Step-by-Step Cob House Construction
Building a cob home helps the planet and your pocketbook. Constructed of a clay, sand and straw mixture, cob homes have a history that goes back thousands of years. Travel to Great Britain for a glimpse of these sustainable homes. Similar to adobe buildings, cob homes require little upkeep and little construction expense.
If you hire a professional cob home builder, expect to spend a lot of money. Building a cob home on your own, however, may cost a mere $3,000 (in 2010). Even novice builders have successfully built cob homes, relying heavily on the existing community of green homeowners for guidance.
Begin your cob home with a good dose of study time. Look for books devoted to cob building and natural building techniques. These books also list experts who offer classes and training workshops. Tap into cob building forums and organizations. Knowing the financial and construction details before you begin is beneficial.
Design your home after you have a good foundation of knowledge in green building. Prepared cob home plans also are available for purchase. Decide between a dome or vertical-walled structure and between a one-story or two-story home. Ask for advice on planning the electrical and plumbing systems. Many cob homes have curved walls because the clay/straw mixture allows more flexibility than wood. Planning windows and walls for a curved structure, however, is challenging.
Ignoring the climate of your area may lead to disaster. Colder climates require thick walls and heating systems. Thick cob walls (2- to 3-feet thick) store heat well. Many cob homes require only a wood-burning stove for heat.
Check into local building codes thoroughly as you plan your cob building. States and municipalities may have no codes in place for a cob structure. Use expert guidance from cob homeowners who already tackled these problems.
Pick a dry, well-drained building site. Some cob homes begin with a trench, 2-feet to 3-feet deep, outlining the foundation. Lay gravel at the bottom of the trench. Using feet or hooves to mix the clay, straw and sand remains the traditional way to create cob. Modern machinery and materials, however, make cob building much easier. A large vertical auger mixes about 7,000 lb. of cob in 30 minutes. Cement mixers and rototillers provide other mechanical mixing options.
Filling earth-bags, usually old feedbags or 50-lb. polypropylene sandbags, has increased in popularity with cob constructions. Each bag is filled with the cob mixture. The top 10 inches of each bag remains unfilled and open.
To use earth-bags, lay them on the gravel bottom of the trench. Place the bags in a line, one after the other, on top of the gravel in the foundation trench. Fold each bag's opening so the open end rests under the bag itself. Place the next bag tightly next to the first to completely seal the first bag's opening. Lay another row of bags on the first row, staggering the rows like brickwork. A metal tamper must flatten the mixture inside each bag before placing the next row.
Add a roll of barbed wire between each row. Place the wire in two rows about 2 to 3 inches from the edge of the bags (toward the middle). Use rocks or tampers to press the wire into the bags.The wire helps the wall stay firmly in place. This process of laying bags continues until achieving the desired height.
Construct wooden or metal frames for windows and doors. Installing doors and windows follows the same steps as in traditional housing, though some cob builders add plain or stained glass to simple supports built into the walls. Roofing installation usually requires a truss or beam foundation laid across the cob structure, from wall to wall. Apply plaster over the dried earth-bag walls.