An important and usually expensive part of any building project, whether it’s a house, a bunker or a shed, is the walls and their foundations. Using conventional techniques, you’ll need materials like bricks, 2×4 wood beams, poured concrete for the foundations, and insulating foam if the building will be heated. Also, even a minor mistake in the carpentry can spell disaster, so a lot of care and experience is necessary to make a solid wall. So how did dirt poor European peasants manage to build their own homes in the past?
One of the most popular medieval techniques for building walls was cob. Basically, it means sculpting the walls with mud. You might think with horror that this would result in the dreaded African mud huts…
But of course, even mud, when shaped with European ingenuity, can be used to make wonderfully solid walls and a comfortable habitat. Some cob buildings built in the middle ages are still in use today, and are quite cozy even by modern standards.
Hippies, always on the lookout for ways to save the earth and avoid getting a job, started experimenting with the cob technique back in the 1970’s, seduced by the prospect of having a free house and using natural materials. Many people started building with cob, and improved on ancient techniques. One of the main sources of this experimentation was the Oregon Cob Company, and the modernized, improved cob technique is sometimes called “Oregon cob”.
While I referred to cob as “mud” previously, in fact it’s a combination of three ingredients: sand, clay and straw. The clay makes the material stick together, acting as a cement; however, clay shrinks and cracks as it dries. The sand’s role is to prevent the cracking (sand doesn’t shrink as it dries.). The straw acts like a reinforcing fiber that makes the material much stronger, and resistant against earthquakes (unlike adobe bricks, which crumble during earthquakes). All three of these materials can be acquired for free, or very cheaply.
As a general rule, you won’t want to ferry loads of sand and clay to the construction site; the dirt on the site will be used as a base. All soil already has a certain ratio of sand and clay. You have to test this ratio by putting the dirt in a jar, adding water, shaking the contents vigorously and letting it settle. The result will be a series of layers (sand and clay have different densities, and so the sand will be at the bottom and the clay on top). Then if the ratio is off, just add an appropriate amount of sand or clay (good cob usually has 3-4 parts sand for 1 part clay).
Then straw and water is adder to the mixture until it forms a thick, fibrous paste. Since every soil is somewhat different, it’s a good idea to do preliminary tests with soil from various places on the building site, and experiment with slightly different ratios of materials. Make tiny little walls, let them dry and check which one is stronger to know your best bet for the final product.
A structural cob wall (meant to support a roof) will be very thick, often 1.5 to 2 feet wide. It’s widest at the base and tapers as it’s built up. The extremely wide base means that no foundation is necessary; the wall can be built directly on the ground without danger of sinking into the earth.
However, while it’s not necessary to dig a foundation (beyond just equalizing the terrain), placing the cob on a stem wall is necessary. The greatest enemy of cob is water accumulation; if the cob wall soaks water, it can collapse. A stem wall, which is just a short wall built of stones or some other water resistant material, which serves as a platform on which to start laying the cob. A drainage ditch (a ditch filled with gravel) should also be placed if there’s any threat of water accumulation at the base of the wall.
The top of openings (for windows and doors) should be supported with a wooden brace, to avoid the material above the opening from collapsing.
The ingredients for the cob are traditionally placed in a basin, and mixed with bare feet by trampling it until a good texture is felt. The Oregon Cob people have better techniques, such as placing the materials in a canvas sheet and mixing it with two people holding the corners.
It’s also possible to use a cement mixer.
Cob is generally placed in lumps, laid down like wet bricks. A layer of cob should be placed on a rough, uneven surface to insure a good bond. For this reason, each layer is poked with a stick to create countless small holes before letting it dry. Each layer should dry completely before placing the next; otherwise gravity will flatten the material, and the wall will “bulge”. If this happens, you can just cut off the excess material with a machete; most cob mistakes can be corrected easily like this, which is why it’s such a forgiving technique. It’s more of a sculpture than a construction in this way.
Cordwood (debarked round logs) can be stacked to make the walls, using the cob as a cement to hold it all together. Since mixing cob is time-consuming, this method will be faster than a pure cob wall, and will give better insulation (since the r-value of wood is better than earth). However, if there’s no wood available on the construction site, this can add hundreds of dollars to the total costs.
Appraisal and further readings
The point of this introduction to cob is to allow you to judge if the technique is appropriate to your project. From the perspective of a fascist group needing to erect some sort of building, the technique offers the following pros and cons:
- Dirt cheap (ha!).
- Can be achieved by amateurs.
- Can be done in total secrecy (you won’t NEED to hire a contractor).
- Extraordinary thermal mass.
- Even a rpg probably wouldn’t get through a 2-feet thick wall (… I haven’t tested that one yet)
- Building will last forever if maintained once every few decades
- More labour intensive than conventional construction
- Above ground, so harder to hide than an underground construction system
- Weak insulation unless you add vermiculite to the cob (a volcanic stone)
- Needs careful protection against water in high rainfall areas
If you’re going to go ahead with this system, you should read up on all the technicalities. There’s several books available, and much information online as well. I recommend The hand-sculpted house, which includes information on roof systems and all sorts of interesting advice related to construction, planning and finding cheap/free land.