From the Ground Up

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Cob easily facilitates the addition of wiring and plumbing through the insertion of four-inch pipes through the walls or foundation. Ann's hermitage will be lit by candles and kerosene lamps, and there's an outhouse in the woods not too far away, so wiring and plumbing installation was not an issue.

For Ann, the foundation and the roof were the only parts of the house she decided to hire help. Building the roof included selecting, harvesting, and preparing ten large cedar poles. Because of the hired labor, the roof was the most expensive part of the building. The hermitage roof is a good example of how any kind of roof can work on a cob structure. Ann designed it to be shaped a bit like an octagonal shell, and conceptually it is quite interesting and unusual. The back slopes downward at an angle that takes it almost into the ground. The front sweeps upward and a separate but lower piece forms a rectangular roof over the entryway. While it is interesting to look at, it posed many challenges for the stooping cobbers who worked on the back wall, bending over very low to place the wall under it, bumping heads, shoulders, and bruising backs. The roof may also prove problematic later as the builders bring the walls up to create some strange angles where the two sections of oddly shaped roof come together. Simplicity is a matter of taste, but may also be a practical consideration for inexperienced home builders.

Though Ann's roof was built before the walls went up, you can add your roof last. Since cob is load bearing, you can simply build your roof right on top of your walls. I liked working with the roof up. We knew where we were headed, and the roof gave the shelter an identity early. Roof insulation, ceilings, and sheathing can easily go the way of conventional buildings. Some interesting natural ceding materials Becky mentioned during school include fabric, cob plaster, and woven bamboo mats.

The Mix

"Imagine the cob as a miniature mortared reinforced stone wall. The sand particles, like the stones in a stone wall, provide strength. The clay serves as the mortar. The straw does the reinforcing job of the rebar," says Becky.

The Cob Recipe:

50% to 85% sand
50% to 15% clay
straw to taste
water

Cob is made from sand, clay, and straw. Every cob mixture is a little bit different, however, and you may vary the ratio of ingredients to suit what your soil naturally has in abundance. You'll want to get to know your soil and try out a few sample mixtures before you can come up with the right recipe for your land.

Several people brought soil from their land and had Becky help them do a test to find out if their soil was sand rich or clay rich. To see the proportions of clay to sand in the soil, we put a little in a glass jar, added some water, shook it well, and then let it settle. The sand goes to the bottom and the day hangs around on top. Silt is in the middle. Silt weakens cob, so if your test shows it in abundance, you'll want to dig down further in search of a better clay/silt ratio. It is possible, if you have a lucky natural soil ratio, that mixing clay and sand might not even be necessary. You can simply take your dirt, add water and straw, and put it on the foundation and start shaping it into a wall.

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