Cob is one of a number of earthen materials used in green
building. Essentially, walls are sculpt
ed by hand, from the ground up, of a mixture of clay
, straw, sand, and soil. Houses created from cob have a unique look, often like something out of a fantasy story.
This method was recently featured on an episode of Dream Builders on HGTV
, which prompted me to look into it further. My findings:
The process of building a cob structure starts with a good foundation (often poured concrete or stone) and a stone base for each wall up to a foot or two high. This is part of the "hat and boots" strategy for preventing erosion of the walls. The clay walls can breathe somewhat, so a little water won't hurt, but soaking for weeks during a rainy season might cause some damage. The "hat" component is a roof with a wide overhang to keep off the bulk of precipitation. A wood frame may or may not be used, depending on the size of the structure and the preference of the builder, but it is a good idea if you plan to have conventional doors and windows. Door and window frames are usually built before the walls are formed. A frame will also support the roof, which is sometimes erected before work begins on the walls. This shelters the incomplete walls, allows builders to work in the rain, and allows residents to live in the house before construction is completed.
Now for the yummy part. The cob material is a combination of clay, sand, soil, and wheat straw or other plant fibers combined in some predetermined proportion. The word cob refers to the softball-sized lumps of material that are pinched and packed together to build walls. The raw materials for the walls and stone for the foundation are often taken directly from the site of the home, in keeping with the earth-friendly lifestyle espoused by many cob builders.
The materials are mixed by foot, like grapes for wine, on a large tarp until the mixture reaches the correct consistency. Then it is packed into small clumps and added to the walls. Cob is applied wet, so builders can pinch and shape a wall as they add to it. This is part of the beauty of cob building. Walls take on shapes that a wood and siding house would never imagine. Curved walls are common, as are round and other non-rectangular windows, niches in walls, and even bas relief sculptures. As an alternative to traditional windows, many builders place art deco glass blocks or panes of clear or colored glass directly into a wall and pack the clay mixture around them, creating a unique type of stained glass window. Cob is too heavy to use for roofing, so a roof is generally made of wood, thatch, or something more green, like recycled aluminum, tires, or sod attached to a wood frame.
Plumbing and electrical wiring are up to the builder. They may be placed under the floor boards or cobbed directly into the walls. Walls can easily be cut open and rebuilt, but I would still be leery of sealing any wiring where it would be difficult to upgrade or install another phone line later. For some cob owners, this is not an issue because they choose to live simply with a minimal impact on the environment, using little electricity and water, or only use their cob dwellings part-time. I suspect that some builders encase wires in some form of inconspicuous tubing or hide them behind ceiling beams.
Because the walls need to remain breathable, it is not recommended to paint them with conventional interior or exterior paint. Homeowners may leave the walls bare, which is nice if the local soil has a pleasant color, or use a wash of lime or clay, which creates rich earthen tones or pastel colors. Cob is also low-maintenance, compared to something like an adobe building. Adobe varies from cob in that adobe consists of sun-baked bricks dry-stacked and covered with a stucco plaster, while cob can be described as monolithic, one solid piece. Stucco is notorious for falling off and needs to be repaired from time to time. Cob will not fall off. The most serious problem I have read about is the potential cracking of a wall, if the foundation is not sound. Because wet cob can be applied to an older wall, the repair of cracks is not terribly difficult.
What about building codes?
The details are sketchy because many states have no laws that deal explicitly with cob construction, so it depends on the local building inspector. Because cob is popular among DIYers, some people manage to build a home, install their own generators or solar panels, and dig a well and septic system without coming to the attention of the local government. They sometimes run the risk of having their homes torn down if they don't comply with building codes, so it is a good idea to read up on local and state laws. In some states, cob is apparently not a problem. The HGTV cob feature showed both a private landowner and a cob construction crew building in Texas, so this is probably one of the supportive states. I suggest researching the laws before you start planning. In some states, like the mostly swampy Louisiana, a cob house might not hold up well even if the laws allow it. Cold and dry places seem to work well. For anybody who is interested in trying it out, you can go to a cob workshop, which is usually inexpensive or free, and learn the basics. The class collaborates on building a small house or cottage, and many people go on to build their own homes without much further training. Oh, the cost is another advantage. There are accounts of cob houses built for only a few thousand dollars, but of course it depends on your needs and budget.