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," said 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 said 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 said 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 said. "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 said. 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 said.
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," said 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 said. "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.
"We think it's going be a nice flagship to help more cob designs go through the permitting process," said Anthony Dente, an engineer working to get the project permitted. "It's a nice project because the clients are on board with the struggle it takes to get nonstandard building materials through.
"We believe cob by its nature is better structural material for earthquake country," he added. "It's been pushed to the backwoods on a nonpermit basis because it's hard to fit into the boxes of code permitting for what is called a safe structure."
Claudine Desiree, founder of Cruzin' Cob in Santa Cruz, Calif., says she spent $20,000 getting her 200-square-foot cob studio to meet building code requirements – although only for day use. The structure had to be reinforced with a wire mesh and covered with French lime plaster, she said. It's the first engineered cob structure to receive a permit in Santa Cruz County, said her engineer, Brad Streeter.
"New Zealand has a cob code they use," Desiree said. "There's the same earthquake risk, and it's a developed country. That's the direction we need to be going in to save trees, save concrete and to build ecologically harmonious buildings."
Contemporary cob structures are built on rubble-filled trenches or concrete foundations, and they require a roof with an overhang. Layers of viscous cob are applied to form the walls. Smooth reinforcing steel can be used to create a tension system to tie the wall together for protection against earthquakes.
In other countries, including the U.K., Germany and Ireland, cob has a long and distinguished pedigree. Much of the village charm in Southwest England comes from whitewashed, thatched homes built with cob, dating back to Elizabethan times.
"In England, they're grandfathered in," said Massey Burke, who is assisting with Tong's guesthouse. "They know how they stand up, whereas it's kind of a new thing in our culture." Burke is a designer and an advocate of natural building methods like straw bale and cob.
Turner's studio was designed by Burke, whose own cob structure in El Sobrante won a 2013 green building award from Sustainable Contra Costa. The organization called it "a beautifully creative expression of cob building traditions, one of the oldest natural building methods of the earth."
"You have to experience one to see what it's like to live in an earthen building," Burke said. People's views of them change, "once they see you can dig these materials out of the ground and make something beautiful with them."
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