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Preservation of Historic Adobe
Buildings
Whether built in the 17th century or in the 20th century, adobe buildings
share common problems of maintenance and deterioration. This brief discusses
the traditional materials and construction of adobe buildings and the
causes of adobe deterioration. It also makes recommendations for preserving
historic adobe buildings. By its composition, adobe construction is
inclined to deteriorate; however, the buildings can be made durable
and renewable when properly maintained.
What is Adobe?
The adobe, or sun-dried brick, is one of the oldest and most common
building materials known to man. Traditionally, adobe bricks were never
kiln fired. Unbaked adobe bricks consisted of sand, sometimes gravel,
clay, water, and often straw or grass mixed together by hand, formed
in wooden molds, and dried by the sun. Today some commercially available
adobe-like bricks are fired. These are similar in size to unbaked bricks,
but have a different texture, color, and strength. Similarly some adobe
bricks have been stabilized, containing cement, asphalt. and/or bituminous
materials, but these also differ from traditional adobe in their appearance
and strength.
Traditional adobe construction techniques in North America have not
varied widely for over 3-1/2 centuries. Adobe building methods employed
in the Southwest in the 16th century are still used today. Because adobe
bricks are not fired in a kiln as are clay bricks, they do not permanently
harden, but remain unstable--they shrink and swell constantly with their
changing water content. Their strength also fluctuates with their water
content: the higher the water content, the lower the strength.
Adobe will not permanently bond with metal, wood, or stone because
it exhibits much greater movement than these other materials, either
separating, cracking, or twisting where they interface. Yet, many of
these more stable building materials such as fired brick, wood, and
lime and cement mortars are nonetheless used in adobe construction.
For example, stone may be used for a building's foundation, and wood
may be used for its roof or its lintels and doorways. In the adobe building,
these materials are generally held in place by their own weight or by
the compressive weight of the wall above them. Adobe construction possibilities
and variations in design have therefore been somewhat limited by the
physical constraints of the material.
Preserving and rehabilitating a deteriorated adobe building is most
successful when the techniques and methods used for restoration and
repairs are as similar as possible to the techniques used in the original
construction.
Adobe Construction Techniques
The Brick: The adobe brick is molded from sand and clay mixed with
water to a plastic consistency. Commonly, straw or grass is included
as a binder. Although they do not help reinforce the bricks or give
them added long-term strength, straw and grass do help the bricks shrink
more uniformly while they dry. More important for durability, however,
is the inherent clay-to-sand ratio found in native soil. The prepared
mud is placed in wooden forms, tamped, and leveled by hand. The bricks
are then "turned-out" of the mold to dry on a level surface covered
with straw or grass so that the bricks will not stick. After several
days of drying, the adobe bricks are ready for air-curing. This consists
of standing the bricks on end for a period of 4 weeks or longer.
Mortar: Historically, most adobe walls were composed of adobe bricks
laid with mud mortar. Such mortar exhibited the same properties as the
bricks: relatively weak and susceptible to the same rate of hygroscopic
(moisture absorptive) swelling and shrinking, thermal expansion and
contraction, and deterioration. Consequently, no other material has
been as successful in bonding adobe bricks. Today, cement and lime mortars
are commonly used with stabilized adobe bricks, but cement mortars are
incompatible with unstabilized adobe because the two have different
thermal expansion and contraction rates. Cement mortals thereby accelerate
the deterioration of adobe bricks since the mortars are stronger than
the adobe.
Building Foundations: Early adobe building foundations varied because
of the difference in local building practices and availability of materials.
Many foundations were large and substantially constructed, but others
were almost nonexistent. Most often, adobe building foundations were
constructed of bricks, fieldstones, or cavity walls (double) infilled
with rubble stone, tile fragments, or seashells. Adobe buildings were
rarely constructed over basements or
crawlspaces.
Walls: Since adobe construction was load-bearing with low structural
strength, adobe walls tended to be massive, and seldom rose over 2 stories.
In fact. the maximum height of adobe mission churches in the Southwest
was approximately 35 feet. Often buttresses braced exterior walls for
added stability.
In some parts of the Southwest, it was common to place a long wooden
timber within the last courses of adobe bricks. This timber provided
a long horizontal bearing plate for the roof thereby distributing the
weight of the roof along the wall
Roofs: Early Southwest adobe roofs (17th-mid-19th centuries) tended
to be flat with low parapet walls. These roofs consisted of logs which
supported wooden poles, and which in turn supported wooden lathing or
layers of twigs covered with packed adobe earth. The wood was aspen,
mesquite, cedar, or whatever was available. Roughly dressed logs (called
"vigas") or shaped squared timbers were spaced on close (23 feet or
less) centers resting either on the horizontal wooden member which topped
the adobe wall, or on decorated cantilevered blocks, called "corbels,"
which were set into the adobe wall. Traditionally, these vigas often
projected through the wall facades creating the typical adobe construction
detail copied in the 20th century revival styles. Wooden poles about
2 inches in diameter (called "latias") were then laid across the top
of the vigas. Handsplit planks (called "cedros" if cedar and "savinos"
if cypress) instead of poles were used when available. In some areas,
these were laid in a herringbone pattern. In the west Texas and Tucson
areas, saguaro (cactus) ribs were used to span between vigas. After
railroad transportation arrived in most areas, sawn boards and planks,
much like roof sheathing, became available and was often used in late-19th
and early20th century buildings or for repairs to earlier ones.
Next cedar twigs, plant fibers, or fabric were placed on top of the
poles or planks. These served as a lathing on which the 6 or more inches
of adobe earth was compacted. If planks were used, twigs were not necessary.
A coating of adobe mud was then applied overall. The flat roofs were
sloped somewhat toward drains of hollowed logs (called "canales," or
"gargolas"), tile, or sheet metal that projected through the parapet
walls.
Gable and hipped roofs became increasingly popular in adobe buildings
in the 19th and 20th centuries. "Territorial" styles and preferences
for certain materials developed. For example, roof tiles were widely
used in southern California. Although the railroad brought in some wooden
shingles and some terra cotta, sheet metal roofing was the prevalent
material for roofs in New Mexico.
Floors: Historically, flooring materials were placed directly on the
ground with little or no subflooring preparation. Flooring materials
in adobe buildings have varied from earth to adobe brick, fired brick,
tile, or flagstone (called "lajas"), to conventional wooden floors.
Traditional Surface Coatings
Adobe surfaces are notoriously fragile and need frequent maintenance.
To protect the exterior and interior surfaces of new adobe walls, surface
coatings such as mud plaster, lime plaster, whitewash, and stucco have
been used. Such coatings applied to the exterior of adobe construction
have retarded surface deterioration by offering a renewable surface
to the adobe wall. In the past, these methods have been inexpensive
and readily available to the adobe owner as a solution to periodic maintenance
and visual improvement. However, recent increases in labor costs and
changes in cultural and socioeconomic values have caused many adobe
building owners to seek more lasting materials as alternatives to these
traditional and once inexpensive surface coatings.
Mud Plaster: Mud plaster has long been used as a surface coating.
Like adobe, mud plaster is composed of clay, sand, water, and straw
or grass, and therefore exhibits sympathetic properties to those of
the original adobe. The mud plaster bonds to the adobe because the two
are made of the same materials. Although applying mud plaster requires
little skill, it is a time-consuming and laborious process. Once in
place, the mud plaster must be smoothed. This is done by hand; sometimes
deerskins, sheepskins, and small, slightly rounded stones are used to
smooth the plaster to create a "polished" surface. In some areas, pink
or ochre pigments are mixed into the final layer and "polished."
Whitewash: Whitewash has been used on earthen buildings since before
recorded history. Consisting of ground gypsum rock, water, and clay,
whitewash acts as a sealer, which can be either brushed on the adobe
wall or applied with large pieces of coarse fabric such as burlap.
Initially, whitewash was considered inexpensive and easy to apply.
But its impermanence and the cost of annually renewing it has made it
less popular as a surface coating in recent years.
Lime Plaster: Lime plaster, widely used in the 19th century as both
an exterior and interior coating, is much harder than mud plaster. It
is, however, less flexible and cracks easily. It consists of lime, sand,
and water and is applied in heavy coats with trowels or brushes. To
make the lime plaster adhere to adobe, walls are often scored diagonally
with hatchets, making grooves about 1-1/2 inches deep. The grooves are
filled with a mixture of lime mortar and small chips of stone or broken
roof tiles. The wall is then covered heavily with the lime plaster.
Cement Stucco: In the United States, cement stucco came into use as
an adobe surface coating in the early 20th century for the revival styles
of Southwest adobe architecture. Cement stucco consists of cement, sand,
and water and it is applied with a trowel in from 1 to 3 coats over
a wire mesh nailed to the adobe surface. This material has been very
popular because it requires little maintenance when applied over fired
or stabilized adobe brick, and because it can be easily painted.
It should be noted however, that the cement stucco does not create
a bond with unfired or unstabilized adobe; it relies on the wire mesh
and nails to hold it in place. Since nails cannot bond with the adobe,
a firm surface cannot be guaranteed. Even when very long nails are used,
moisture within the adobe may cause the nails and the wire to rust,
thus, losing contact with the adobe.
Other Traditional Surface Coatings: These have included items such
as paints (oil base, resin, or emulsion), portland cement washes, coatings
of plant extracts, and even coatings of fresh animal blood (mainly for
adobe floors). Some of these coatings are inexpensive and easy to apply,
provide temporary surface protection, and are still available to the
adobe owner.
Adobe Deterioration
When preservation or rehabilitation is contemplated for a historic
adobe building, it is generally because the walls or roof of the building
have deteriorated in some fashion--walls may be cracked, eroded, pitted,
bulging, or the roof may be sagging. In planning the stabilization and
repair of an adobe building, it is necessary:
* To determine the nature of the deterioration
* To identify and correct the source of the problem causing the deterioration
* To develop rehabilitation and restoration plans that are sensitive
to the integrity of the historic adobe building
* To develop a maintenance program once the rehabilitation or restoration
is completed.
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