Foundations for Cob Building

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 The Cobber's Companion: How to Build Your Own Earthen Home (Cob Cottage Co., 1998) and co-author of The Art of Natural Building: Design, Construction, Resources (New Society, 2002) and The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage (Chelsea Green, 2002). He teaches practical workshops and provides consultation to owner-builders on a wide variety of natural building techniques, site selection, and design. He lives in an intentional community in Northern California. His informative website is www.strawclaywood.com.

Questions and Answers

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.

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: 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 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.

How deep in the soil should be the the concrete reinforced foundation (approximately)? The best choice is to come with the foundation up to the soil level or start building the steam wall below the soil level?

I would dig down a minimum of 45 cm. I'm assuming that you have no issues with frost there? If there is a soil level where organic content reduces and the soil becomes firmer and harder to dig, you should dig down at least that far. Again, consulting with local builders is recommended. You have several choices in where to make the transition between the concrete footer and the stone stemwall. One would be to pour just the footer in concrete and have the stone start below ground level. Another would be to bring the concrete up to ground level and then transition to stone. A third would be to bring the concrete all the way up to the level of the cob, and simply face it with stone for aesthetic reasons. Concrete will tend to be much faster and stronger than stone masonry. It is also more environmentally damaging (due to the cement manufacture) and could be more expensive.

Q: We are going to build a cob structure and plan a rubble trench. The soil in this part of the country (northern WI) is compacted sand from the last ice age. Is it possible to dig the trench for drain tile placement, gravel to cover drain tile, then back fill the remaining trench with the sand instead of more gravel? I can image a barrier may need to surround the drain tile / gravel so the sand does not infiltrate, but isn't sand itself permeable (like the gravel) and not subject to frost upheaval? The frost depth is about 4 feet, so that's a lot of gravel to backfill.

A: It may be possible to get away with filling a rubble trench with sand rather than gravel. The purpose of the gravel backfill is to provide structural support for the building while at the same time allowing water to easily drop down into the drainpipe where it will flow away from under the building. Coarse, rounded drain rock does the best job of combining those 2 functions. Finer sand may either not have the desired structural properties (it may compress and settle under the weight of the building, for example), or it may not be well-draining enough to achieve the desired result. Cob buildings weigh a lot, and could potentially compress the sand over time back into a density similar to how you find it in its undisturbed site. Is the drainage in your soil so good that when you pour water on the undisturbed subsoil, the water almost instantly percolates and disappears? If so, you probably have nothing to worry about as far as drainage goes; the rubble trench is practically superfluous. If your drainage is not already ideal, you would be taking a gamble to use your site sand in place of drain rock. Consider what the consequences would be if the drainage fails. A rubble trench under a cob wall is virtually impossible to clean out and replace. If you're just building a shed or auxiliary structure, it might be a reasonable risk. If you're building a house that you want to last for many decades, I wouldn't personally want to risk it.

Q: I'm hoping someone might know if a rubble trench and stem wall foundation is suitable for a cob home built on sugar sand. We are in the burned area of Bastrop, Texas, and the ground is semi-compactable sand at least down 6'.

A: I'm afraid I don't know anything about the properties of sugar sand. Building a rubble trench foundation on loose soils could be problematic. I would ask the opinion of local builders who are familiar with the conditions there. If they don't know about rubble trench foundations, ask about how they would approach a French drain on your soil type. The issues are the same, but the negative consequences of drain failure are higher with a rubble trench. If you do decide to go ahead, I would recommend using filter fabric to keep the sand and gravel separate. Worst case I can think of is that the drainage fails and you have to install a new French drain around the perimeter at some point in the future. This is a possible (or likely) outcome eventually with any rubble trench foundation.

A: (Kelly) I built an earthbag house on very fine sugary sand in southern Colorado over a decade ago, and I used no foundation whatsoever. This was possible because of the relatively arid climate and the fact that the sand drains extremely well. You never see standing water, even after a heavy rainstorm. If this is the case with your soil, then I think that a rubble trench without even installing a French drain would work well as a foundation. You still need to get the cob off the ground, so you will need a concrete, stone, or earthbag stemwall before starting the cob.

Q: What size rubble trench foundation would you recommend for a 160 square foot cob structure built in sunny Western Colorado @ 6000' elevation.

A: A rubble trench should be just slightly wider than whatever kind of stemwall you are going to put on top of it. The depth is determined by frostline in your location, which is defined as the greatest depth that the ground is likely to freeze to. If you don't know what that is where you are, ask a local builder or your building department. You may have read that a rubble trench can theoretically work even if it is less deep than the frostline. Personally I would not want to risk it if I cared about the longevity of the building. The upward force exerted by freezing water underneath a building is monumental. At the very least I would expect it to push soil into your rubble trench, which would likely interfere with the correct functioning of the drainage.

Q: My family and I have been researching cob homes for 5 years. We are a year away from pulling the trigger. One question will help me decide where to buy my land. I need to build above the flood plane. I live near Galveston TX where we're are almost at sea level. Can I build a cob on stilts? Can I build my base, lay concrete down on the platform and build on concrete? If not, is there a better way, or should I find a different piece of land.

A: Cob walls are very heavy - more than 10 times heavier than a standard light framed wall. While it is conceivable to build a cob building on stilts, it would have to be very heavily engineered and would require a huge amount of wood and/or concrete to support it. What I would recommend instead, in this order, would be: 1) find another piece of land where flooding is less likely. 2) Build a very high foundation out of a non-permeable material such as stone. In most of Britain cob buildings typically have stone foundations about 3' high. Sometimes the entire ground floor is made of stone and cob is used only for the second floor. 3) Grade your site in such a way that you have higher land immediately surrounding your house. A good way to do this is to build a high foundation out of stone and/or concrete, say 4' above existing grade, and then push earth up all around it so you have a new grade 3' above the old grade. This could possibly be combined with a cellar, although I'm not sure of the advisability of building a cellar in a flood-prone area.

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