(MCT)—Making homes out of clay, sand and straw—a material called “cob”—might raise eyebrows in earthquake country, but California devotees are hoping to gain acceptance for what they say is an inexpensive, environmentally friendly and safe way to build.
Right now, their projects are limited by code restrictions to elfin structures that often look like the illustrations in a book of fairy tales. But advocates note that large cob homes have been standing in England for 400 years.
“This is a wave of the future,” says John Fordice, a Berkeley, Calif., architect who operates the nonprofit Cob Research Institute from his Berkeley home. “The advantages to the general public are vast, to say the very least.”
Cob walls offer some advantages over conventional wood framing. Clay can be taken directly from the building site, and the owner can do much of the work. It’s similar in composition to adobe brick, but it is used to make thick, solid walls that are says to be much less vulnerable to shaking.
Ellen Turner, a retired Silicon Valley tech writer, is building a small cob studio behind her home in the east foothills of San Jose, Calif. With its circular footprint, round windows and spiral roof, it’s half sculpture. Turner says it’s going to be a weaving workshop.
The name “cob” comes from an early English word for loaf or lump. Turner thinks there are two reasons for that.
“When I mix the stuff together, it’s very much like kneading bread,” she says. “Then when the walls get higher, they often make loaf-sized lumps and toss them to the person who is making the wall.”
The clay for her studio came from her property. “We hired somebody to dig down 15 feet to reach the clay, then filled the dirt back in,” she says. The walls rose gradually as she added layers. The roof is being finished now by a small crew that includes Jessica Tong, 23, an environmental design graduate of the University of British Columbia.
Tong also is building a cob guesthouse in her parents’ backyard in Berkeley.
Cob “is not something people are widely aware of, and I do think it should be one of the tools in the kit,” she says.
The problem is, there’s no building code provision in much of California for cob houses, so most of the legal structures are 120 square feet or less to avoid the requirement for a building permit. Three California counties—Nevada, Humboldt and Mendocino—allow cob to be used for dwellings under a state code provision for rural limited density housing.
“It’s has been around forever and is one of the most conventional ways to build a building,” says Craig Griesbach, Nevada County’s director of building. But they have to be “engineered and backed up with structural calculations showing they can stand the wind and seismic load,” he says. “They reinforce them with rebar, strapping and threaded rods. It’s not just rammed earth.”
Tong hopes the guesthouse she is building will meet Berkeley’s alternate materials standards for an occupied dwelling.