walls to
shrink as they dry out, a shrinkage amounting to as much as one inch
Exterior
■stucco
Metal
flashing over
wood lintel
Helad
Nailing block
Z**4" strip
along jamb ■Sp£
Side.
Stucco
Reioforciog
Head
SO d. spike 3 every
8 inches aloncj jamb.
SIDE.
Fig. 22. — Two details of lintel and jamb construction for door openings.
At the right is a concrete lintel and rabbeted jamb, the latter held in place by
spikes extending into the wall. This method of support is more suitable with
sun-dried brick construction than with any of the other wall construction
methods.
At the left is a detail of a built-up wood lintel and wood frame extending
through the wall thickness. The nailing block and vertical strip set into the
wall should be placed in position when the wall is erected; the jamb members
may be placed then or later, as desired. There should be at least three nailing
blocks on each side of the opening, extending 18 inches or more into the wall.
All vertical members placed when the wall is erected should be cut approx-
imately one-half inch short to allow for the settling of the wall, otherwise they
will not allow the lintel to settle with the wall and will cause cracks above
the opening.
for every ten feet of height, it is necessary to place the lintel above
the height desired for the finish frame an amount depending on the
height of the opening.
Self-supporting arches may be erected, sun-dried brick being used ;
but such arches complicate the building of the frame and should not
be attempted unless constructed by experienced builders.
Bul. 472]
Adobe Construction
35
Providing for the Roof Loading. — Because of the low tensile
strength of the puddled earth material, the walls will not withstand
lateral or outward thrusts. For this reason the roof trusses must be
so designed that they will impart only vertical loads on the wall ;
they must be so well tied as to have no tendency to spread (see figs.
25 and 26).
Interior
planter
Wood lintel
T-shaped
nailing block
Wire mesh,
reinforcing
around all
corners
Metal
flashing
strip
•Shaped and
stained to
give wood
beam effect
Stucco
moidinq
5jde
Nailing block '-zjfrj
Damp 'proof
course
Concrete si//
poured under
window frame
Fig. 23. — Typical detail for a double-hung window set midway of the
wall thickness.
For the best construction it is advisable to place a reinforced con-
crete collar beam at the top of all earth walls (see fig. 25). This
provides a continuous reinforcement or tie around the wall and also
gives a secure means of fastening the roof to the wall. When the
earth wall has come within six inches or a foot of the height desired in
36
University of California — Experiment Station
the wall, it is smoothed off to an approximately level surface, and 20
penny spikes are driven half their length into the top of the wall,
spaced six inches to a foot apart, and staggered across the width of the
wall. These provide a tie between the earth and the concrete, which
is poured in a continuous layer around the perimeter of the wall, a
minimum of six inches thick. Two ^-inch reinforcing rods should be
jfi cqcrf it? concrete
for reinforcing.
---'-"-•^1
iSide y/eiv of Jirrral
Extra timber for
thick /raffs or oyer
rride open,
nail
ALTERNATE. DETAIL
Three. Z~*4 or Z'*6''
blocked and nailed of ends
and center cf <sjean.
Fig. 24. — Lintels are used to carry the load above door and window open-
ings. The upper detail shows a reinforced concrete lintel for a three-foot-wide
door or window opening in a wall 12 inches thick. The lintel is long enough
to extend into the wall nine inches at either side of the opening. Four
reinforcing rods are used, the two inner being bent up at 45° one foot either
side of the center of the lintel. The reinforcing should be so placed as to be
just one inch from bottom (and top) surfaces of the concrete slab. The con-
crete for the lintel should be mixed in the proportion 1:2:3. The completed
weight of the lintel shown will be approximately 225 pounds.
Two designs for built-up wood lintels are shown in the lower detail. Solid
timbers, such as discarded but sound railroad ties, are sometimes cheaper than
built-up lintels. Struts should be placed under all lintels when building over
them by the rammed earth method, in order to minimize vibration. Wood
lintels can be covered with metal lath and plastered if desired.
placed in the concrete, one near each face of the wall, about the center
of the concrete layer and an inch from the outer face. These rods tie
the beam together and strengthen it against lateral thrusts. Standard
reinforcing rods should be used ; barbed wire, woven wire, steel cable,
and strap iron are not good reinforcements. Before the concrete has
Bul. 472]
Adobe Construction
37
Z*4 plate
'/z*6" bolt,
6'o.c.
Cement
plaster
Ventilator
Grade
Light weight
insulating material
placed between
floor Joists
5 /s reinforcing bans (Z), continuous
Z.O d nail driven into yrall
Reinforced concrete bond stone,
continuous,
h— Lime plaster
'//'• s- Quarter -round finis
Oamp-proofin,- £ )^^ £ Sff
myJMtif f . • • '
Quarter -round finish molding
nish floor
tiding paper
Rough floor
Z' sill -Floor Joist
fjir space under floor
.;. W&//rW^"'*
Fig. 25. — Cross section of adobe wall construction, showing typical concrete
foundation and collar beam, an insulated ceiling, and a light-weight roof of
standard construction. There should be at least one screened ventilating open-
ing of one-half square foot cross-section area on each side of the structure,
placed through the foundation as shown. Such ventilation under the floor
obviates possibility of buckling of the flooring, or dry rot of the floor joists.
The concrete for both foundation and collar beam should be of 1:2^: 4 mixture.
The 2-inch sill supporting the floor joists should be bedded in a concrete
mortar to level on the foundations. Some builders dispense with the sill and
wedge under the joists to level.
GraveUd roll roofin
4-" Rafter
/-r
fldoba
,wall
/ /
held
bu *if - IE" bolta
ev«ry 4 or 6 feet.
Bolts secured with
A' wood or m«tal
platw in -tbe adob«.
Fig. 26. — Details of cheaper roof plate designs where reinforced concrete
bond stones are not used along the top of the adobe wall. A continuous strip
of metal lath 12 inches wide, bent around the corner between wall and ceiling
as shown in the detail to the right, will prevent corner cracks.
At the left is shown a detail of a construction which transmits the roof
weight to the center of the wall and gives an interesting roof line. Bolts
buried in the adobe wall should be used to fasten the plate in regions subject
to high winds.
38
University of California — Experiment Station
set, bolts with large washers should be buried every four to six feet
along the top of the beam, with enough exposed to hold a 2-inch wood
plate. Fastening the roof members with spikes or lag screws to a plate
so secured is one of the surest means of tying roof and wall together.
In case the concrete collar beam is not desired, the wood plate
member should be fastened to the earth wall by long bolts with large
washers buried ten inches or more in the wall. A variation of this
method is to bore 2-inch holes down 10 inches from the top of the wall
with a soil auger and set the bolts in these holes in concrete. Wire ties
Pig. 27. — Earthquake damage, Calexico, California, January, 1927, showing
that well-built adobe walls can withstand earthquakes without being destroyed.
In these structures, however, the heavy tile roof of the bungalow and the
second story of adobe in the apartment house violate good design practice for
regions where earthquakes may occur. A few one-fourth inch cracks and
some crushed adobe brick where partition walls had pounded against outer
walls constituted the main structural damage. One minor crack was the only
apparent damage in the second-floor walls of the apartment. Good concrete
foundations and the use of concrete, bond stones at the tops of the walls (and
at the second floor level in the apartment) undoubtedly minimized the
damage. There was no integral reinforcement in the walls. The. stucco was
applied over nails driven into the adobe walls and had to be completely
removed and replaced except on the second-story walls of the apartment. The
interior plaster suffered damage similar to the stucco.
put in place across the wall a foot or more from the top as the wall is
erected, and then bent up and over the plate, and spikes driven down
through the plate into the wall, are sometimes used on small, cheap
structures.
Designing Walls to Increase their Resistance to Earthquakes. —
Earth wall construction is inferior to most standard construction
materials in earthquake resistance. Well-built adobe structures do
not disintegrate so readily under earthquake shock as is commonly
believed (see figs. 27 and 28) ; but where earthquake resistance alone
Bul. 472]
Adobe Construction
39
is the deciding factor other materials known to be superior for this
purpose should be employed. 29
If a builder desires to erect an adobe structure in a region subject
to earthquakes, serious consideration of each of the following design
features is advised. Skillful and intelligent workmanship is also much
to be desired.
a. The plan of the building should be compact and regular. One-
story construction, with exterior walls not less than VI inches thick,
is recommended. If a second-story is added, the first-story walls
should be not less than 18 inches thick, the second-story should be not
Fig. 28. — Earthquake damage due to poor design and construction, prin-
cipally the lack of a foundation and of a bond stone at the top of the wall.
The structure was being salvaged when the picture was taken by erecting wood
studs inside to support the roof. The adobe walls were to be tied to these
studs to form a veneer wall. Calexico, California, January, 1927.
less than 12 inches, or may preferably be constructed of light, well-
braced wood frame. Bearing walls should not exceed 30 feet in length,
unless intercepted at intervals of 30 feet or less by earth or masonry
cross walls at least 8 feet long.
b. A substantial one-piece foundation is essential, and is best pro-
vided by reinforced concrete laid on a dense sub-grade.
c. The puddled earth material used in the wall should have a
minimum crushing strength of about 500 pounds per square inch. 30
d. Mud mortar of such composition and consistency that it will
dry to a hard, dense mass is suitable for construction of pre-cast
units. Lime and cement mortars may also be used.
29 Long, J. D. Adobe structures for earthquake regions. Agr. Engr. 8(7):
173-175. 1927.
30 This is the value specified in the Santa Barbara building code of 1926.
40
University of California — Experiment Station
e. Integral reinforcement of the wall is advisable.
/. Window and door frames should be firmly anchored to the walls.
g. Partition and adjoining walls of other materials should be of
light construction to avoid such rigidity as might crush or displace
the bearing walls of earth by oscillating at a different period.
h. Cross pieces for the roof and upper floors, or members at right
angles to them, should be so anchored to the walls at six-foot intervals
as to provide a cross-tie for each wall at the ceiling line.
i. Ceilings of wall-board, lumber, or sheet metal are preferable to
those of plaster, as there is less danger of their dropping from position.
12.* adobe brick wall
for second vfory
\\\\\^\\\\\\\^\^
F/oor J°>-*+
EZZZZ
*■ Z" p/afe
^--Concrete bond <r/one
reinfbrcea? mtb f»vo
ff? rods.
18" adobe brick wa/f*
fbr f)rsr •r/o/y.
Fig. 29. — Detail of concrete bond stone at second floor level.
j. A continuous, reinforced concrete bond stone of six-inch mini-
mum depth should be placed on top of, and bonded to, all earth walls.
Such a bond stone should also be used at the top of first-story walls
where a second-story is added (see Hg. 29).
k. The roof trusses should be so designed and constructed as to
impart no side thrust ; they should put only vertical compression loads,
evenly distributed, on the wall.
I. Roofs should be of light weight, and firmly anchored to the walls.
m. A good protective covering should be used on the exterior of
the walls to prevent excessive weathering, particularly at the ground
line. Lime and cement plasters and stucco should have a wire netting
reinforcement (18-gauge minimum), well stapled to the earth wall
with 2-inch staples.
Bul. 472] Adobe Construction 41
PLANS FOR ADOBE STRUCTURES
In any type of building work it is always advisable to plan the
structure carefully (see fig. 30), before the actual work of construction
is undertaken. Careful planning is even more necessary for adobe
structures than where the more common construction materials are
used, because the workmen will naturally find it somewhat harder to
adapt their methods to the new material. Also it is more difficult to
fasten the earth materials together and to secure other materials to
them, and so it is harder to make corrections, alterations or additions.
METHODS OF FINISHING WALLS
Most earth walls are sufficiently soft and porous to dust off when
brushed against and to absorb water coming in contact with them. For
this reason it is advisable to put on the walls, both interior and
exterior, a protective coat which will provide protection against
mechanical wear and, on the exterior, against moisture.
The damage done to a bare earth wall when rain falls against the
vertical surface is very slight; destruction results if water is per-
mitted to soak down into the wall from the top, or to splash from
the eaves and undermine the wall at the bottom. A tight roof and a
high foundation or exterior coating at the base of the wall to prevent
undermining are essentials for permanent construction. A wide over-
hang to the roof and an eaves trough to prevent roof drainage from
dropping at the foot of the wall are advantageous. Copings, cornices,
exterior trim, and frames of all openings should be designed to carry
the water away from the wall.
The various materials which have been applied as finishes to earth
walls may be listed under the headings : plaster and stucco, paints, and
whitewash. None of the finishes should be applied until the wall has
thoroughly dried out. It is usually advisable to wait at least two
months after the completion of the walls before applying finish coat-
ings. Under California climatic conditions this delay should give
sufficient time for the wall to reach an "airdry" state.
Plaster and Stucco. — Mud may be plastered against earth walls in
the same way that cement and lime plasters are applied, and usually
adheres much more closely than these standard plasters. Earth from
the same source as the walls may be used, provided it has been care-
fully screened to remove small gravels which would ' ' tear ' ' the plaster
under the trowel. Where the soil is a clay or a clayey loam, screened
42
University of California — Experiment Station
Bul. 472] Adobe Construction 43
sand must be mixed in to break the tendency of the mud to crack in
drying". The proportion required for best results can only be deter-
mined by experiment with the individual soil in question.
Lime plasters are used directly on earth walls; and if the wall
is slightly rough and care is taken to work the plaster firmly against
the wall, there is usually no difficulty about its adhering. Lime
plasters should not be used for exterior walls, because they are subject
to washing by the rains.
Cement stuccos have been most widely used as exterior finishes in
this state and are probably the best where both water tightness and
resistance to mechanical wear are desired (see figs. 10, 33 and 34). In
almost all instances, however, there is difficulty in getting the cement
stucco to adhere to the natural earth surface even when it has been
roughened prior to the application. Apparently because of the
difference in thermal expansion of the cement and earth or the
moisture absorbed by the stucco, the two materials separate after a
short period of exposure, leaving the "skin" of stucco standing un-
supported. This frequently breaks and comes from the wall in large
slabs. There appears to be little or no difficulty of this nature on
interior walls.
Reinforcing and better workmanship in application are recom-
mended to overcome this fault. Fastening metal lath or one-inch mesh,
18-gauge poultry netting to the wall surface with large staples gives a
reinforcement for the stucco even if it should loosen from the wall.
Another method of tying the stucco is to drive eight-penny nails into
the walls, spacing them about six inches apart and leaving one-quarter
inch of the head exposed to bond to the stucco. Countersinking the
nails by striking the wall with the hammer and driving the nail in the
depression so formed obviates the difficulty of plastering over the nail
heads protruding from the wall. Wire ties laid in the joints as the
wall is erected may be used to hold the wire reinforcing'. Some
builders bury sharp stones and broken bits of tile in the outside of the
mortar joints of sun-dried brick construction to bond the stucco.
Reinforcing" need not be used on interior walls, but all adobe wall
edges and corners, including ceiling corners, should certainly be
reinforced by bending a six-inch strip of metal lath around the
corner and nailing it with eight-penny nails to the adobe walls.
Similar reinforcement should be given exposed edges on the exterior.
For the best work this reinforcement should be used in addition to
any wire reinforcing placed over the face of the wall.
A few contractors appear to be successful in getting stucco to
adhere to the walls by carefully raking out the mortar joints of
44 University of California — Experiment Station
sun-dried brick work to a depth of one-half inch or scoring the sides
of other earth walls to the same depth, and then throwing' and firmly
troweling the stucco into the depressions so formed. Sun-dried brick
with the special plaster groove cast on the side which was described
in the section on methods appear to give little difficulty in this regard.
Mixing not more than 10 per cent of lime with the cement for the
stucco increases the workability without materially decreasing the
strength. Two coats of cement plaster of 1 to 3 mix and a third coat
of stucco are recommended for the best work.
In one known instance where there was no roof overhang, a very
heavy rainstorm wetted the .stucco to such an extent that moisture was
conducted through the wall and showed as damp spots on the interior
plaster. Two coats of liquid stucco waterproofing applied to the
stucco corrected this fault.
Paints and Liquid Waterproofings. — A number of paint compounds
have been applied to the natural earth walls with some success. These
include the common oil and lead paint, a mixture of equal quantities
of light mineral lubricating oil and linseed oil, a mixture of kerosene
and linseed oil, proprietary waterproofing paints, gypsum plastic
paints, and tar paints. All but the last-named have been used on the
interior as well as on the exterior walls. A standard glue or commer-
cial sizing, or a diluted linseed oil is sometimes applied to the wall as
a first coat to decrease the amount of finish material required and to
give a firmer surface to work on.
The paint compounds give finishes relatively easy of application
and renewal. Their chief disadvantages are that they do not provide
so firm a coating against mechanical wear as do the stucco and plaster
coats, and that their length of life is uncertain because there are no
tests to indicate what may be expected of their wearing qualities over
a period of years.
Liquid waterproofing solutions to be sprayed or brushed on to the
wall have been suggested as cheaper finishes. These include a solution
of silicate of soda, hot coal-tar pitch, or asphalt, or a solution of
bitumen, resin, or paraffin wax in light oils. The duration of the
wearing period of these materials is also uncertain. Because of their
relative cheapness and ease of application, renewal, if necessary after
three to five years, may prove an economical procedure.
Whitewash Applications. — In most of the ancient work and in
much of the modern, a coating of whitewash, renewed each year, is
the only finish given to earth walls. It is not uncommon, in desert
locations, to hear of the Indian and Mexican builders mixing in
cactus juice to make the whitewash coating more permanent. The
Bul. 472] Adobe Construction 45
same objective is accomplished in our present day formulas for white-
wash which specify glue or skim milk as one of the ingredients' 51 (see
frontispiece and fig. 31). In some of the modern work a wash of
cement and water, mixed to a cream consistency and applied with a
brush, has served as a temporary finish.
Fig. 31. — The stables on the Libby Estate, Ojai, California, provide a good
study of simple architectural treatment and whitewashed walls. (Wallace
Neff, architect.)
Wall Papers and Canvas Coverings. — Wall papers, and various
wall paper substitutes, have been successfully applied to earth walls
for interior finishes. The details of application are similar to those for
the application of wall papers over lime-plastered walls. If the
surface of the wall is uneven, it may be brought to a smooth, even
surface by a mud plaster coat. The actual details of hanging the wall
paper vary but little from standard practice.
The wall is first sized with a glue or commercial sizing, two coats
being required on the more porous surfaces. The sizing fills the
pores between the soil particles, binds the particles together, and
gives a firm surface for the additional treatments. After the sizing is
dry, the paper is hung, using the paste and methods common to the
paper-hanger's trade.
Canvas makes a particularly attractive and suitable finish for
earth walls, the weave of the fabric giving a pleasing texture and the
strength of the cloth resisting cracking and mechanical wear. Light
weight canvases may be used. After the wall has been sized, the
canvas is applied similarly to the hanging of wall paper, using glue
to secure it. A paint coating on the canvas wall gives a pleasing
finish:
3i Whitewash and cold water paint. Bul. 304-B:l-8. National Lime Associa-
tion, Washington, D. C.
46
University of California — Experiment Station
ADVANTAGES AND DISADVANTAGES OF USING EARTH
FOR BUILDING MATERIAL
No construction material can be entirely suitable for all types of
structures or all localities; there are always advantages and disad-
vantages to be considered in making a selection of a material. The
characteristics of earth as a building material may be discussed under
the headings of availability, labor required for building, strength,
costs, durability, sanitation and comfort, and attractiveness.
Fig. 32. — Small farm structures of adobe construction. (A) Entrance gate-
way and (B) manure pit on the R. H. Baker ranch near Saugus, California.
(C) The fruit sulphuring house and (D) the gasoline and oil storage house on the
Kearney Park ranch near Fresno, California.
Availability of the Material. — Not all soils are suitable for earth
wall construction, but it appears that most of them can be used in
some one or more of the various methods of wall building, thus per-
mitting adobe construction to be used almost universally insofar as
material is concerned. A thorough study of the characteristics of the
soil available is always necessary before construction work is under-
taken. One advantage, of major importance in some instances, is the
availability of the material near to the site, which eliminates trans-
portation troubles and costs. 32
The Erection Labor Requirement. — One of the disadvantages of
this form of construction is that it requires a large amount of physical
32 Pise de terre mine buildings. Engineering and Mining Jour. 106:615.
1918. 8 * /
Bul. 472]
Adobe Construction
47
labor. Farm equipment already owned by the farmer and standard
construction equipment owned by the small contractor can be used to
a slight extent in some of the methods of adobe building, but it is
doubtful that any great advance in labor saving machinery will be
made unless some contractor secures sufficient work of this nature to
justify the expense of the development and construction of special
machinery.
Another disadvantage to the person who does not wish personally
to undertake the construction work is the difficulty of securing work-
men skilled in this form of construction.
Fig. 33. — Adobe is sometimes a satisfactory material for small urban
structures. Offices of the Santa Fe Land Improvement Company, Eancho Santa
Fe, San Diego County, California. Adobe brick construction with cement-
plastered exterior walls. (Eequa and Jackson, architects. Photograph by
courtesy of Miss Lillian J. Rice.)
Strength Developed in the Completed Walls. — The relatively low
structural strength of puddled soil makes it imperative that all major
fastenings be provided as the wall is formed. Because it is hard to
join new work with old, corrections in the work and additions are
difficult and expensive. Other structural materials must be provided to
assume tensile stresses, such as occur over door and window openings.
The material is unsuitable for structures where the goods stored
within exert lateral pressures against the wall, or where door and
window openings make up a very large part of the wall area, partic-
ularly if doors are to be hinged or fastened to wall parts in such a way
as to put them under tension loads.
Largely because of the low strength, it is necessary to use thick
walls. These have many advantages, but they also have the disadvan-
48 University of California — Experiment Station
tage of being bulky and increasing the ground (or plan) area to secure
the same net floor space inside, thus requiring more expense for
foundation and roof.
Costs of the Completed Adobe Structure. — The matter of initial
cost of earth wall structures has frequently been mis-stated. Over a
wide range of conditions and structures it appears that the first cost
of a residence of such construction, including cost items for all labor,
commercial materials, and equipment necessary to put the structure
in readiness for use, is about the same as for a similar quality wood
frame and stucco or weather-boarded structure. Some contractors
figure adobe residence building costs as 10 per cent more than wood
frame for the same net floor area.
Where the builder desires to utilize his own labor in the building,
the cash expenditure necessary can be reduced very materially. One
of the chief advantages of this form of construction is that it does
permit a "self -builder" to provide shelter with his own labor and a
minimum of capital.
Further economies can be effected in the elimination of much wood
trim, the utilization of second-hand and second-quality materials, the
development of low cost wall finishes, and simplification of the design.
For example, second-hand bricks, lumber, doors, etc., from wrecked
buildings are sometimes procurable in good condition and at low
cost ; knotty lumber, rough beams, and imperfectly burned brick will
often work in quite as satisfactorily as the more expensive, better
quality materials ; and adobe may be used for fireplace and chimney,
with fire-brick lining for the opening and terra cotta or concrete pipe
lining for the flue, at a cash cost materially less than that of standard
construction. Most builders, too, are of the opinion that their further
work with this material, after the initial effort, can be done much more
efficiently and cheaply because of time-saving expedients learned on
the first job.
Table 4 gives the results of a survey of costs of typical structures
of recent date. It is impossible to make an accurate, generalized
estimate of construction costs, because a large part of such costs is
for unskilled labor which has a wide range of efficiency. Then too,
very few builders keep an accurate record of their costs. Frequently
the recorded price is the contract price and so includes the builder's
profit. Costs per square foot are not reliable because of the divergence
of opinion as to how much of the interior finishing, equipment, and
furnishing should be included in the estimate. Basic information on
costs is given in the section on the choice of construction method as
data on cubic feet of wall erected per man-hour of labor.
Bul. 472]
Adobe Construction
49
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50 University of California — Experiment Station
For outbuildings such as barns, garages, and sheds the probable
costs with earth construction, including all labor, is perhaps higher
than for standard wood construction. In such structures the cost
of the earth wall itself will be low because of the relatively small
proportion of area in openings and the straight, simple structural
lines. The cost of the exterior walls of such structures represents,
furthermore, a larger per cent of the total cost of the structure. But
in such buildings the cost competition is with a lighter, cheaper con-
struction than in residence walls.
The Durability of Earth Walls. — As with any other material
likely to fall into the hands of careless, incompetent, and ignorant
workmen, there are many examples of crude, insecure, and insanitary
structures of adobe. Several examples of structures a century or more
old attest, however, the fact that when properly used and given the
slight attention they require, adobe walls are as durable as can
reasonably be expected of any material. One good illustration of this
fact is the Palace of the Governors which was erected of adobe brick
in what is now Santa Fe, New Mexico, in 1609. 33
The chief point to be considered to insure the durability of earth
walls in small structures is protection against structural defects,
moisture, vermin, surface destruction by accident, weathering, and
physical or chemical decay.
Lack of a suitable foundation to prevent uneven settlement of the
wall and to prevent water and wind erosion at the ground line has
apparently undermined many neglected structures. The susceptibility
of such walls to destruction from inundation is not so great as might
be supposed. For example, a house in Ventura, California, erected in
1857, stood through a clay or more of flooding from an adjacent river,
and two adobe brick houses in the vicinity of Santa Paula, California,
flooded at the time of the St. Francis dam disaster in 1928, suffered
no structural damage. 34
As the weight of earth walls is greater and the tensile strength less
than with many of the standard materials, earthquake shocks of equal
intensity might reasonably be expected to damage the earth walls
more. There are examples of earth structures which have been
damaged by the vibration induced by temblors, some wholly wrecked ;
but there are also structures which have come through quakes of
considerable intensity with little or no noticeable structural damage. 35
33 Twitchell, Ealph Emerson. Old Santa Fe. 43 p. New Mexico State
Historical Society, Santa Fe, N. M. 1925.
34 Fairbank, J. P., and J. B. Brown. Some aspects of the St. Francis Flood
damage of interest to agricultural engineers. Agr. Engr. 9:237. 1928.
35Soule, Winsor. Lessons of the Santa Barbara earthquake. American
Architect. 128(2482) : 1-112. 1925.
Bul. 472] Adobe Construction 51
Examples of the latter are to be found in San Francisco, Santa Bar-
bara, Los Angeles, and the Imperial Valley, California. A good
foundation, a low design with light weight roof, and a reinforced
concrete collar beam at the top of the earth walls are the most
important precautions to take against the possibility of earthquake
damage.
Adobe construction is fireproof, but only insofar as the earth walls
themselves are concerned. Floors, partitions, and roof members will
still be subject to destruction by fire unless constructed of non-com-
bustible materials (see fig. 34). There are examples of fires gutting
such buildings and leaving the earth walls standing unharmed.
Records of earth-walled structures on the Atlantic Coast and in the
mid-west states show such structures to be capable of resisting severe
wind stresses occasioned by cyclones and hurricanes. 36
Sanitation and Comfort of Adobe Structures for Human Occu-
pancy. — There are no absolute standards for housing sanitation and
comfort, but earth wall structures are commonly cited for their excel-
lence in this respect. The influence of the thick walls on temperature
and humidity is largely responsible for this feature. Other important
points adding to the comfort of living in earth wall houses are that
they are practically soundproof, dustproof even during heavy wind
storms, and markedly free of drafts. All these features are conducive
to restful interiors.
Properly designed and constructed earth walls make for dry in-
teriors. In some of the crudest structures without foundations, or
without a water-proofing layer in the walls, moisture conducted from
the ground has been found sufficient to promote a feeling of dampness.
In some instances of structures near the sea coast where the
humidity is high, the cooler air within the structure causes an in-
creased relative humidity and consequently a chill feeling. In but one
known case has this action been sufficiently great to pass the dew point
and cause actual condensation on the interior of the walls. Opening
the house for ventilation and keeping a small fire burning during those
parts of the day when the outside air has both high temperature and
high humidity are suggested as remedies for this condition.
The fact that earth walls are thick and that the conduction of heat
through dry soil is relatively slow, results in the well-appreciated fact
that the interiors of earth-walled houses are cooler in summer and
36 Miller, T. A. Report on the condition of rammed earth buildings built
1820 to 1854 near Sumter, South Carolina. Mimeographed report of the United
States Department of Agriculture, Division of Agricultural Engineering.
52 University of California — Experiment Station
warmer in winter than those constructed of standard materials, unless
special provision has been made for insulation in the latter. Con-
sidered on a unit thickness basis, earth walls are a relatively poor
insulating material, inferior to the same thickness of solid wood
construction. It is the mass of the earth walls and the fact that they
permit little, if any, air nitration, that causes a modification of interior
temperatures. 37 Prevention of sharp changes in temperature makes
the structures more livable.
Insulation of floor and ceiling is also essential to minimize interior
temperature changes, the ceiling particularly requiring attention in
this regard. Such insulation should tend to prohibit the passage of
heat by conduction and also by filtration of air through the floor or
ceiling. Tight construction and the use of standard insulating materials
are advised. For some structures a 4-inch layer of dry earth over
the ceiling joists will provide an adequate, but a heavy-weight, insula-
tion 38 (see figs. 3, 25, and 30). Diatomaceous earths, available in some
localities, afford light weight insulation. The use of a second roof built
a foot or so above the first with the space between entirely open has
proved an insulation from the heat of the summer sun.
Because earth walls do not lose their heat so rapidly as lighter
walls, they may prove unsatisfactory for use in sleeping rooms in
such localities as the Imperial Valley. 39 Here a combination of a lower
story of earth walls for living quarters and a light frame and screen
superstructure for sleeping should prove preferable.
The infiltration of air through walls is a factor of considerable
importance in ventilation and heating. In high-quality construction
with well built earth walls and tightly fitted doors and windows it
37 Unpublished data secured by H. L. Belton and J. E. Dougherty of the
University of California show that a shed-roof, open-front poultry house with
12-inch adobe walls had approximately 7 degrees less variation, for both
maximum and minimum temperatures during the summer and winter respec-
tively of 1928, than a similar house alongside, which had one thickness of
tongued and grooved sheathing for the side wall. A temperature lag in the
adobe house of approximately 1^ hours behind the temperature variations in
the wood house was apparent on days free of wind. A tightly closed front
wall and an insulated ceiling would undoubtedly have lessened the temperature
variation in the adobe structure still further.
38 An incubator house erected by the Sutter City Hatchery (see fig. 3) with
18-inch adobe walls and 3^> inches of soil over the ceiling maintained a nearly
constant temperature of 65° during the winter of 1928-29, according to Mr.
John Lind, owner.
Adobe structures with sand insulated ceilings and equipped with stoves and
ventilators have been found very satisfactory sweet potato curing and
storage houses in Arizona. See Orider, F. J., and D. W. Albert. The adobe
sweet potato storage house in Arizona. Ariz. Agr. Exp. Sta. Bui. 106:393-410.
1925.
39 Unpublished data from tests made for the author in the Imperial Valley
by J. P. Fairbank in 1926.
Bul. 472]
Adobe Construction
53
has been found advisable to include provision for ventilation by the
installation of ventilating flues and manually operated registers.
There are very few cases of vermin or insects working in well
constructed earth walls with masonry foundations. Mixing powdered
glass or arsenic with the soil used in the lower layer of the wall has
been tried in some instances as a precautionary barrier. Hornets have
been known to work through mud or lime plaster used on the exterior
wall surface.
Fig. 34. — Typical adobe residences in Californiia.
Upper left. Mr. and Mrs. P. T. Smith erected this adobe brick structure for
their home near Yuba City, California, in 1927. It served for the demonstration
of the local committee which won first prize in the rural communities and
small towns competition of the National Better Homes in America contest in
1928.
Upper right. Adobe brick home of Mr. and Mrs. L. W. Taylor, erected near
Bakersfield in 1928. A plaster dado is used as a splash guard at the foot of the
wall. The exterior walls were given a spray coat of linseed oil diluted with
kerosene, and a brush coat of a gypsum plastic paint.
Lower left. Residence on the Kearney Park ranch near Fresno, California,
erected of sun-dried brick in 1906. There have been three fires in the interior
of this structure. Special precautions were taken in erecting the walls to add
to their insulating effect; the interior is said, in consequence, never to get
warmer than 80 degrees, even though a thermometer on the porch has indicated
outside temperatures as high as 116.
Lower right. An adobe brick residence in Santa Monica, California.
(John Byers, architect.)
The Potential Attractiveness of Adobe Walls. — Architects and
others with a feeling for the aesthetic are interested in the possibilities
of design in earth wall construction. The deep reveals at door and
window openings, and the heavy style promoted by the massive wall,
have a refreshingly novel appearance.
54 University of California — Experiment Station
In general, any of the popular historical styles of residence archi-
tecture which make use of masonry construction can be erected with
earth walls. The low, rambling' styles are more in keeping with the
nature of the material (see frontispiece and figs. 4, 10, 11, 31, 33, and
34).
CONCLUSIONS
1. The practical and economical value of earth as a construction
material for small structures in California, particularly farm
buildings, has been proved through the actual erection and use
of such structures.
I. Three methods of incorporating the earth into walls appear prac-
tical for use by present day builders in this state : the sun-
dried brick, the rammed earth, and the poured earth. All the
methods of using earth as a structural material are termed
' ' adobe construction. ' '
3. Any intelligent person familiar with standard construction prin-
ciples can utilize any of the methods of earth wall construction
without fear of failure after studying the peculiarities of
such construction and making a few tests to determine the
characteristics of the soil to be used. The apparent strength
developed in the test specimens and the amount of cracking
occurring as the mud dries can be used as an index to the
suitability of the soil. Amateur builders should complete the
construction of some small structure before attempting any
large building project.
4. The selection of the method to be employed depends on the type
of soil to be used, the climate, the number of workmen to be
employed, and the preference of the builder. The final results
secured 'in the building are similar, regardless of the method
employed.
5. Because of the characteristics of the material and the manner of
its use, certain modifications of standard construction practice
are necessary. A study of the soil itself is the first necessity
before any building project is undertaken. The effect of ad-
mixtures to the soil, of reinforcements to be placed in the wall,
and the design of foundations, roofs, and openings, should be
understood.
Bul. 472] Adobe Construction 55
6. Earth-wall construction is inferior to most standard construction
materials in earthquake resistance, but adobe structures have
withstood earthquakes in this state with little or no apparent
damage. Skillful and intelligent workmanship and the incor-
poration of certain reinforcing design features will help to
minimize the damaging effect of earthquakes.
7. Careful planning is necessary to secure the most economical and
satisfactory results with adobe construction.
8. A protective coating should be used on adobe walls to guard
against moisture and mechanical wear. Cement, lime and mud
plasters, and various paints have been used with success.
9. The advantages of adobe are that it is a native material of
adequate strength and durability for residences and small
structures, and a material generally economical to use. At-
tractive, sanitary, comfortable, fire-resistant, dry, sound-proof,
and thermal-insulated structures may be erected of the material.
10. The principal disadvantages attending the use of the material are :
a large amount of physical labor is involved in such buildings ;
those not wishing personally to undertake their erection jobs
are likely to find it difficult to secure builders skilled in the use
of the material; the low tensile strength requires particular care
in securing door and window frames; and additions or altera-
tions in the plan after the work is once started are difficult.
11. The cost of earth wall structures varies widely under different
conditions. In this state the complete cost of an adobe res-
idence, including items for all labor, commercial materials, and
equipment necessary to make the structure available for use, is
about the same as for a wood frame structure of similar quality.
Economies in the cash outlay required may be effected where
the builder desires to supervise and do much of the work him-
self, or where he is willing to eliminate much of the trim and
other decorative features frequently employed in residence
design.
12. Adobe construction proves attractive to those builders who desire
the novel architecture it affords, to those who have a senti-
mental appreciation of historic methods or desire to erect a
structure directly from the soil, to those who desire to utilize as
much of their own labor as possible in the erection, and to those
who desire to combine cheapness and comfort in their houses.
56 University of California — Experiment Station
ACKNOWLEDGMENTS
The author acknowledges indebtedness to Farm Advisors L. W.
Taylor, H. J. Baade, V. W. De Tar, W. E. Gilfillan, and M. A. Lindsay ;
to Professor C. F. Shaw, Division of Soil Technology, University of
California ; to Mr. J. P. Fairbank, extension specialist in agricultural
engineering, University of California; to Dr. H. B. Humphrey and
Mr. James Townsend of the United States Department of Agriculture ;
and to Mr. John Byers, Mr. E. D. Herreras, Mr. Karl J. Ellington,
and many others who have evinced interest in and given assistance in
the study of adobe construction. The frontispiece of this publication is
a view of the Santa Monica, California, office of John Byers, architect.