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Foundations for Cob Buildings |
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| Q: I am building on a hill that is mostly rock and gravel. I am wondering how deep I must dig the trench for my foundation. I'm thinking that I shouldn't have to dig very far since what I'm digging out is rock and gravel. A: The depth of your trenches depends on several factors. One is that you need to reach very solid, undisturbed subsoil and/or bedrock which will not settle when you put the weight of the building on it. The second is that if you have high water tables during the rainy season, you want to size your trenches to carry away as much water as is likely to get into them. And of course you want to get down below maximum freeze depth to avoid frost heave. It's hard to generalize without knowing more about your site geology and weather, but my initial guess is that 2 feet deep should be plenty. Ask local builders for their input. Q: I've read a great deal so far about building with cob and remain stumped on one point regarding foundations. I read the foundation should come up several inches above ground before cob is built on top of it, and mostly this foundation is stacked stones. From the exterior I see this looking nice, but from the interior I am confused - if I had an earthen floor, wouldn't my floor butt up against that stone foundation on the interior - and wouldn't the wind and rain blow through those stones in to my house? If I cobbed down on the interior to wall-off that stone foundation, then I am defeating the purpose of the stones in the first place because moisture is going to wick up through that low level of interior cob. Could you please clarify all this for me? A: There are many possible ways to make a foundation for a cob building. Stone is a common solution, especially in places without strong seismic activity. However, in most cases the stones are joined with mortar, both to increase the strength of the wall and to keep weather and critters from being able to get through. Mortared stone can be an attractive feature on the interior as well as the exterior. Or you could bring your plaster down over the stone to cover it up. This does create a small danger of moisture traveling up through the plaster and into the cob, but not a severe one. If the stone foundation is more than a few inches above floor level, the likelihood of enough water wicking up through the plaster to cause any damage in the cob wall seems very low to me. For a much fuller discussion of foundations and drainage issues as they relate to cob, see my book (with Ianto Evans and Linda Smiley) "The Hand-Sculpted House." Q: I am planning to build a cob cottage using a post and beam construction to hold up the roof as I am in a seismic area. However, I am confused as to whether the posts should be buried in the ground adjacent to the foundation or should they be nestled into the foundation itself (I plan to have a rock stemwall and a rubble trench beneath it). In the later case, how do you avoid burying the post in the cob since it shouldn't be more than halfway buried and also how does the post avoid the perforated pipe in the rubble trench? Also, how can one avoid rotting in the post where it contacts the ground - I live in an area with a lot of rainfall. Q: Where would I find more information on straw light-clay? A: That's a tough one. Unfortunately there is not a lot of written information available in English. The technique is German in origin. If you read German,"Lehmbau-Handbuch" by Gernot Minke would be a great source. I hear that an English translation called "Building with Earth" is about to be published, but is not yet available. In the meanwhile, there are several possible sources in English. The man most responsible for introducing straw light-clay to the U.S. is Robert Laporte, whose company, EcoNest, is based in Santa Fe, New Mexico. Robert and his wife Paula Baker-Laporte came out with a book last year, also called "EcoNest" and featuring lots of nice pictures of their work with timber framing, light-clay, clay plasters and floors. Unfortunately, the book contains very little technical information. Robert and others do teach workshops on straw Q: There's a bit on the net. Not comprehensive instructions, but the general gist. I guess the key is in making a good clay slip? Which would involve using the correct proportion of clay etc, as in cob building? A: Actually the clay slip is the easy part. You want a slip about the consistency of heavy cream or paint; just thick enough that when you dip your hands in and pull them out, you can't see your fingerprints. You can achieve this with almost any clay soil, as the parts of the soil that are not clay (sand, silt, stones) will drop to the bottom of the barrel. A: There are a number of ways to reinforce a stone foundation. The specifics depend on the type of stone you are using and the likelihood of severe earthquakes, among other factors. One approach I commonly use is to start by pouring a concrete grade beam at the bottom of the foundation, about 6" thick and reinforced with 2 or more continuous strands of barbed wire. While the concrete is still soft, if possible, set the first course of stones down into it. At the top of the footer, create a channel by using smaller stones on the inside and outside faces of the wall and fill the void between with concrete with another strand or two of rebar. You can do the same thing halfway up if you like. If your stones are small or poor quality, or if you are very concerned about earthquakes, you could essentially use the stones as formwork on the inside and outside faces of the wall, and fill the entire middle of the footer with reinforced concrete. Half-inch rebar (number 4) is adequate and the easiest to bend by hand, but you could use anything from there up to three-quarters inch. You definitely want to surround the rebar by a few inches on concrete wherever you use it, to get the maximum value from its strength and to prevent it from rusting out. Q: I am considering a regular framed house with interior cob walls for mass. Having built a cob playhouse, I am aware of the considerable weight and drying time of cob walls (I live in humid Indiana). So, 2 questions: what kind of structural support would my interior walls need, and when in the house-building process should I build them so that they get enough air movement to dry? I have not worked in Indiana, so I don't know what to recommend as far as building sequence. As you know, drying time is aided by direct sun, air movement, and low humidity. Definitely build the cob walls before the building is fully enclosed (siding, windows and doors) but you may want to put the roof up first for sun and rain protection (the sun is a concern more for the workers than for the wall). You can also improve drying by the use of fans if necessary. Q: I'm building a 200 square foot cob house on a south facing slope. I would like to build the curved north wall right into the cut of the earth - it will be aprox. a 4 foot tall cut. I will be building a rock foundation with local stone. Any suggestions on how to correctly build the tall north side foundation/retaining wall? Should it be very much thicker at the base? Step into the hillside? Types of mortars? Can I get away with not much portland cement? Do I really need a moisture barrier? The house will have a cob floor and I'm in southern Oregon, but the hillside is pretty dry and I plan to dig a swale or curtain drain around the site. Rock advise needed! A: I don't recommend building into the slope like that except in a much dryer climate than Southern Oregon. If the soil up-slope from the building becomes saturated, you will have hydrostatic pressure forcing water through your rock wall and into the building. It's much safer to grade back from the building, leaving at least 2 or 3 feet between the building and the bank, and then put a French drain at the edge of the cut and/or a rubble trench under the building foundation. Depending on the soil type, there can be a huge amount of pressure on a 4' high retaining wall. In conventional building, that is the limit where special engineering is typically needed. That's another reason to have your retaining wall separate from your building foundation. Build it far enough from the building that if the retaining wall fails you have space to get in there and rebuild it. Retaining walls will be more stable if built at a slope rather than vertical. The greater the slope, the better, though of course that requires more material. If the retaining wall is not part of the building, you should be able to do it without mortar if your stone is decent for building. Q: I am interested in learning more about Cob building and as my first project want to build a garden wall between my house and my neighbors house. It will be approx. 15-20 ft long, and 4-5 ft tall with a gate. I plan on making it a very organic, flowing shape, taking advantage of the characteristics of the Cob. For a wall such as this, how critical is it that I have a rubble drainage trench below the wall? We live in California (Bay Area) and our property is flat. I've accumulated a good stack of urbanite that I plan on using for the foundation. A: No, it really isn't necessary to have a rubble trench for a garden wall in a climate where the ground doesn't freeze. I would remove any topsoil and create a level surface to place your urbanite on, and then you could just start stacking urbanite in the trench. Bring the foundation up a foot or more above grade. That should be enough to prevent ground moisture from working its way up into the cob wall. Q: As extra protection, would it be necessary to apply tar or other protection to the foundation wall to prevent moisture from being drawn up into the cob wall? A: That depends on what the foundation stemwall is made out of. If it is something porous, like concrete, and you are in a wet climate then yes, this would be a wise precaution. Q: When setting the foundation for interior cob walls, does it need to be set at the same depth as the outside foundation walls or can it be set directly onto tamped subsoil inside of the house? A: Cob walls are very heavy. If you do not have a continuous foundation under both exterior and interior walls, you can fairly easily get differential settling of the soil under the 2 types of walls, with a resulting crack at the junction. The main thing is to spread the load of the wall at all points onto undisturbed subsoil and to have a strong continuous foundation with no breaks in it. Q: Can I build the cob walls on top of the raised wood frame/wood floor base which is our current plan. A: It's typical to make a stem wall out of mortared stone or concrete which brings the bottom of the cob wall a foot or two above the surrounding soil level. It is possible to build cob walls on a raised wooden platform, but remember that cob is extremely heavy, so you will need to use very large beams to support it. Also, the cob should last longer than the wood, and it will be nearly impossible to replace the wooden floor underneath the cob should it begin to rot or be attacked by insects. Q: Is it feasible to create a mobile/portable structure out of cob? If so, how would you go about constructing a stable yet portable base in lieu of the usual fixed foundation? What materials would you recommend using for the base? A: Cob is extremely heavy, on the same order as concrete. So while it is possible to build a portable structure out of cob, it is generally not practical. I have heard of people building mobile earthen ovens on trailer beds, and some friends of mine built a cob bench on wheels a couple of years ago. The bench was so heavy that it took many people pushing and pulling with ropes to budge it. Q: I am building a cob house. The foundation is gravel with earthbags beginning at 6 inches below grade. I plan to use bricks (from 6" below grade up to 20" on the wall) on the exterior to cover the earthbags. (I'm planning the bricks because I need something between the earth/lime stucco and the ground.) How do I connect the earthbags and the bricks? We've never had earthquakes in my area but there have been tremors in the city 40 miles north. A: Hmmm.... an interesting problem. I would definitely want to attach the bricks in some way. Here's one possible solution: attach loops of tie-wire to the barbed wire between courses of earthbags. Use that to cinch stucco netting or hardware cloth to the exterior face of the earthbag coat. Then attach more tie wire loops to that at the heights corresponding to mortar joints between brick courses, and sink those into the mortar. An additional possibility would be to first stucco over the mesh before building up the bricks, but leaving the wire loops sticking out. Another possibility would be to forgo the bricks entirely, make the earthbag foundation the full width of the wall, and stucco over the exterior of the earthbags in the fashion described above. Q: We will be building our cob home in an area with large sunken rocks or boulders. Some will need to be leveled. The final building surface will not be solid rock. There will be soil between many of the rocks, sometimes up to a meter or two, sometimes just 20 cm. I believe that we need to dig a trench between the boulders and fill the trench with stone and cement, but do we need to somehow tie in the trenches of gravel with the boulders? A: That depends on the likelihood of earthquakes in your area, and on how attached you are to having the building stand up for a long time. If you want the foundation to be fairly earthquake proof, your approach sounds problematic, as you will have a lot of discontinuity between the various sections of the foundation between boulders. One approach would be to drill into the boulders and epoxy rebar into the holes, which you would then pour concrete around to make a continuous foundation. This would admittedly be a lot of work and industrial materials, but it may be the most secure option. If you're not worried about earthquakes, you could simply fill the spaces between the boulders with stone and mortar. |
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Michael
G. Smith
has a background in environmental engineering, ecology, and sustainable
resource management. In 1993, along with Ianto Evans and Linda Smiley,
he started the Cob Cottage Company, a research and teaching group focused
on reviving and improving traditional forms of earthen construction. He
is the author of 
