Plastering or Waterproofing Cob
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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 noticed that cob homes in England that are 200 and 500 years old do have stucco outside to protect the cob. Maybe that is a more recent solution to other problems though?

A: I think there's a misperception about the stucco on old cob buildings in Europe. The traditional protective plaster on the exterior and usually the interior as well was a mixture of lime putty and sand - no Portland cement. Lime plasters are waterproof in the sense that they do not get soft when soaked, but they are also very breathable. They are traditionally maintained by periodic applications of limewash. Some cob buildings have recently been re-plastered with cement stucco, but this very often causes serious moisture issues in the earthen wall, and has in several cases contributed to collapse of ancient buildings.

Q: To protect an exposed cob wall from heavy tropical rain, is growing a thick ivy on it a possible solution? I thought the roots into the wall may bind the wall and the leaves may shed the water. Is there any information on, experience with and opinion about the subject?

A: An interesting suggestion, but probably not a good idea. Climbing vines are known to cause damage over the long term to masonry buildings such as brick and stone. Some vines send tendrils into the wall which can slowly crack them apart. The best solution to protecting cob buildings from heavy rain is a good roof with a wide overhang, combined with a high foundation to protect the bottom of the wall.

Q: We have access to good lime-sand renders for sealing..is this sufficient to avoid moldy walls?

A: A lime-sand plastered cob wall should be completely mold resistant as long as there isn't a lot of water getting into the wall from either above or below.

Q: I would like to know if I put light clay straw in my 2x4 walls which have pine board siding outside and tar paper on top if I could then add a stucco with lime mix on out side wall? I also plan to put on an earth base plaster on inside walls but the overhang out side is not out far enough for the same earth plaster would it cause a problem with drawing in water through wood and tar/ felt paper?

A: The problem is not so much with drawing water in through the exterior stucco, paper, and siding. The problem is that any moisture that gets into the wall, either through cracks in the stucco or from interior vapor, will not be able to evaporate to the outside. If you live in a very dry climate, that might be OK. If you live in a wet place, I would not recommend it. It's almost always preferable for straw and clay walls to be able to "breathe" or release water vapor, to both sides, because the relative levels of humidity and temperature inside and outside can reverse themselves seasonally.  In your scenario there is a good chance of mold growth due to trapped moisture in the wall. A better option would be to extend you roof eaves and use lime plaster on the exterior.

Q: I have a cob oven. It has a roof over it to protect it but as it is England and the position that it is in it still suffers from erosion by rain. Is there any way to help make the oven waterproof? I have heard that linseed oil or lime plaster may work and which would you suggest works best?

A: Linseed oil and lime plaster are both sometimes used to seal cob against weather. However, I wouldn't recommend either of them for an oven. When you bake something like bread in an oven, it releases an enormous amount of water vapor as it cooks, much of which is absorbed into the oven wall. That vapor needs to be able to travel through the wall of the oven and evaporate to the outside, rather than being trapped in the oven wall where it can cause damage. Linseed oil especially will slow down this process and could lead to moisture problems. Lime is more vapor permeable. However, an oven also needs to be able to expand and contract as it heats and cools. Lime plaster will expand and contract at a different rate than the earth oven, and will tend to crack off. What I would recommend is extending the roof to protect the oven better and/or covering it with a very waterproof tarp during rainstorms.

Q: Why is it that lime is not mixed into the actual cob during building stages to enhance the weather repellent properties, thereby saving a step in plastering?

A: It would certainly be possible to add lime to a cob mix. I can think of several reasons why this is not typically done. First, mixing in lime throughout would increase the cost and ecological footprint of the cob wall far more than using a fairly thin layer of lime plaster on the surface. Second, because lime sets chemically (like cement, although not as quickly) it would reduce the amount of time you have to work with your cob after it's dry, and make re-using leftovers and trimmings more complicated. Third, because lime is caustic you would need to suit up with boots, long sleeves and rubber gloves to work with it, whereas one of the nice things about cob is that it is so safe and easy to handle. Finally, it would be really difficult to build in such a way that plastering would be unnecessary anyway, unless you were extremely, slow, careful, and fastidious. I have rarely seen a cob wall come out smooth and even enough in the building that it didn't need plastering. Because only the surfaces of the cob wall are exposed to weather, it makes much more sense to concentrate high-embodied energy materials like lime there, where they will be doing some good.

Q: I have build a small cob house in outskirts of Mumbai, India. This place gets very heavy rainfall. I have built the house on a stone foundation with overhanging eaves. If I apply a cow manure/clay plaster and then coat it with linseed oil, will the cob walls be waterproof and breathable?

A: Linseed oil will substantially reduce the breathability of your walls, and I have seen it lead to mold growth on the oiled surfaces. The best idea would be to stick with clay and cow manure and replace it as necessary, or else to go with a lime/sand plaster. Lime will be much more durable because it does not get soft when it gets wet like clay does, yet it still breathes well. However, lime is more expensive, much higher embodied energy, and more difficult to use than clay. Take your pick.

You are right; after checking in stores here I found out lime is more expensive than clay. In that case can I do the first coat with clay/manure and the second with lime/sand fallowed by whitewash?

Probably not a good idea. Lime/sand and clay/manure are very different materials that don´t bond very well with each other. If you're going to start with clay, you should ideally finish with clay, the same goes for lime.

Thanks for the advise. I was out on a trip and when I reach my site today after a gap of 2 weeks something unexpected had happened. My care taker has already applied cow manure + ash without asking me. Upon confronting he said "this is what we have been applying for ages." Can I still apply clay/manure? Will the manure, which is now my scratch coat, hold on to the second coat-clay/manure?

What is your concern about the manure and ash mixture? If your caretaker says that is what they have been using for centuries, it seems likely that it´s an excellent solution to your problem. When looking for effective, practical, tried-and-true building techniques, the best resource is always local traditional craftspeople.

Q: Is lime plaster or earthen plaster better for the interior and exterior of a cob home? I also plan to mix various clay earth pigments into the plaster to achieve a variety of earthy colors. What do you think of this idea?

A: Both clay and lime plasters work well on cob walls. Because clay is cheaper, easier to apply, and has much lower embodied energy, I prefer to use it except in very wet conditions. Places where I would use lime include the inside of a bathroom or the exterior of a house in a climate with wind-driven rain. Each type of plaster has its special tricks and techniques. There are good books available to describe how to achieve different colors, textures and so on. I would especially recommend "Using Natural Finishes" by Adam Weisman and Katy Bryce in England.

Clay is not necessarily a good additive to lime plasters. You would be safer to use mineral pigments such as iron oxides and yellow ochres. The usual technique is to apply a pigmented lime wash over the lime plaster rather than to tint the plaster itself. In the case of clay plasters, you can either tint the plaster itself (especially if you start with a very light colored clay) or apply a colored clay paint (known as an "alis") over the top. This is a fairly complicated matter. Feel free to experiment, but don't expect to get good results immediately. Consulting a good book or an experienced plasterer will speed up your learning curve.

Q: I have just started decorating my walls with Cob. I am putting on a inch of cob on the existing wall. The problem is that the layer of cob starts to crack. I am working with cob with about 20% clay content and straw I add more or less as i feel is fit. What is my problem?

A: I would refer to what you are doing as an earthen plaster, rather than cob. The word cob is more properly reserved for a monolithic mud wall. If your plaster is cracking, it is because you have too high a ratio of clay in the mix. For such a thin application, a lot of fiber is extremely important. I would usually add huge amounts of chopped straw or equivalent fine fiber to such a mix - up to 50% by volume. Adding sand will also of course reduce cracking, but at the expense of also reducing stickiness and workability. Another concern is good adhesion between the wall and the earthen plaster. If your earthen mix is not sticking well to the wall, that can cause cracking.

Q: I want to build a one-and-a-half storey cob house. The architectural restrictions in the area I live in in Cape Town call for clipped eaves; so no overhang. But the area, in winter, has quite a lot of wind-driven rain. This combination got me worried about the walls lasting. But then I saw many pictures of old cob buildings which are double-storey and have double pitched roofs; so 2 of the 4 exterior walls are completely exposed, and the ground floor is exposed all the way around; and they have lasted a couple of centuries! So is good lime rendering alone sufficient to protect walls? Based on the anecdotal evidence are overhangs reassuring to have, but not essential to have?

A: I would conclude the same as you: that cob buildings with lime plaster resist damage from blowing rain, even when the eves are fairly short. However, I am sure that the long-term maintenance would be higher on a building with short eves. In addition, working with lime can be tricky. Those traditional Devon cob homes were plastered by people who knew what they were doing and had access to excellent raw materials. Is there a tradition of limework in South Africa that you can draw on, and access to good quality lime putty? If not, I wouldn't expect to immediately have excellent results with your lime plaster.

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