Miscellaneous Q & A about 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 needing to build a home for my son and me. I will not have much budget to work with overall. I want to find out if there is someone who can come and build a home for us such as a cob home? How much is the cost to do this? It would be a simple design home I need a home about average 1600 sq feet? I am by Dallas Texas.

A: I wish I could tell you there was an easy way out of your predicament. Natural building techniques such as cob can be very inexpensive for owner-builders because the materials (clay, straw, etc.) are so cheap. Cob is so simple that almost anyone can learn to build it. However, it does require a lot of time and work. If you have to hire people to build your house, a cob home may not come out any cheaper than conventional. This is due not only to the number of hours of work required, but also to the fact that there are relatively few experienced professional natural builders, and so therefore you are usually paying for a lot of on-the-job training.

My suggestions would be these: First, take some natural building workshops to find out how much of the building process you can do yourself, or with family and friends. You might even find a natural building instructor willing to lead a workshop helping to build your home. If not, then find an experienced builder who you trust, and work with him or her as much as possible. Keep your home as small as you possibly can. If there are only 2 of you, you should be able to cut the size down to 800 sq. feet or less and still have plenty of room. I live in a community where the average house size is 400 sq. ft. (including for families with small kids). Try to phase your building so that a core area with the absolute necessities (kitchen, small living and sleeping spaces) can be built first, and then any additional spaces added on later when you have the resources.

Q: I'm still learning about cob, and so far I love the idea. I have one major question. What about mold? I have kid's with asthma so mold is a big deal.

A: The best strategies I know of to avoid mold problems: 1) build a home with breathable walls so you don't end up with condensation build-up; 2) use a lot of clay in your finishes (floors and plasters), which tends to absorb and distribute moisture; 3) use careful siting and passive solar design to keep your building warm and dry in the winter. Cob lends itself very well to all of these strategies. In my experience, moisture problems (of which mold is a symptom) are practically non-existent in natural buildings with breathable walls. Make sure you have good perimeter drainage and avoid moisture barriers in the floor and walls (sometimes they are helpful in the ceiling.)

You also want to do your earthen building in warm, dry weather. Where I live in Northern California, we have a 6 to 8-month window where we can do earthen building. In the winter it is too wet. If you build cob in wet weather it can take a very long time to dry, and mold can grow on the wall during that time. However, my experience is that even in that case, once dry weather eventually comes and the cob wall dries, the mold will die and not reappear.

Q: We live on the Big Island Of Hawaii and are getting ready to purchase 4 acres here. We will need to put up a quick structure and be able to live in it within 3 months. Do you think that a small cob structure would be the best choice? What do you think about using lava in the building of a cob home? How difficult is it to build your roof before the house?

A: I have heard conflicting reports on the presence of clay on the Big Island. Some people say the volcanic soils of the island aren't old enough for clay to have formed. Others say that there is nonetheless something sticky in the soil that allows for making cob. I can only suggest doing some sample batches and seeing whether you can make a fairly strong material. Lava rock ground up fine could work well as the aggregate, if any is needed. It will be very hard on your hands and feet so you will need to use suitable protection.

Cob is not usually the fastest building technique. If you have suitable materials on site it could be a good choice. I would also look into wattle and daub with bamboo or other local materials as the framework. That still assumes you can find a decent clay source. Or you could just take your lava rock, stack it up and mortar it together. If you don't have clay you could use a cement-based or lime-based mortar.

You can certainly build the roof before the walls of a cob structure, supporting the roof beams on either temporary or permanent posts. That's a good choice in hot and/or rainy areas, because the roof will protect both the workers and the cob walls from sun and rain.

Q: My question is regarding cob, adobe house building...if you merely "pile up" the mixture to form a wall, is there a "best" height you should pile up at a time? And how long would it take (to dry) before you could add to the height of the wall (I have land in the desert of Arizona where it is dry and can get hot.)

A: The drying rate for a cob wall is highly variable, depending not only on local climate and weather but also on your mix, wall thickness, and other factors. In the Arizona summer, the cob is likely to dry out just about as fast as you can get it onto the wall, unless you have a large crew working on a small building. Your challenge is more likely to be keeping the top of the wall moist so you get good bonding between layers without cold joints. For a much more complete discussion of drying issues (including ways to speed up or slow down the drying time when necessary) see "The Hand-Sculpted House."

Q: I want to put a cob mural on an existing brick wall. Each cob image will be no wider that 10 inches and only as thick as need be: small cob stars on a brick wall. Will the cob stars stay? Should I use "anchors" of some sort?

A: If the brick wall is fairly rough (for example, if the mortar joints are incised), you should be able to make a good, sticky earthen plaster stick. As you are doing that, you can build out with a cob-like mix to make your relief mural.
I would be concerned about trying to stick on just a very small cob image onto a brick wall without anchors. The cob is likely to pull away from the brick surface as it dries and could come loose. But it sounds like a very simple experiment to see how well a small cob image sticks to the particular brick wall. If you do need to use anchors, you could use any kind of masonry screws or bolts.

Q: I live in South India, where I work with eco-restoration of the local Mountain. We have also just started a small farm/forest based school, and are about to embark on constructing the various buildings for that. I love the concept and feel of cob, but have one very major doubt, and nowhere do I find any mention on the topic in any of the pro-cob literature. Termites. Here it is a very real problem even in conventional construction, and I envisage the termites simply eating all of the straw out of the walls, and worse still, using that as a pathway to access the roof materials.

A: I don't have direct experience with termites eating cob walls, and I don't know whether they would. Do they cause damage in other earthen structures in the region? If you're concerned, I would make the following suggestions based on my experience with wood-eating termites. First, make sure there is a termite-proof barrier between the ground and the cob wall. A concrete or stone foundation would serve this function well. You could make it fairly high for added protection. Second, make frequent visual inspections at the base of the building inside and out to make sure that the termites aren't building a pathway across the foundation. If they are, scrape off their pathway and apply borax or another insect-deterrent. I would expect that the combination of these strategies would protect your building from termites very well.

Q: Can you run electricity in a cob home?

A: Sure you can. As far as wiring goes, you have several options. You can either bury your electrical cable directly in the cob wall (simplest) or bury conduit to allow you to change the wiring later. Or you can build an accessible chase for wires in pipes behind a removable piece of wood either in the floor or at the base of the wall.

Q: I want to build a cob home in Brazil. Its a dry climate, hot with cold nights. A friend of mine built with adobe and he says cob wont work because the termites will eat out the straw. Is this true?

A: I would expect it would take a very long time for termites to eat all the straw out of a cob house. Even if they did, the straw is most important in the construction process, and less important after the walls are dry. At worst, if the termites did eat out all the straw, you would be left with the equivalent of an adobe house with no straw.

Since I don't know much about conditions in the particular part of Brazil where you want to build, I'd recommend doing some on-the-ground research. Look around for earthen buildings, which I imagine you will find in rural areas. If you can get permission from the owners, or if you can find old, abandoned earthen houses, cut into the wall and see whether you can find any straw or other organic material inside the wall, and whether there is any evidence of termites or termite tunnels inside the wall. If you can find straw inside old earthen walls, that should disprove your friend's assertion. In any case, talking with local people about their experiences with earthen construction should give you a great deal of useful information. If you can find no earthen buildings at all, there is probably a good reason for that, since adobe, wattle-and-daub and other earthen wall systems are common around the world, especially in dry climates.

Q: I'm living in a cob house that I completed last year, and this year I am installing a new wood stove. The house has a living roof with an EPDM membrane, and I hope to run the pipe through this membrane. (It is not really feasible to run the stove out the wall, unfortunately.) How do I make a strong seal between the pipe and the membrane? I want to do this right.

A: I don't know of a way to get a seal directly between stove pipe and EPDM membrane. You could do a web search and see if anyone else has a system for that, but my guess is that it's not possible because the metal and EPDM both expand and contract with temperature change, but probably at different rates. Also, you really need to keep anything flammable like wood, plant matter etc. at least an inch away from double-wall stove pipe. You probably wouldn't want to cut your roof decking back and leave the EPDM floating there with no support.

Here is how I have dealt with a similar situation: Before the EPDM went on the roof, I built a box on top of the roof decking that was wide enough for the double-wall stove pipe to pass through with at least 1" clearance on all sides and that extended up a few inches above the future soil level. I installed beveled blocking on all 4 sides of the box to eliminate the sharp angle between the roof surface and the sides of the box (which I figured would reduce strain on the glue joints as the whole roof membrane expands and contracts). When laying the membrane in place, I cut a snug hole allowing the EPDM to drop down over the box. Then I very carefully glued scraps of EPDM to the roof membrane and brought them up the sides of the box and over the top. I used the primer and glue sold by the EPDM manufacturer for that purpose. It's somewhat expensive and you will probably have to order way more than you need.... maybe someone else at DR has some lying around from another project? Finally, I cut a hole in the EPDM patch at the top of the box, bigger than the stove pipe, installed the pipe, and then used aluminum stove pipe flashing over the top of the box, which can be folded down over the edges of the box and sealed to the pipe with caulk. I did that over 7 years ago, and there has been no leakage yet.

This was a somewhat tricky operation, and will be even more so in your case since the soil is already in place on your roof. If you remove a large enough area of plants and soil, you should be able to retrofit a solution similar to the one described above. My biggest concern is that if the EPDM is not extremely clean, you might not get a good seal when you go to glue down the patches, and you could end up with some leaking through the membrane around the stove pipe. To get the membrane really clean, you may need to remove all of the soil above it as far as the peak of the roof.

I want to report that the stovepipe through living roof installation was a a success. Here's how I did it: http://small-scale.net/yearofmud/2010/10/07/installing-stovepipe-through-living-roof/

Q: How to make a cob/mud roof waterproof? I know there are lots of buildings with a cob/mud roof (e.g. arabic countries). This is probably no problem there because the lack of rain. But how to solve this problem when there's rain sometimes (Spain, Valenciana). I am building a strawbale home there with a flat cob/mud roof. I have covered the roof with cob/mud. After this became dry I used waste kitchen oil (some 160 liters) to impregnate the roof and roof/walls and it does the job as far as there are no cracks. But is this the way?

A: There is no proven method for waterproofing mud roofs in rainy climates. That is why you only see them in deserts. In rainy climates, we always rely on a waterproof membrane such as EPDM (a heavy-duty flexible rubber sheet) underneath the earth layer to keep water out of the structure below.

Your oiling method may work for a couple of years. However, the oil will break down in the sunlight. Also, your roof will expand and contract with changing temperature, and the mud layer is likely to crack as a result. Then there will be a way for rainwater to infiltrate the mud, which could have several different unpleasant results. Most likely you will just get slow leaks. Hopefully these will be in a place where you can see them rather than directly over your bale walls, where the leaking could cause serious damage if undetected. The worst case scenario is that the mud layer on your roof could absorb so much water so quickly (say in a freak heavy rainstorm) that it would take on a huge amount of additional weight and the roof could collapse. I know of at least one case where this happened here in California, killing the builder who was asleep inside.

Q: I have a question regarding "Solar Oval One" cob plan. Would a basement work for this plan and, if so, would it count as additional square footage even if it's used as storage?

A: (John Fordice) If the basement has a ceiling over 7' tall & is accessible, then it will count as habitable area & will exceed the 120 s.f. limit that allows the "no premit" status of the design. Certainly, it can be built over a basement, but depending on the conditions, cob walls for the basement that are below grade are NOT a good idea. Any portion of the basement walls that are below grade will need to be concrete, mortared stone, concrete block, or some other material that will withstand moisture.

Q: I've read quite a bit about building mud ovens using glass bottles in the foundation. Currently, I'm building a grill/smoker by stacking used brick. I've read rock is a poor insulator, so assume brick is, as well. The brick is laid 2 deep, length wise. Can bottles be encased within a bit of clay/mud against the brick (on the outer wall), without becoming a bursting hazard? I am not sure how much heat they would actually be exposed to in such a situation.

A: I have not used bottles myself in an application such as you describe, although I have heard of it. I don't know enough about your design to guess how much heat the bottles would be subject to. I would imagine that bottles can withstand a reasonable amount of heat without breaking. If I understand you right, and there are 16" of brick between the heat source and the bottles, that sounds safe. Also, if they do break, but are surrounded with mud, what would happen? I don't think they would explode with any force. The only purpose of the bottles is to hold a volume of air; even if the glass breaks, the clay will still do the job, so long as you are not relying on the bottles for any structural support. So my first response is, "Give it a try. What do you have to lose?" My second response is that even if they never break, bottles filled with air are not really great insulators. Again, not understanding your design I don't know how important good insulation is in your case. I can't remember ever seeing a grill or smoker that had any insulation in it at all. But if you really want good insulation, the way you get that is with very small pockets of air. An air cavity the size of a bottle is large enough to form a convection cell, which will transfer heat across the space by the movement of warmed air. To increase their insulation value, you would want to fill the bottles with something very light which nonetheless prevents air movement. Possibilities include perlite, vermiculite, sifted wood ash, or sawdust. For safety in this case you might want to stay away from sawdust or anything else that could burn.

Q: I have been wondering whether people ever build with cob in a form. Can't imagine that it hasn't been tried. Do you know anything about it? I have been reading about cast earth with cement, but would like to build a small wall without cement, and thought it would be interesting to try cob in a form.

A: Yes, some people build cob in a form. In the Southwestern States there's a tradition called "puddled adobe" which is essentially cob in a form. I don't know many people who've tried it, for a couple of reasons. You need a quite sturdy form to go up very far in a single course (similar to a concrete form) because the wet cob will exert a lot of pressure on the form, so the energy and materials going into the formwork could be considerable. Also, many people like to make their cob walls curved rather than straight, which complicates the formwork considerably.

One solution to this problem is the one developed by Rob Pollacek of California Cob (californiacob.com). He uses a small form about the size of an adobe block form, fills it on the wall and then moves it along to its next position. The form has 3 sides and the open end clamps onto the existing wall with bar clamps. Even though the form is rectangular, it is small enough that by the time the wall is plastered it looks more curved than faceted.

Another thing to note is that the wetter you make your cob mix, the more likely it is to crack as it dries. So there may not really be a big advantage to formed cob rather than unformed.

Q: I am finishing the waiting room for an outdoor sauna. Two of the walls will be constructed out of strawbales. One has been started in cob. It is about half finished, with about 3' constructed and 3' to go. It is around 8" thick. I live in Ohio and freezing overnight temperatures are probably not too far away. I do not think that I could finish the wall with cob and have it dry before freezing weather arrives. I have thought about finishing it using another construction method like wattle and daub or some form of light straw. Would these building methods allow for a quicker drying time than cob? Would it be possible to build them on top of a cob base? How would I learn more about these building methods?

A: It's late in the season to be doing any kind of earthen building. Any technique involving wet mud is vulnerable to damage if there is a deep freeze while it is still wet. The force of the expanding water can break the wall apart. Wattle and daub or light straw-clay have less volume of clay in them than a cob wall, so they would theoretically dry faster. But when temperatures are low and/or humidity is high, drying can slow down almost to a standstill. If I were in your position I would be considering wrapping up the project for the winter and getting back to it after the likelihood of deep freezes is past.

There are ways to transition a wall from cob to another system. The easiest option would probably be the wattle-and-daub. You could simply implant the vertical sticks in the cob wall and begin weaving your horizontals from there up. Light straw-clay would be more tricky, especially with a curved wall, since you need some sort of structure to attach your formwork to.

I wish I could recommend a thorough source of how-to information on either of these techniques, but unfortunately there really isn't one. You can find some details in "The Art of Natural Building," edited by Kennedy, Smith, and Wanek.

Q: I live in East Texas and need to build a TORNADO PROOF home that also keeps my family cool in the summer and warm in the winter, and do it as cheaply as possible. I wanted to do a cob home, but do I need to do the shell of the home with strawbales, and would the tornado proof roof be a living roof or not? I am thinking that cob homes are tornado proof, but I am not all together sure.

A: (Kelly) The best way to achieve safety from tornados, as well as comfort in all seasons in that climate, is to go underground, or at least earth-bermed. Cob structures could be made resistant to tornados with appropriate engineering (especially for the roof), but cob does not provide much insulation from the heat and cold.

Q: I'd like to find out if it's possible to build a cob house without using wood. I would like to build a small, 800 sq ft cob structure that will be utterly unattractive to the ferocious termites we have in our region (Sub-Saharan Africa). Is it possible to use steel where wood is needed? Will rust be an issue? Are there any other types of materials to consider?

A: Yes, absolutely. There's no reason you couldn't use steel instead of wood for door and window frames, roof framing, etc. Of course steel will rust away eventually, but no faster in a cob building than in a concrete one - probably less quickly, since clay tends to pull water away from other materials. I don't know whether there would be any issues with termites getting into the cob walls to eat the straw. I'd recommend doing some experiments to find out.

Q: I am a structural engineer from Ireland and I have been asked by my local Heritage group to provide a design for a replica star shaped fort which previously existed in the area. I think this is a great opportunity to teach people about cob building and get the community involved in this interesting project. Using cob would also be similar to the original construction which I think is important if it is to be successful. The replica fort will be 2.5m high and the longest straight length of wall before there is a return will be be 2.5-3.0m. The star shape helps with the wall restraint from horizontal forces.

I would love to construct the wall using cob and lime plaster but having read a lot about it there are a number of questions I have. I know the most effective cob construction has a good hat and boots but I think putting a roof on this structure would take from it and would not match the original design. I can't find any successful alternative but I wonder if there was a breathable waterproof membrane put over the wall and then the lime plaster on top of that could this work. Also if the wall was constructed without a roof and with a shape that would encourage water run off could the structure last if properly maintained every few years? Would posts be required to restrain the wall from horizontal forces or do you know where I could find any design guidance on cob construction as this structure will require planning permission and I think I will have to build a strong case to get it to pass as it is such an unfamiliar construction material even though it has been used for thousands of years!

A: I don't have any direct experience with the kind of membrane-and-plaster approach you're suggesting. My guess is that it would be impossible to attach the membrane and the plaster (which would require some sort of lath to adhere over the membrane) without putting lots of holes into the membrane. Lime plaster wicks moisture, so then you would have moisture working its way through the holes in the membrane and unable to escape. This is similar to the consequences of putting cement stucco over cob, which is often disastrous.

But yes, if the cob is shaped so that water runs off in a dispersed way, the cob could last a long time if maintained when necessary. I'd recommend putting a little coping of stone or brick (or even concrete) on top of the wall to protect the vulnerable flat surface, and lime plastering the vertical parts of the wall inside and out. If the coping could overhang the walls by just a few inches, that would reduce the amount of water running down the surface of the walls.

Posts are certainly not necessary for the wall to stand up; as you probably know, cob walls in Britain were almost always load-bearing, with no additional structural posts. Convincing your local council of that may be another matter. I know there has been a fair amount of structural cob build in Britain over the last couple of decades, but I'm not sure what the current situation is either in the UK or Ireland regarding planning permission.

Q: Interested in a way to production build cob homes with trained workforce.

A: Cob is like any other building system in that the more training and experience you have on the crew, the faster and better the results. The challenge might be coming up with enough cob projects in one geographic area in a relatively short period of time to be able to fully train (and retain) your crew.

Q: I live in Texas and we have fire ants. How big a problem would building cob and them be? How would I keep them from trying to build in my walls?

A: I have to say I have no first-hand experience with fire ants. You might be better off finding a cob builder in your region to ask. However, I imagine that you could probably keep the ants out of your cob walls by ensuring that they have no direct pathway to get from the ground into the cob walls. You would want to bring your foundation (made of concrete or stone with cement mortar) up high enough that you could easily see an ant trail across it. From a quick search online I see that there are various substances recommended as effective barriers for fire ants - one is a mixture of talcum powder and alcohol painted on to a surface. Design your foundation in such a way that you have a vertical area to paint the barrier on - perhaps this could be protected from the weather by a strip of overhanging flashing or the like.

Q: I am in the very beginning stages of designing my dream off-grid house with two containers and a cob walled center that spans from one container to the other. I'd like to use a cob oven/fireplace, cob accent walls throughout. How realistic is it to think my son and I can complete this in a summer?

A: I’m glad to hear about your project, and I wish you the best of luck. It’s hard to know how realistic it is that you and your son can complete the project in a summer. I have seen some very determined and hard-working people complete a small cob living space in a single season. I’ve also seen a lot of people start similar projects that were ┬ánever finished because they underestimated the amount of work required. I would suggest that you break the project down into manageable chunks, and take them on one at a time. And try to line up some help. If you can possibly get some hands-on cob building training before you start, that should go a long way. You may have to travel for that. Check out cobcottage.com and cobworkshops.org for some workshop listings.

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