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.

A: Since writing "The Hand-Sculpted House" I've become much more comfortable with the idea of using cob as infill in conjunction with a load-bearing post-and-beam system. In seismic areas I now think it's a pretty good idea. There are several possible ways to arrange the posts in relation to the cob walls. One of the best options is to leave the posts exposed and visible inside the building and have the cob walls continue around the outside of them. Or the posts could be partially exposed and partly surrounded by cob. In either case, the foundation should be wider at that point to accommodate the post as well as the cob wall. It is generally not advisable to bury a post in the ground or to have structural wood in direct contact with the
ground. The next question that this brings up is the nature of your foundation. Under active seismic conditions, dry-stacked or mortared stone may not be advisable. If you are planning to use stone, I would recommend reinforcing it with a concrete-and-steel "bond beam." The concrete can be concealed inside the stone foundation for aesthetic reasons. The posts can be anchored to the foundation by the use of steel tie-downs manufactured for that purpose, which are embedded in the concrete.

Q: We live in Sub-tropical Queensland, Australia and have built a round pole frame with an iron roof. We're planning to build a suspended timber floor as our land is a little low with very sandy soil that gets seasonally waterlogged (or used to - things are drying up these days!) Can we use cob to fill in the walls between the round pole frame if we put in an extra (round pole) bearer around the outside of the timber floor to bear the weight of the cob walls? If not, what's another low cost natural wall fill in that you'd recommend?

A: You should be able to infill your round pole frame with cob provided that the floor support structure is sturdy enough. I would recommend asking the advice of someone familiar with construction engineering. You may not want to use enough wood in your floor structure to support the cob safely. A couple of more lightweight options would be straw light-clay or wattle-and-daub. The straw light-clay also has the advantage of being a better insulator than cob, which
would be especially advantageous if you have either very high or very low temperatures there.

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
light-clay, but probably not in Australia. Back to books. Probably the best choice for you if you can't wait for Minke's translation is "Alternative Construction" edited by Lynne Elizabeth and Cassandra Adams, which contains a substantial chapter on light straw-clay technique.

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.

The more challenging parts of the process are getting the slip to straw ratios right and the right amount of tamping. Novices tend to add way too much clay slip and tamp too hard. This compresses the straw and reduces the insulation value of the wall while at the same time increasing the amount of work, materials, and money involved. Unfortunately the "right amount" of slip and of tamping are very difficult to describe in words. My recommendation would be to start with what feels right and then reduce both the slip and the tamping bit by bit until the wall doesn't hold together anymore, then increase it a bit.

Q: We are trying to build a cob house with a stone footer and stemwall in an effort to minimize our concrete usage. We have a 12 inch deep rubble trench (no frost issues here) about 31 inches wide. We are building on sand in a very hot climate. We hope to build a 36 inch tall footer/stemwall to be buried 18 inches underground to help keep our structure cool. The footer would start at about 28 inches and taper to 18 where the cob starts. Could you please help explain how to use stone and rebar to make the strongest footer and stemwall possible. The building is an oval if that helps any and about 300 square feet. Some questions that come to mind: Do we run rebar in cement mortar between courses of stone? How many bars or rebar and of what size? How many levels of rebar should we place (ie, two pieces six inches apart every 2 feet of wall)? How thick would you recommend the mortar to be? Should we use 1 large rock or are two leaning towards each other just as good? Are there other techniques we can use to stabilize and be as close as strong to a poured footer as possible for potential settling down the road?

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?

A: Your interior cob walls will need a sturdy masonry foundation just as you would build for an exterior cob wall. If you are planning a platform floor, you could theoretically build the cob walls on top of the floor and just provide lots of extra structural support below the wall, but I don't recommend it, as the cob wall will probably last longer than the wooden floor, and you will want to be able to replace the floor without tearing out the wall.

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.