The first time I saw people building with cob I remembered making mud pies, that quintessential summer activity of every childhood lived where there is dirt. Cob, from an old English word meaning lump, is the oldest technology for building with earth, utilizing gooey straw-laced mud clumped handful by handful into thick walls. Since they are all of one piece, cob buildings are strong, and their mass makes an interior that stays cool in summer and holds heat in winter. There are cob houses built in Britain in the 1500s that are not only still occupied but in greater demand than ever. More recently, rounded sculptural cob houses have begun to appear like mushrooms after rain in the back woods of Oregon and Northern California, built by their owners without permits or prior experience.
But then, cob building is so simple, any three-year-old can participate. The materials--earth, sand, straw, water--are not only cheap and near at hand but nontoxic and renewable, making cob the most eco-friendly extreme in building technologies. The hitch? It's as slow as you might imagine building a house out of mud pies could be. Even a structure small enough that a building permit is not required--120 square feet or less --could use the labor of not only the whole family but the entire neighborhood.
The cob itself imposes a height limit of eight inches per day--any higher and it starts to "splooge"--a cobber's term that describes how the wet mud slumps from its own weight. But look on the bright side--cob building could be the perfect antidote to a rushed work life. Building a cob potting shed, say, could be a backyard summer project that provides hard physical exercise, a full social life, and a meditation practice all rolled into one, with the added bonus of gaining a permanent garden structure. Don't need a potting shed? Then try a curved bench that fans out from your house or a covered sitting nook--organic designs are a cinch with cob.Back in 1997, Northside community gardener Jim Cisney thought about building a cob toolshed at the garden's BARTside plot, a block north of Hopkins Street on Northside Street. At the Peralta Community Garden on the next block, architect John Fordice was designing a wooden toolshed but looking for an occasion to try cob. Environmentalist Karl Linn, whose work had helped shape both gardens, connected the two up. By August 1998, construction begun on Northside Community Garden's cob toolshed, with John Fordice in charge and Linn providing midwifery services in the background. The project was supposed to take three months to complete, but here's where cob can really mess with your expectations--it took two and a half years. The cob "tool temple," as the structure has come to be called, was formally dedicated at a ceremony attended by over a hundred neighbors on June 3. Visitors strolled past burning sage sticks on the welcoming "arms" that make a seating area at the little building's entrance, into the cool, roundly triangular interior, still so sparsely furnished with garden implements that it really does seem more temple than toolshed. Recycled beams fan overhead, and the cavelike embrace of earth is lightened by a diamond-shaped skylight and two windows, also recycled. The earth roof was in full bloom with the pink shades of native clarkias, while California poppies peeked through, next in line to bloom. The effect is both whimsical and massive, an artful folly built to last for centuries.
Examining the toolshed's construction explains both why it took so long to build and why it's likely to last so long. First, an eight-inch-deep reinforced concrete foundation was laid for the building; then a one-and-a-half-foot base wall was built up of broken sidewalk--called "urbanite" by the eco-builders--to keep the water-soluble cob up away from the ground. There are not yet any accepted standards for building with cob--though the technique is ancient, it has not re-entered our construction vocabulary --so every builder is on his or her own. Karl Linn insisted that Fordice overengineer the toolshed, since it was to be a visible educational edifice on public property. For seismic stabilization, Fordice adapted techniques the Getty Conservation Institute had developed for use on historic adobe buildings. He attached one-quarter-inch diameter wires every two feet around the perimeter. The wires ran vertically from the concrete foundation, through the urbanite layer, all the way up the walls embedded in the cob, and tied into the reinforced concrete bond beam ringing the top. This treatment is very likely much more than is necessary--but we don't yet know what is necessary with cob. Traditionally, the earth used for cob buildings comes from the site--you can create a backyard cob teahouse and a frog pond at the same time. Northside Community Garden had no room for a pond, so a total of fifty cubic yards of soil was trucked in, mainly from the R.C. Knapp Co.'s Seabreeze Soil Depot at the base of University Avenue. Since the soil comes from building site excavations, it had to be sifted at the garden to rid it of rocks, big chunks of clay, nails, glass, and oddities. Sifting reduced the volume of usable soil by a third, and the tailings were returned to the depot for a less exacting use. The next step was to create just the right mix of soil, sand, straw, and water. Fordice made test lumps, carefully measuring proportions as he added sand to the mud until the lump fell apart, then backing off on the sand just enough that the lump would hold. The amounts had to be retested with every batch of soil. West Berkeley topsoil doesn't need much sand, but soil from several feet below the surface, or from the hills, contains much more clay, and without added sand would shrink and crack as it dried. Straw is added to further control cracking and add tensile strength to the glop. The proper consistency is an easy thing to learn by experience and impossible by the book.
On weekends, Fordice and garden volunteers used a small cement mixer to make batches of soil, sand, and water. They dumped the mix onto a tarp and worked the straw in with their feet (straw would clog the mixer). Then they built up the walls, mashing the new layer into the old with a wooden tool invented by Fordice and called a "cobber's thumb." Fordice used a height-to-thickness ratio of seven to one for the shed--one-foot-thick walls that are seven feet high--but he recommends even thicker walls, ideally four to one, especially for structures not seismically reinforced as this one was. To keep the cob from drying out too quickly, Fordice put wet burlap on top of the walls and then wrapped them in tarps between cobbing sessions.
At every stage of the construction, materials were salvaged, found, and donated. The hauling of soil, sand, cement, and lumber were likewise volunteered. After a year of weekends, even the enthusiasm of the stalwart Northside gardeners understandably waned, and fresh volunteers needed to be recruited from the wider community. A similar project in your backyard might move along considerably more smoothly.
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