Green Home Building: Foundations for cob buildings

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.

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.

Q: I´m planning to build a cob house in Brazil, in a very sandy place. It means that I will have very good drainage on the soil. My doubt is about foundations since sandy soil is not very stable. I already dug 1.5 meters and the soil is still sandy. The stem wall will be done with stone. It is necessary to make a rubble trench on this soil? How deep should the stem wall be to provide a good structure for the building? Should it be completed with concrete and iron bars?

A: It's difficult to give you precise information without visiting the site. I would recommend consulting other builders in your area. However, here are some ideas: If you can't dig down to solid, compact soil, you will want to build a very wide footing below your stemwall to distribute the weight over a larger area. Concrete reinforced with steel would be the best material for this footer. I would not recommend a rubble trench for your situation. You may or may not need to install perimeter drainage. When you dig your foundation trenches (or even a test hole), if they drain naturally after a heavy rainstorm, you don't need additional drainage. If the trenches hold water, you will want to create a way for that water to get out and away from the building.

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've read "The Hand-Sculpted House" and I'm in the process of planning my cob cottage. I have some doubts regarding the stemwall above the rubble trench. I'm in a seismic zone and I'd like to build it with tires because it seems like the cheapest option and we're very short on budget. My concern is, if they are rammed earth, couldn't there be problems of this sucking up humidity from the ground and into the cob wall? If I understand correctly the main purpose of the stemwall is to keep the cob wall away from ground-humidity, so wouldn't rammed-earth tires not solve this problem? One possible solution I thought of is filling them with gravel. Have you ever tried this and would you recommend it? Any special thoughts about it? What measures should I take to make the cob wall stick solidly to the tire foundation?

A: There is no problem with moisture wicking up through rammed tires, because the way the tires are stacked prevents that. The opening of each tire goes on top of the sidewalls of the 2 tires below, so there is no earth-to-earth contact.

It will not work to fill the tires with gravel. You need a soil with some clay in it, just slightly dampened, so that it will stay in place when you ram it into the tires. Be warned that this is extremely laborious work - more so even than mixing cob. A person in good physical condition might expect to fill about 6 tires in a day, and feel exhausted at the end of it.

I have mixed feelings about tires in a seismically active zone. On the one hand, being made of rubber, the tires should be able to absorb some of the shock during an earthquake and not transfer it into the cob walls. On the other hand, in a large earthquake with enough uplift force, the courses of tires could separate from one another because there is nothing holding them together except gravity. It's nice to use tires for both budgetary reasons and environmental ones, but I would not expect a tire foundation to perform as well in an earthquake as a concrete one.

As far as locking the cob wall down onto the tires, that will mainly be achieved by bringing the cob down over the joints in the top course of tires and onto the tires in the second-highest course. In addition, you could set bolts into the tops of the tires and/or drive rebar spikes into the rammed earth in the top tires if you wish.

Q: We are planning to build a number of COB structures here in el Salvador where we live. We are by the beach, however the soil is not hugely sandy because we are in a part where the mountains come down to the ocean. There are 2 seasons here: wet and dry. They both last about 6 months. During the rainy season it rains about every day usually at night with days being hot and sunny. The dry season is completely dry and very hot. During the rainy season water will collect on the land but usually drain out very rapidly. However, the water table is very high.

My question is about foundations and floors. While the soil is not sand it is not very densely compacted. Because of this I am thinking that a broader foundation is more important than depth. I am planning on making a foundation trench not very deep say 18 inches but significantly wider than would normally be used to give the base on the soil more stability. I have many thoughts on the composition but have been thinking of a volcanic rock base, then gravel, and then earthbags with stone to about a height of 18 to 20 inches.

I will obviously have a substantial overhang for the roof. For the floor I plan to excavate and use the same lava rock and gravel. The floor will be built up to at least a foot above grade. However, as much as I want to avoid cement, I have also been wondering if a concrete pad under the whole structure might be a better idea. With the high water table any hole you dig is immediately filled with water during the rainy season and I am concerned about settling. I question a basic rubble trench foundation because it seems to me all this will do is collect water that will immediately flow to any open area. I will be putting in several drains on the land to move water to a small wet season creek on the property. I am committed to COB because of its sculptural possibilities. I should also add that El Salvador is a seismic area but less so in the area I am in.

A: Here are my thoughts on your situation. First of all, I agree that a rubble trench is not a great idea if the soils are not well compacted underneath. However, really no foundation system is likely to hold up well under those conditions, especially with a heavy building during seismic activity. Your foundation is not a place to skimp on work, materials, or expense if you want the building to last a long time. And cob walls are very heavy. So I would strongly recommend digging your foundation trenches down deep enough until you hit some really solid, stable material that is not going to shift around under the weight of the building. Once you've done that, you can either use a rubble trench, or else pour a reinforced concrete bond beam and build up on top of that with stone or earthbags or what have you. Or you could start with your earthbags at the bottom of the trench and skip the concrete. If it seems like a ridiculous mount of work to dig down to stable soil -- say you had to go 6' down -- then you might be able to get away with digging holes down to that depth every 4 or 6 feet around the perimeter of the building, filling those with concrete, and connecting them at the top with a fairly beefy concrete bond beam.

As far as the concrete slab idea goes, I don't think it's necessary. Concrete in the foundation will help hold the building together in an earthquake or as soils settle and move. But an earthen floor in the interior of the building should hold up fine in very wet conditions provided you have several inches (6" would be good) of drain rock underneath it, between it and the soil. This also presumes that the soil inside the building has been compacted and is not going to settle much under the relatively minimal weight of the floor (compared to the weight per square foot of the exterior walls.) You would want to compact the soils even if you were going to pour a concrete slab, or it would likely crack.

Another critical part of the picture is drainage. With such a high water table, you need to install drain pipes to get the water away from under your building. Hopefully your site has enough of a slope that you can take the foundation drains downhill "to daylight", even if you end up having to dig deep trenches.

Q: I have just started building our cob home in Zimbabwe. I need to make a physical termite barrier between the stone foundation and the cob walls. I have bought galvanized sheet metal which I intend to place onto the walls with metal rods protruding up from the foundations. Will this be enough for the cob to grip to as I build up from the barrier? Is gravity sufficient or do I need something to bond the metal with cob? If you have any information on making a home-made termite barrier this would be great as I am struggling to find any information.

A: I have limited experience with termites, so take my advice with a grain of salt. Nonetheless, here are my thoughts on the matter.

It seems to me that you don't need a termite barrier between the stone foundation and the cob. Presuming you use enough mortar to fill the voids between stones, there is no way the termites can burrow through the foundation. What you need is a way to stop them from climbing up the foundation and getting into the cob. You could use a fairly narrow strip of your galvanized metal, cast into a bed of mortar at the top of the foundation and protruding outwards, with a downward slope so that water won't catch on it and run back under the cob. This way, you aren't using so much metal, you aren't interfering with the bond between the cob and the stones, and you aren't creating spaces underneath the sheet metal that termites or other insects could get into.

My guess is that even a galvanized metal barrier won't be enough to deter the termites for long. What will stop them from making a passage over the metal and getting into the cob anyway? Only your constant vigilance in removing their trails, I suspect. Or is there some substance you can put on the sheet metal that termites will not pass over?

If you nevertheless find it easier to use a whole sheet of metal across the foundation and extending past the outside surface, I suppose that would work. Yes, it will reduce the adhesion between foundation and cob (as will flattening the top surface of the foundation, which would probably be necessary with your barrier design), but how much does that really matter? Cob walls are very heavy. Under normal circumstances, gravity will hold them in place just fine. If you have a very large earthquake, the cob walls could conceivable come away from your foundation, but in that case what other problems would you have? Mortared stone is not an especially earthquake resistant foundation system, unless you have a really good way to reinforce it. Could there be an earthquake strong enough to separate the cob from the foundation but not crack the foundation itself, or cause other major structural damage to your home? I doubt it, but I don't know for sure. If there is such an earthquake, it would probably overcome the additional friction you would get by surfacing the sheet metal with something to improve adhesion (I don't know off hand what that would be.) I like your idea of leaving rebar sticking up out of the foundation and into the cob; I can't think of a better solution except not to run the metal all the way across the width of the foundation.

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