Building with Earth
(Page 3 of 6)
April/May 2002
By Dan Chiras
Like adobe, cob consists of mud and straw. Mixed in pits, usually tromped by feet, the cob is then formed into small-loaves, which are deposited on the foundation, then massaged and shaped by hand.
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Slowly but surely, a wall emerges with smooth, curved surfaces. Finished cob walls may be left unplastered or may be painted with a traditional lime wash to provide protection and beauty. In many in cob is coated with an attractive protective layer of earthen or lime plaster
Advantages of Cob
• Wall building is fun, easy to learn and relatively inexpensive.
• Encourages artistic expression and lends itself to curved walls and other usually exciting features, such as niches
• Durable, fireproof and nontoxic.
Disadvantages of Cob
• In most of North America cob is a "new" building concept: obtaining approval may be difficult.
• Can be time-consuming.
Rammed Earth
Rammed earth is one of the oldest of all natural building methods. Rooted in North Africa and the Middle East, it dates back to the time of the pharaohs. Today commercial builders in California, New Mexico, Arizona and Colorado are using this time-tested technique to construct modern homes.
To build a rammed-earth home, wooden or steel forms are mounted on a 1- to 2 foot-thick foundation. Six to 8 inches of slightly moistened dirt are shoveled into the forms, then tamped with a pneumatic tamping device. More dirt is added, then tamped. Once the form is filled, it's removed. The moistened soil dries, creating a thick, strong, sandstone-like wall. "Raw earth walls, especially those containing native subsoil and clay, are remarkably durable and handsome.
Traditional rammed-earth construction relies on a mix of 70 percent sand and aggregate (small stone) and 30 percent clay. The clay acts as a binding agent.
In areas where moisture and earthquakes are a problem, rammed-earth builders often use a mix of sand and Portland cement, although the results lack the visual appeal of raw earth walls. Rammed-earth walls may be left as is, or plastered or stuccoed to provide an additional measure of protection.
Advantages of Rammed Earth
• Produces an extremely attractive and enduring structure.
• Suitable for many different architect ural styles.
• Often cheaper to build than brick, stone and adobe structures.
Disadvantages of Rammed Earth
• Requires special skills and construct ion of forms; generally not suited for owner-builders.
• Costly, if labor costs are high.
• May be difficult to gain building dep artment approval.
Tire Homes
While some builders ram or pour earth into forms, others are building walls using earth that's packed into old automobile tires. Called rammed-earth tire homes. or simply tire homes, this distinct architecture makes use of an abundant waste product(Americans discard nearly 250 million car tires each year.)
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