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Mixing and Applying Cob |
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Q: I am an architecture student at Smith College in Northampton MA. My class has been given an assignment to design, and then build a small (8 x 8') cabin/shed/shack place-space of repose.... my group is proposing cob construction. What is the ratio (i.e. earth, sand, clay to straw) that we would need to order...yes, we'd have to order the materials... no on-sight materials available. We've purchased the Hand-Sculpted House book, but I haven't found a ratio of materials. A: The reason there is no standard recipe in The Hand-Sculpted House is that there is no standard recipe. This is because clay soils vary so much from site to site. Even if you are buying material, you are unlikely to buy pure clay; that would be very expensive. The book describes a process for determining your ideal mix through a process of testing. Even if you are not going to actually construct the building, this process would still be very instructional for you. Sometimes you can find/buy a soil that already has the ideal ratio of sand to clay. More often you will come up with a clay soil which must be tempered by the addition of more sand. The ratio might range between 2 parts clay soil to 1 part sand and 1 part clay soil to 3 parts sand. When you take 1 bucket of clay soil, mix it with 1 bucket of sand, some water, and some straw, you end up with less than 2 buckets of clay. Sometimes a lot less. That's because through the mixing process you are compressing out most of the air. A lot of the clay takes up what was previously air space in the sand. Make a mix, measuring your ingredients carefully, and then measure how much cob you end up with. As for straw, it will be a minimal percentage, by volume, of the wall. When purchasing straw for a job, I typically buy straw bales to equal about 10% of the volume of cob I will be mixing. This is usually quite a bit more than sufficient, but bales are very useful to have around the site for scaffolding, to make soaking pits, and as ingredients in adobe floors, plasters, etc. Q: I just watched the 28 minute video COB Philosophy Building 1007 Are there any other videos on COB. I was amazed at the amount of hay in the mixture. A: Unfortunately, there are no good instructional videos on cob that I'm aware of. Might I recommend a book, instead. The latest and most thorough how-to guidebook is "The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage" by Evans, Smith, and Smiley. Available from Cob Cottage Company (www.cobcottage.com) or your local bookstore. By the way, straw is preferred over hay for cob and almost every other building application. Remember the adage: "Hay is for horses; straw is for houses!" Q: I am considering building a somewhat large project in Colorado out of cob. Can you give me an estimate of the number of worker hours needed to construct and finish a linear foot of an 8' cob wall (or any other kind of generalized time estimate). I need to know how much work I will be biting off. A: The speed of cob mixing and building depends on many factors. Assuming all the materials are collected, processed as necessary (e.g. clay soaked, large stones screened out) and arranged in a convenient fashion near the building site, the biggest factor to consider is the method of mixing. Using only their bodies and a tarp, two experienced and energetic cobbers might mix and apply 30 to 50 cubic feet of cob in a day. Use of a mortar mixer might double or triple this amount. With a tractor or backhoe, huge batches of cob can be made fairly quickly. For example, two people mixing cob with a Bobcat on the street of Eugene, OR made 17 cubic yards in 6 hours. Machine-made cob tends to be of poor quality compared to hand-made, but with care this need not be the case. Once the cob is mixed, the building rate depends on other factors, like the amount of sculptural detail and the height above the ground. Work out a good delivery system for getting your material to the top of a wall. If you use a tractor with a bucket to mix, the cob can be scooped up in the bucket and lifted high onto the wall. The other limitation on building speed is drying time. Build in warm, dry weather to allow the wall to dry out as quickly as possible. Normal height rates are in the range of a foot or two per day. This can be speeded up in various ways, including the incorporation of lots of rocks or other solid objects. For more details on all of the above, check out the new book "The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage" by Ianto Evans, Michael Smith, and Linda Smiley. (Available above.) A few words of advice: take a cob workshop or work with an experienced cobber to learn their systems of production building. Be very organized. And don't take on a large cob project until you have one or two small ones under your belt. Forming systems may also speed up the building rate. For an explanation see the November, 2002 issue of The CobWeb, (from Cob Cottage Company, PO Box 123, Cottage Grove, OR 97424; 521-942-3021). Q: Hi I'm 17 and hoping to build a cob house next year in Washington state.(West side) I have a ton of questions and have to get a lot of information to convince my mom to let me build it (it will be her house). My dad and I will be working on it full time. We hope to start in April.the big thing I'm worried about is the time and not many people to help. We would need a place to live in before the winter. So do you think it would be impossible to build a comfortable sized space between April and October? Comfortable is like 40 feet by 30 for starters. A: It sounds like you have zeroed in on what may be the biggest obstacle: not having enough labor to get the building finished in time. This is a problem that has beset many first-time cob builders (and many owner-builders regardless of technique.) Here are a few suggestions: Q: I am building a small cob house in washington state and the roof and foundation are done we just started cobbing this fall and we have only got about a foot or so of cob on the foundation and it is cracking a lot ..small and larger cracks...I'm thinking it could be too much water or clay..in the mix...we do it all by foot and tarp...and also I'm wondering if this time of year is ok,we have had some heavy frosts and the walls are dying really slow. A: There are two reasons why you might be getting a lot of cracks. The most likely is that you have too high a clay content in your mix. Try reducing the clay content significantly, and/or increasing sand and/or straw. It's always advisable to start by testing a number of different mix proportions before you begin building your walls. Form each mix into large bricks, carefully labeled, and let each dry completely. Then check for cracking, hardness, and tensile strength before deciding your final mix. The other possibility is that the freezing temperatures is contributing to the cracking. This is especially likely if the cracks are mostly on the surface, and if they appear immediately after a heavy freeze. It's best not to expose wet cob to subfreezing temperatures, or your walls could be severely weakened. In general, it's preferable to build in warmer, dryer weather, so that the walls will dry out more quickly (this allows you to build much more quickly) and to avoid frost damage. Q: I was wanting to know a basic cob recipe with clay, sand, straw, and water. If you have a lot of clay, is a lot of sand necessary? I would like to mix it in the 5 gallon bucket size to start a small project. A: Glad to hear you want to try building with cob on your new property. Although there is no standard recipe for cob, you are exactly correct in principal. The more clay you have, the more sand (and/or straw) you need to add to prevent your earthen wall from cracking as it dries. I would recommend mixing in a tarp over mixing in a bucket. It is much easier, even if you are only making a small batch. The Cob building is not terribly technical, but if you have never done it before I would strongly recommend one of two things. Either read a book or take a workshop. The most complete guide to cob construction is "The Hand-Sculpted House" by Evans, Smith, and Smiley, published by Chelsea Green in 2002. There is no standard recipe for cob, but from the book you will learn how to test your soil to determine the best range of mix ratios, and then how to test your mix to make sure it will hold up well. The book will also tell you how to go about protecting your earthen wall from water and weather. One good place to go for a workshops is the Cob Cottage Company at www.cobcottage.com or 541-942-3021. Depending on where you live, there may also be more local resources. Q: What kind of mixer can I use to mix cob for building. Would a Mortar Mixer work, or should you have a special mixer. How would a Pan Mixer work? Q and A (Kelly): Would it be possible/effective to create a paper-cob, similar to papercrete or paper adobe? You could certainly do this, but how effective it would be is questionable in my opinion. You might experiment a bit with this to see what kind of characteristics it has. What would be the pros and cons of this? The pros would be that you would be recycling paper, but the cons are that it would not strengthen the cob, nor would it really make it much more insulating. Also, the paper could, under some circumstances, support mold, which you definitely don't want. Would the mixture still retain the sculptability properties and some of the massive properties? It would probably be quite sculptable, and would have some mass properties, but not as much as real cob. I live in Ontario, Canada; would papercrete or one of these other building techniques be suitable? In Canada, you need a well-insulated shell for housing; so for this I would recommend strawbale, cordwood, earthbags (filled with an insulating material), or possibly papercrete or light-weight concrete. All of these materials need to be used carefully to assure that they are not susceptible to rot or other problems. Generally this means protecting them from the elements with substantial eaves and a good foundation. Q: I am thinking of building a small guest house/shelter from cob, our property has sand, silt, gravel, clay, peat moss and sphagnum easily gathered, but straw bales in our area have become quite expensive. I am thinking that because the sphagnum moss is a very stringy and rot resistant plant fiber it might be a suitable substitute for straw. If I were to use it, should I cut the fibers to a shorter length? A: It is possible to substitute other fibers for straw, and it makes great sense to look around for what you have locally. I don't have any experience with sphagnum moss. The question would be whether it is strong enough to provide much tensile reinforcement to the cob. You can test the quality of the fiber just by holding onto both ends and pulling. With good quality straw, it should be very difficult to break just by pulling with your hands. Another good test is to bend it a few times and see how long it takes to break. Different kinds of straw have different amounts of strength, but ideally they should not snap in the first couple of bends. So play around with your moss, do some tests. If you think it might work, make a small batch of cob using it for the fiber, form the cob into bricks and try to break them. Well-reinforced cob bricks should be impossible to break in your bare hands. The other big issue is the amount of fiber you will need. When estimating the amount of straw bales to buy for a cob project, I figure out the volume of the cob (length x width x height) and then divide by ten. That gives me an approximate volume of baled straw that I will need. But keep in mind that baled straw is tightly compressed. If you cut open a bale, the straw will expand to many times the original volume. I imagine your moss is very loose, and therefore takes up a lot of volume per amount of fiber weight and strength. Is it really feasible to gather that much moss? Q: I've found a place with good ceramic quality clay. How do I mix this as I'm only one person, and can I use any aggregate to build with? I'd like to build a house, and add on to it over the years, but I can't imagine a good way to mix up that much cob as one person. A: I would strongly recommend that you read "The Hand-Sculpted House" or one of the other how-to guide books available on cob building. There is far more to the cob mixing process than can be explained in a short email. Besides mixing by foot, there are several ways to mechanize the cob mixing process, including the use of a roto-tiller, a mortar mixer, and a tractor. For very large scale applications, tractor mixing is often the best way to go. For up-to-date articles on mechanical mixing, a good source is the "CobWeb" newsletter published by the Cob Cottage Company. You can call them at 541-942-2005 to order back issues. It's ideal to have your walls breathable on both sides. Depending on your specific climate situation, it is sometimes OK to have only one side of the wall breathe. I didn't quite understand the rest of your question about "if used only as interior walls." . Q: I'm an Australian living in a remote area of the Northern Territory - a desert region. Would I be able to use spinifex instead of straw to make cob? A: I don't know what spinifex is. However, any strong flexible fiber can be used in making cob. If you're uncertain (and I won't be able to help you much, never having heard of, let alone seen, this material), try making some cob batches with it and see how it performs. Then let us know! Q: I am about to teach a class on cities of the future, and would like each of my students to be able to build a small cob model. For such a small scale, could I use one part clay (from a store selling pottery supplies) to one part sand, water and some straw to make an appropriate mix? Would you suggest digging for the clay instead...any other suggestions for my young friends? A:
You will have to do some experimentation to find appropriate ratios for your cob mix. Different clays have different properties, even bought clay. If you are using a fairly pure purchased clay, you will most likely need to add more parts sand than clay - typically 2 or 3 parts sand to 1 of clay, by volume. You can also of course use found clay, which may be a more interesting experience for your students. You can find complete directions on how to go about prospecting for and harvesting clay soil in "The Hand-Sculpted House" (Evans, Smith, Smiley, 2002) which would also be a great reference for the students. Q: I receive questions daily about mixing cob, mostly concerning which clay they should buy. We are in Portland, Oregon and there is a great resurgence of cob building going on. I understand that traditionally you would use the clay from where you were going to build, but this being a city, its not an option. We sell all different kinds of clay ( raw materials, i.e. ball clay, fire clay, bentonite, etc) for ceramic art application and I am curious as to what would work best. I have been recommending fire clay (lincoln 60). A:
Almost any sort of clay can be used for making cob. Non-expansive clays are best, including those generally sold as "fire clay" and "mortar clay." Lincoln clays are ideal - very sticky but not highly expansive. Highly expansive clays such as bentonite should be avoided, as they tend to make cob that cracks a lot as it dries. A: I've never heard of using shredded carpet or any other kind of plastic fiber to reinforce cob or adobe. There's a good chance it would work. The main question from a structural perspective would be about the kind of bond you would get between the plastic fiber and clay. It is possible that the fibers would be too slick for the clay to grab onto well, or that the fibers would be too short or too weak to provide much reinforcement. You could do some simple experiments. Find out what a good cob or adobe mix would be using your local clay soil and straw. Make some test bricks with that mix, and some others substituting half or all of the straw with carpet fiber. Then test tensile strength of the different blocks by supporting both ends of the block and weighting the middle of the block until it breaks. Relatively long, thin blocks are best for this sort of test. The other question that occurs to me is, what's the point? Straw is readily available in most places and quite inexpensive in the quantities you need for a cob or adobe building. It's great to know that your walls are made entirely out of natural materials which will return harmlessly to the ground once the building is no longer needed. I shudder at the thought of a cob wall disintegrating and littering the landscape with little bits of plastic. And shredding carpet sounds to me like a fairly energy-intensive, high-tech operation. What are you trying to accomplish in this scenario? C: The thing that people don't like so much and keeps cob from growing faster is the mixing techniques. When I learned how to do it, I was turned off, because my hands,and feet were bleeding ever time I mixed. It was from the sharp gravel and straw. My back, neck, arms, and body was hurting. I had to figure out a way to solve these issues. Living in LA I do junk yard tours where I find everything to build rocket stoves to mixing tarps that are way more sustainable. Using permaculture design I was able to clean up the cob mixing problems once and for all. How much time do we spend cleaning our hands and feet? We need more people to come together on the weekend and play with the earth, not to work and be worn out. Here's a sample of how I see we all could take it to the next level.
This tarp keeps the legs active without having to step in the sharp, cold , wet, and slippery mix. It's advantage is that it gives you more foot edge and forces you to keep mixing while dancing. The key is the plastic handles that make it easy to control. This other handle is made of recycled polycarbonate connected to a bill board tarp and makes mixing cleaner, safer, less muscle pain, more productive, lasts 10x longer...able to drag tarp, speed up production. Q: Is there a specific type of clay used in cob homes? Someone told me that the red clay here in Georgia wasn't real clay or the right type of clay for building a cob home. A: Any kind of clay can be used for making cob, although highly expansive clays can be hard to deal with. What you do need to determine is that the substance in question is actually clay. Lots of people confuse silt with clay. There are some basic tests you can perform which will tell you whether your soil is suitable for building. "The Hand'Sculpted House" describes these tests, as do many other books on earthen building. Q: I am from India and I want to construct a cob home. Is it a must to add straw to cob or can I use just mud? If I use straw, how can I avoid the ant menace as I am afraid the house would be attacked by ants? A:
The addition of straw is necessary for making good cob. Not only does it provide structural reinforcement for the walls, but it also gives the cob mix the proper consistency for building with. It also helps to distribute moisture within the wall so that the cob dries out more rapidly and there is less chance of catastrophic moisture problems. Q: I live in Bali, Indonesia and I'd like to build a house with a combination of Cob and reclaimed wood. As we have no access to straw here, is it possible to substitute rice husks for the straw in cob? The other option would be rice stalks (which are long and dried out post harvest and traditionally burned here i.e. plentiful and free). A: Here in California my preferred source of fiber for cob is rice straw, which is the bundled stalks of rice. I don't know whether the rice in Bali is the same as here, but I would give that a try. Rice hulls are too short to give you the fiber strength you are looking for, but they can be used in finish plasters and floors. Q: I have a question about possibly building a type of cob but hybridizing it a little. My idea is using both the basic cob recipe but combining it with a papercrete recipe. Use your clay/earth, sand, and straw but reduce some of the straw with shredded paper and cardboard. I'm considering mainly cardboard. Reason being is my place of employment throws away tons of cardboard boxes a day. How feasible do you think this is? The paper would probably cause a little shrinkage once it dries, but I'm wondering if it would compromise the integrity of the cob any? A: I have never heard of anyone doing exactly what you're suggesting. I have no doubt that it would work, but it may be more complicated than necessary. Certainly people have experimented with "fidobe," a combination of paper pulp, clay and sand, and have used this mixture like cob. I suppose adding straw to this mixture as well would get you some additional structural strength, but at the same time it would complicate the mixing process. By all means experiment with this mixture and report on how it performs. Q: Can gumbo clay (gray clay) be used for making cob? I live in central FL and that is the only clay I have around. I have been making test bricks out of it and they all get soggy when wet and than crack when they dry. A: I don't know the specifics of "gumbo clay" as found in Florida, but just about any clay can be used for making cob if you find the right ratio or clay, sand, and straw. Sounds from your description like you need to add more sand and possibly straw as well to your test mixes. Follow the testing procedure as outlined in Chapter 11 of "The Hand-Sculpted House." Q: I have been building test bricks with cob and have finally found the right ratio of sand to the clay found in my area to make a solid brick without cracking. However, I am concerned about one thing. I have noticed that when it gets wet, (after it has dried) and if I dabble and rub it with my fingers, the surface will start to dissolve. Is this normal with cob? And if it is normal, will a good roof, foundation, and hydrated lime plaster fix that? A: Any kind of unstabilized earthen mixture (cob, adobe, etc.) will turn to mud if it absorbs enough water. Some mixes are soft and crumbly even when dry, which is generally not good. Others can sit in water for a while without getting soft. It depends primarily on the qualities of the clay in your soil and the amount of silt in your soil. Without testing your soil and touching your block, it's hard for me to say if it's within the normal range or not. If it is really crumbly, you are probably low on clay. Adding straw to your mix will increase the proportion of clay you can use without significant cracking. Q: We are going to build a light clay straw cabin. We have on our land much hay and would like to utilize that instead of straw. Just wondering what you think of that. The only thing that we can come up with is that it would sprout. A: When grass is harvested to make hay for animal feed, it is cut when the plant is still green and the seed heads are mostly not mature. This maximizes the nitrogen in the plant, which makes it most nutritious for the animals that eat it. Unfortunately, this same nutrition is highly attractive to bacteria, fungus and other micro-organisms which will break down the hay when they get a chance, causing it to rot when it gets wet. In contrast, straw is the stalks of mature grains, left behind as a by-product when the grain is harvested. Straw is made mainly of cellulose and lignin, the same compounds that make up wood, with very little available nitrogen, because the nitrogen has almost all been deposited in the mature seeds. This makes straw much more rot-resistant than hay. I wonder what you mean when you say that you have hay on your land. If you mean that you have a hay field that was cut and baled for animal feed, then I would strongly caution you against using it as a building material. If you mean that you have a lot of wild grass growing on your land, and if you wait until it is fully dry and then harvest it, that could be an OK material to use. You may have some problems if there are a lot of seeds in the grass, either with sprouting (which is not necessarily a big problem) or with animals such as mice trying to eat your walls (also not usually a problem if you do a decent plaster job). On the other hand, if you leave the grass standing in the field long enough, most of the seed heads will probably fall off by themselves.
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Michael
G. Smith
has a background in environmental engineering, ecology, and sustainable
resource management. In 1993, along with Ianto Evans and Linda Smiley,
he started the Cob Cottage Company, a research and teaching group focused
on reviving and improving traditional forms of earthen construction. He
is the author of 
