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Creating a Homestead Out of Sun-Dried Mud
Adobe, or sun-dried mud, is among the most ancient and widely used building materials. The ingredients for making adobe—soil, water, and straw—are cheap and abundant, and the bricks themselves are easy to make, easy to use, and durable. Perhaps most important, adobe is an energy saver. Thick-walled adobe homes provide interior environments that are thermally stable, resisting both the penetrating heat rays of the sun during the day and the outward flow of warmth from within during cool nights.
Adobe is an arid-climate material; unless it is specially treated, it tends to decompose in humid conditions. But in regions to which it is suited, adobe brick will last almost indefinitely. Remains of adobe structures 700 years old and more still stand in parts of Arizona and New Mexico. The famous Thos Pueblo of New Mexico, an enormous adobe complex, has been the home of unbroken generations of Indians since long before their first encounter with the Spanish conquistadores in 1540. With such a long history it is not at all surprising that the architectural style of adobe that evolved over the centuries—a blend of ancient Indian technology with later Spanish stylization—is both in harmony with nature and perfectly adapted to the resources of the region.
How to Tell If Your Soil Is Suitable for Adobe
The ideal soil for making adobe has little or no organic matter and contains sand, silt, and clay in roughly equal proportions. The ingredients work much like ordinary concrete: the sand and silt act as filler, the clay as binder. Too much filler results in bricks that crumble easily; too much clay results in bricks that crack as they dry. Organic soil is not acceptable, since the bricks will be weak and harbor vermin.
The knack for choosing adobe soil is not hard to acquire. Start by looking at different soil samples: squeeze them and smell them to detect organic content. Next, perform the tests shown at right. The tests are easily done and will give the approximate proportions of sand, silt, and clay. If you find some likely adobe, check further by digging samples from several spots, mixing them thoroughly to obtain an average profile. Further tests using sample bricks are shown on the next page. As a final step, have a commercial laboratory perform a detailed composition analysis. This will probably be required by the local building code in any case.
---- Mix soil with a little water and roll into cylinder ¾ in. in diameter and 8 in. long. Soil should not stick to hands. Cylinder of good adobe soil will break at between 2 in. and 6 in. when held out straight. If it breaks at less than 2 in., add clay. If it breaks beyond 6 in., add sand
Making Adobe Brick
The traditional way of mixing adobe—in a hand-dug pit is simple but physically demanding. Much labor can lie saved by using a rotary tiller to dig up the soil and a power cement mixer to blend it with water and straw.
There are two ways of casting adobe bricks: gang mold and cut slab. In the former method each brick is separately cast; in the latter a large slab is cast and then cut into bricks. With either method bricks should be at least 4 inches thick and of manageable weight. The most commonly used dimensions are 4 by 10 by 14 inches (~35 pounds) and 4 by 12 by 16 inches (about 50 pounds). Newly formed bricks must dry uniformly; otherwise they will crack. For this reason, wet brick should be protected from uneven exposure to wind and suit. Cover new bricks with burlap after the forms are off, and turn the bricks on edge as soon as they are firm. Cure bricks at least 10 days before stacking them. Do not make bricks when temperatures fall below freezing.
Local building codes usually call for laboratory tests of adobe to determine moisture absorption, compression, and breaking strength. The last two tests can be approximated as shown below. Experiment with sample bricks made of different soil mixes and keep accurate records to help determine the best formula.
2. Smooth off top by drawing trowel or 2 x 4 across form with sawing motion. Remove form promptly to prevent sticking. Brick should sag downward no more than 1/4 in. if mix is correct.
4. Stack bricks against central column to distribute weight evenly. Cover top to protect from rain but leave sides open. For long-term storage, stack on wooden plat forms, cover sides with plastic.
Testing the strength of your bricks
A Strong Foundation For Massive Walls
Building an adobe house requires careful planning, but actual construction is comparatively simple. The building site and concrete foundation should be prepared according to standard building methods. Walls are constructed by laying the adobe blocks like ordinary bricks; adobe mud, without the straw, serves as mortar. Adobe does not hold nails, so wooden inserts must be set in place as construction proceeds. After the walls are up a continuous bond beam of wood or concrete is set in place to tie the walls together and distribute the weight of the roof. Floors are generally made of brick or adobe, al though concrete slab floors or wooden floors laid atop joists anchored to the foundation are also used. Decide plumbing and electrical arrangements prior to construction. Also determine the locations of fireplaces beforehand; fireplaces made of adobe are heavy and require thick foundation slabs.
---- Corner blocks may have to be cut to size with trowel to maintain 4-in, overlap. 1 Bricks and mortar are J laid according to lines on a story pole. Check 12 corners for right angles regularly.
Twine acts as guide, ensuring level courses. Stretch it between nails set in precisely aligned corner bricks. Walls are heavy and must be precisely vertical.
Wall building begins at corners and proceeds toward middle. Lay bricks so vertical joints are offset at least 4 in. between courses. Height of wall to bond beam must not exceed eight times wall thickness. Build no more than six courses per day. Mud for mortar should be screened to remove particles bigger than ¾ in. Traditionally, vertical joints are left open. When walls are plastered, openings will give plaster better grip.
Rough in plumbing before foundation is built. Pipes can be concealed in hollow frame wall, often shared by
Adobe is heavy and needs massive footing. Poured concrete, reinforced with ½-in. rebars (steel reinforcing bars), is cast atop undisturbed earth dug below frost line. Footing must be at least B in. thick and 4 in. wider on each id than wall above. Stem is built of hollow cement block packed with adobe and capped with cement to prevent moisture damage. Stem blocks rise from footing a minimum of 16 in. above grade for ground water protection. Non-weight-bearing interior walls require no footing.
Brick or adobe floors can be laid directly on earth. Wood floors re quire crawl space to prevent rot. A concrete slab floor should be 4 in. thick with insulation between slab and footing to provide room for expansion.
Doors, Windows, Beams, and Wires
Locate openings for doors and windows away from corners to preserve wall strength. Rough frames are set into place as wall is built or attached later to gringo blocks. Allow 3/4 in. for vertical settling of walls and lintels.
Lay wiring inside walls as courses proceed or fit later into channels gouged with claw hammer. Drop cable from ceiling or run beneath wood floor. Have electrician super vise. Anchor outlet boxes with horizontal metal strips embedded in joints and extending 12 in. on each side. Record wiring locations before plastering.
Wooden lintels must be at least 6 in. thick and extend 18 in. on each side of wall opening. Check local code.
Providing surfaces for interior nailing
Rough frames afford surfaces for attaching doors or window frames. Use 2 x 4’s or wider. Recess frames into notched adobe bricks. Drive 16d nails partway into frame at joint lines before blocks are laid to anchor mortar.
From Floor to Roof: The Final Steps
Since roofs and ceilings must be designed with the load-bearing capacities of walls and foundations in mind, it is essential to have professional assistance when planning them. This is particularly true for old-style earth roofs; although they provide excellent insulation and economy, they are extremely heavy. Asphalt or tile roofs are lighter and offer greater architectural freedom. A roof must also protect the walls from water runoff. This is accomplished by channeling roof drainage into protruding gutters called canales.
Canales: Roof drainage is provided by canales. Locate them along low side of roof, level with decking. They must project enough to prevent water from running down the walls.
Details of roof construction
Roof beams can be anchored to nailer or straps embedded on top of concrete bond beam. Use spikes or boils if bond beam is wooden.
Two-ply roof incorporates insulation and makes wiring easier. Interior surface is of standard decking. Use 1/2 in. plywood for exterior.
Grade roof to drain off water line canales with rustproof metal. Seal roof seams and joints by hot-mopping with tar. Inspect frequently.
Your choice of roofing surfaces
Hot-mopped roof consists of three or more layers of treated felt mats sandwiched between coats of asphalt. Felt is dished upward at sides. Cover final tar layer with light colored gravel to reflect heat and protect asphalt from direct sunlight.
Interior and Exterior Plastering
Plastering protects and beautifies adobe walls. Mud with a high clay content is the traditional plastering material (it is still used by southwestern Indians), but it requires more frequent maintenance than modern stucco compounds and is more vulnerable to moisture damage.
The first step in exterior plastering is to nail galvanized chicken wire to the wall with rust-resistant 16-penny nails. Next, trowel on a primary, or scratch, coat, pressing it into joints and gaps for a strong bond. Just before the scratch coat hardens, roughen it with a rake so that the following coat (called the brown coat) will have something to grip. The brown coat is troweled on smooth and left to dry and cure at least 10 days before a finish coat of thin stucco—colored if desired—is applied.
Neither chicken wire nor a scratch coat is needed for interior plastering. Use fibered gypsum plaster mixed with sand for the first coat; un-fibered gypsum plaster and sand for the second. This finish will not rub off on clothes. Since plastering is messy, interior work is best accomplished before the floor is laid. If wood is to be plastered, first cover it with 15-pound asphalt felt and strips of metal lath to keep the wood dry and prevent the i luster from cracking as the wood swells and shrinks.
Adobe mud for floors should be 20 percent clay and have extra straw mixed in. Make floor at least 4 in. thick, built up of packed 2 in. layers. If cracks form, fill with adobe Traditional floor sealant is ox blood. A modern alternative is boiled linseed oil.
Brick floors are laid in 1-in. thick bed of sand atop surface of packed earth. Treat earth with insecticide, then cover it with plastic vapor barrier. Tamp bricks into place with mallet. Fill cracks by sweeping floor with fine sand. Finish with commercial scalers and wax.
Frames of doors and windows are notched with 1/4. groove so that they overlap plaster rather than butt against it. This helps reduce cracking. Baseboard is similarly notched, Install felt and metal lath at wall openings to protect the edges.
Exterior walls require chicken-wire backing and three coals of plaster. Total thickness is about 4 in. Plaster should extend below grade level for even detail. Inside walls need only two coats of plaster and no wire.
Anita Rodriguez, Enjarradora
Maintaining an Adobe Tradition
Anita Rodriguez, of Taos, New Mexico, is an expert in enjarrando—the craft of plastering with adobe. Here she is shown working on an horno, an ancient type of oven.
‘When I was a child in Taos, where I was born, nearly all the buildings were adobe, Now they have been replaced by techno logical construction, but I have never forgotten the lovely soft shapes of earthen walls or the fireplaces that were in every home It is hard to exaggerate the beauty and harmony of that vanished town.
“In this part of the country for over 500 years the craft of finishing adobe has traditionally belonged to women. The Spanish word for such a woman is enjarradora. In English this means only “plasterer,” but enjarradoras also build fireplaces, lay mud Iloors, and paint walls with clays of various colors by an ancient method called alisando.
“A friend who knew my interest in adobe asked me to build her a fireplace. Someone else happened to see it and asked me to build another one, and that’s how I got started. Today I am a professional enjarradora. One can only learn enjarrando from enjarradoras. The techniques are passed on in a strictly oral tradition that has never been written down. I learned from Hispanic enjarradoras who were generous to me with their knowledge; and while I was teaching school at Taos Pueblo, I too passed my knowledge along to other women.
“With the interest in environment and the emphasis on energy saving, there is a revival of interest in adobe construction, It is particularly good for buildings with solar heating. Adobe walls are excellent insulators. They trap heat in winter and keep the interior cool in summer.
“Working with adobe is hard physical labor. Ten years of enjarrando have not made me look like Charles Atlas, but I work with men all day and know that women are much stronger than people think they are. The work has given me a sense of my roots. I identify with my culture and I don’t want it to die.”
---- Adobe holds heat well, making it ideal for fireplaces. Fire place exterior can be anything imagination allows, Shape bricks with dull hatchet and masonry saw; finish with mud spread over chicken wire. (Technical aspects of fireplaces are discussed in Fireplace Construction and Design, pp.62 65.)