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:
1) Start smaller. Design your dream home, but make the design modular. Plan on getting one or two rooms complete so you can move in and use them, then expand from there.
2) Try to get a bigger crew. Ask friends and family to commit to helping you out for a month or more. Advertise in cobber's networks ("the CobWeb" newsletter and Cob Cottage Company's Helping Hands Directory) for cobbers looking for more experience.
3) Consider mechanical mixing. One of the biggest labor sinks in a cob building is mixing the cob. Consider using a tractor or back hoe to mix huge batches of cob at one time, so that your available labor can be expended in the actual building process.
4) Do as much preparation this year as you can. Finalize your design, prepare your site, and collect all the materials you will need. Read "The Hand-Sculpted House" thoroughly so you will be as prepared as you can be. Good luck!

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
compression you get by stomping on the mix improves its quality by a lot, and you can use your weight and the stronger muscles of your legs much more effectively than your arms and hands. Even a small project ends up using a lot more cob than you would initially imagine.

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?

A: A mortar mixer can be used to make cob. It tends to get bogged down trying to make the stiff, straw-rich mixture that is preferred for cob building, so people usually do one of the following: either make the mix wet, then dump it out and let it dry for a while before using it; or else make a stiffer mix without the straw, then add the straw in by foot. I don't know what a Pan Mixer is, but there are certainly other kinds of mixers that can work. The best way to learn how to mix cob is using your feet, and working with an experienced cobber. Then once you know what a good mix feels like, you can experiment with whatever machine you have access to and see whether you can make it work for you.

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.
You just pile it together, add water, and stomp around on it? Furthermore, if used only as interior walls don't you have to insure that the outside is breathable, or can you have your weather proofing, insulation, then cob wall with only one side of it breathing...I'm working off a book by Kiko Denzer, "Build your own earth oven", and am fascinated that it is feasible to build in high humidity with cob. We'll see how it goes.

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."

Plastering or Waterproofing Cob
Q: I am currently researching plaster and rendering materials for cob walls specific to heavy use, public spaces. The surfaces have to be strong enough for kids to climb on and durable enough to endure the spilling of hot beverages, food, etc...I have been hearing about Sodium Silicate, commonly known as waterglass, to seal for water, etc. Have you had any experience with waterglass? I also have questions about Casein for durability, and a good source of recipes.

A: Finding durable finishes for cob has been an ongoing quest. Kiko Denzer has had good results with waterglass to protect cob and earthen plasters from the weather. I have never tried it myself, and don't know what it does in terms of enhancing hardness. See Kiko's article in issue 14 of the CobWeb (available from Cob Cottage Co., 541-942-2005). I have had pretty good results with lime-sand plasters, although they can take quite a while to get fully hard. Another thing I've seen work great is a clay plaster with several coats of linseed oil applied afterwards. It is extremely hard and water-resistant, like an adobe floor. This is probably my best recommendation for your situation. Casein does increase the durability of soft earthen plasters and clay paints. I don't know how it would hold up to a lot of wear and tear and moisture. Good luck!

Q: I want to let you know that there is an environmentally safe soil stabilizer on the market named Earthbind 100 that may be a good alternative to asphalt emulsions used to stabilize and waterproof cob. As you know, asphalt contains many of the carcinogenic polyaromatic hydrocarbons. Earthbind does not. It is a mixture of paraffin resins and tree lignin. Earthbind has been thoroughly tested and does not contain any organic compounds that are considered toxic. In addition, Earthbind was been extensively tested regarding the toxicity on fish, mammals, and plants and it considered very non-toxic. I believe that it will stabilize cob better than an asphalt can because of the resins. I would like to send you free sample. For complete production information please visit our web site at www.enviroad.com.

A: Thank you for your information and the kind offer. I would certainly like to know more about your product and to try it out. I would also like to let you know that I have never used asphalt emulsion to stabilize cob, nor has anyone I know. It is not recommended practice for the reasons you mention and many others. Cob is always built without any stabilizers mixed in. A weather-resistant plaster made of lime, or earth stabilized with linseed oil, is then applied where necessary. Perhaps your product would work in place of linseed oil as a water-resistant exterior finish? Could you please send me a sample and a complete list of ingredients?

Q: I am planning on building a chapel with strawbale turned on it's side with 4 inches of cob on either side. Is there any reason this wouldn't work? Also, what kind of insulation would you recommend for the roof?

A: What you are proposing sounds a lot like a technique that is being promoted by the Cob Cottage Company under the name of "balecob". The current edition of their newsletter, "The CobWeb" contains 2 articles on it. You can get a copy by calling 541-942-3021. I know of no reason why your idea wouldn't work, as long as drying conditions are very good (dry, hot, windy weather) so that the bales don't stay wet for long. I wonder, however, why not just stack up your bales and put a very thick earthen plaster on both sides. That sounds a lot simpler and quicker to me.

As for insulation in the roof, there are many options. One of my favorites is the recycled cotton batting called Ultra-Touch, which has been available in California only for the last couple of years. For other options, check out chapter on roofs in "The Hand-Sculpted House" or the section on natural insulation options in "The Art of Natural Building."

Q: I am about to start working on an old farm house in France that has some of the walls made with Cob. There are a couple of cracks that need to be repaired. I am demolishing an old tractor shed also made with Cob. Can I utilize some of this to repair the walls on the house. How would I go about doing this? When the repairs are done, how should the wall be finished. both internally and externally? Internally, I was thinking of battening, insulating between the battens and then plaster boarding to allow painting or tiling. Is this OK? I have heard that laying new concrete floors with a damproof membrane can damage cob walls. is this correct and if so what is the alternative?

A: Here in the US we have only been building with cob for about 15 years, so we have yet to develop much expertise in cob repairs. However, that expertise does exist in Britain, and probably in France as well. There's a new book out of England called "Building with Cob" by Adam Weisman and Katy Bryce, which has a chapter on restoration that will answer all of your questions about repair techniques. Cob walls are generally rendered or plastered with an earthen or lime mixture. It is not recommended to use a non-vapor-permeable surface such as tile or commercial paints. These sometimes lead to major structural problems as a result of moisture being trapped inside the wall. If you use battens with insulation and then plaster board, make sure the space between the cob and the surface is very well vented to prevent moisture accumulation. The same sort of problem can result from the use of damproof membranes or vapor barriers in the floor,
although it is generally less devastating that in the walls. Make sure that the drainage underneath the floor is excellent, by using drain tile and several inches of drain rock underneath the slap. Earthen floors are generally considered more compatible with cob walls than concrete.

Q: The house in question has stone along the base of the walls to about 2 feet high. As this separates the cob from the soil, will it be Ok to install a damp proof membrane under a concrete floor? Would you recommend injecting a damp proof course between the mortar in the stone work?

A: This would probably not cause any problems for the cob walls. However, you could still end up with a cool, damp building unless you provide additional drainage. Depending on the kind of stone used (whether or not it is porous and can thereby wick water up from the ground) you might seal the top of the stone stemwall before putting cob on top.

Q: I am building a cob bench in wet Vancouver, BC and am looking for an alternative to adding a rain shelter. Have you tried a hydrated lime plaster with linseed oil finish? Would that still breathe? The bench is on a rubble trench (with drain tile) and two layers of drain rock in bags for a stemwall. Decent drainage I think, non? Are you aware of any other waterproof and breathable coatings that would work in this rainforest environment?

A: This is the question of the year. The best luck I have had with unroofed cob benches is to use an earthen plaster and then several coats linseed oil. Oil makes a non-breathable finish but this is less problematic for an outdoor bench than for the wall of a house. The oil will need to be renewed at least yearly - more often if it is in a very sunny location where the UV will break down the oil. In a shady location, you may get mildew growing on the oil during wet whether, which turns the surface black and also breaks down the oil. But with frequent re-oiling it should last a while. How long, I don't know. Depends on a lot of factors such as the quality of your clay, carefulness of detailing with plaster, etc. But probably 10 years - maybe more.

Lime plasters are water resistant in that they don't get soft when wet, but they are not impermeable. I did a really beautiful and time-consuming fresco over lime plaster on an outdoors cob bench. The next winter, water soaked into the plaster on the seat of the bench and froze, severely cracking the plaster. You may be onto something with oil over a lime plaster. I don't know much about the compatibility of those materials. We have one wall here at Emerald Earth where someone put linseed oil over a lime plaster. That reduced the breathability to the extent that we ended up with mold problems both on the inside and outside surfaces of the wall. Again, that will be less of a concern on a bench, where permeability to water vapor is not so critical. But you may well end up with black mildew stains on your plaster.

When it comes down to it, I don't know of any way to plaster a cob bench that is guaranteed to protect it in a rainy climate. You're much safer with a roof. But if you want to experiment with other kinds of finishes, by all means go ahead, and let me know in a few years how it looks.

Q: I have an outbuilding that is presently unfinished and in need of siding over the exposed plywood walls. Can I use cob as an exterior siding? I live in Portland, OR, a cool, rainy climate.

A: What you're referring to is an earthen plaster, rather than cob. Cob is a structural mixture of clay, sand, and straw, used to build walls. The same ingredients can be used to make an earthen plaster. Earthen plasters can be made to stick to virtually any surface (although for non-earthen materials like plywood or concrete considerable surface preparation is sometimes required). However, earthen plasters are not particularly weather resistant. If your walls get much rain running down them, I wouldn't recommend earth as an exterior finish.

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.

The most recent edition of The CobWeb (the newsletter of the Cob Cottage Company) contains an article on "styrocob," a lightweight mixture of clay, styrofoam, and straw that the author found to be especially well-suited to model-making as it weighs less than half as much as regular cob.