Plastering and Waterproofing Cob

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 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. 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 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?

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". 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.

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 than 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 slab. Earthen floors are generally considered more compatible with cob walls than concrete.

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: 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 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.

Q: I am undertaking a project to convert my garage into a living space. Currently, it has no insulation or interior walls: just studs. I live in Vermont and understand that cob is NOT insulating unless it is very thick. I'm wondering if it is possible to use some kind of insulating material (be it cotton batting or conventional insulation) to get the proper amount of insulation, and then cob over the insulation. Basically, I would be using cob instead of sheetrock to form interior walls over the insulation. I don't want to use straw bales because they are so thick, and the space is small, although I know straw is a great insulator.

A: The word "cob" is generally used to describe a structural wall system in which the fairly thick walls (usually a foot or more) hold up the weight of the roof. What you're looking for sounds to me more like an earthen plaster. If I understand you right, you want a fairly thin layer of earthen material to help contain some kind of loose insulation, such as cotton or wool. This is totally doable. What you will need is some sort of lath that you can put in place first to contain the insulation and to serve as a substrate for the plaster. In similar situations I tend to use reed mats, sold at the hardware store as landscape privacy fences. These are relatively inexpensive and quick and easy to install, either by screwing through the wire that holds the mats together into the studs, or by screwing narrow strips of wood or plywood over the mats to hold them to the studs. The reeds should run horizontally for better keying with the plaster, and should be installed as tightly as possible. Another solution is to make your own lath out of thin pieces of milled lumber (say .5" thick by 1.5" wide) like an old lath-and-plaster wall, or even to use round sticks, running horizontally and spaced about an inch apart. Regardless of your lathing system, you can then apply a fairly thick layer of base plaster made out of clay soil and chopped straw, and then a thinner layer of finish plaster made out of finer materials.

Yes, I suppose earthen plaster is what I had in mind. Just something other than sheetrock. Do you think chicken wire would work in place of lath? It might be problematic because I can never seem to get it taut enough. But I had a vision in my head of chicken wire holding the insulation and providing texture for the plaster. How much room is there for creative innovation with things like this? There's this creative being in me who wants to go wild and build sculptures into her walls with the plaster.

By all means let your inner sculptor out! There's no more conducive medium for exploring sculpture than a good clay plaster. However, I don't think chicken wire will work. You need something pretty stiff and strong to support the weight of the plaster. You should be able to push against it fairly hard and not have it move much. Otherwise you will get a lot of movement and cracking in your plaster as it dries.

Is the clay plaster something I buy? or do I gather the materials and mix it myself? Sounds kind like of a stupid question, but my general surroundings are mostly gravel, so it's not like I can dig it out of my yard.

You could buy clay plaster in a bag, although most of what's available is finish plaster, which generally goes on quite thin and would not allow you to do a whole lot of sculptural relief. You'd be much better off making your own base plaster out of a mixture of clay soil and chopped straw. Even if you don't have enough clay in your own soil, you probably won't have to look far to find enough. There are several good books that will lead you through the process of how and where to find clay soil. My book, "The Hand-Sculpted House" has a chapter on this. Other recommended resources for you would be "The Natural Plaster Book" by Guelberth and Chiras or "Using Natural Finishes" by Weisman and Bryce.

Q: I have built two Cob items - one oven (turned out great) and a bench. I was advised to apply boiled linseed/turp oil on it the 1st winter as I ran out of time/money to build a roof over it: I now cannot get the final lime/clay coat to stick on top to finish it. I scored the surface and lightly sanded but the final coat just ‘rolls’ off. HELP! She (the bench’s name is Gloria) is so dark and unattractive right now.

A: Yes, as you discovered, the linseed oil prevents further coats of plaster from adhering. This is in part due to the fact that the oil prevents the cob or base plaster from absorbing moisture, which is necessary for getting another coat of plaster to stick. Your best bet is to completely remove the oil-saturated part of the bench. If you only applied a single coat of oil, it should not have penetrated deeper than about 1/4 inch. However, depending on the hardness of your cob it could be a big task to chip off the surface layer. Good luck!

Q: We have built a natural timber frame with larson trusses and straw/clay walls. We are ready to apply a base coat. We were hoping not to use any exterior window trim. The windows are sealed all around. We are hoping to butt the 1 inch base coat right up against the aluminum frame. (There will be wire lath or burlap over the window buck up to the frame.) Will the clay plaster adhere to the aluminum?

A: Sure you can get the plaster to stick. Do you know the burlap trick? This is essential knowledge for plastering light-straw clay walls. You use it to cover any slick material that would not otherwise accept clay plaster easily: framing lumber, exposed concrete, flashing, etc.

Take some loose-weave burlap (either coffee sacks or landscaping burlap) and cut a strip as wide as the material you want to cover, plus an inch or two extra so that it will cover the seam between materials and come over onto the straw-clay. Then make a paste mixture of 50% thick clay slip (thick milkshake or pudding consistency) and 50% flour paste (see recipe below). Rub some of this pate onto the straw-clay that the burlap will cover (you don't need to apply the paste to the other surfaces, but it won't hurt.) Soak the burlap in the paste, squeeze out any extra liquid and smooth the burlap onto the surface, rubbing hard with the palm of your hand to remove air pockets. Then let the burlap dry completely before plastering over it. This is an amazing technique; I have yet to find anything it won't stick to, and it provides a rough, fibrous, clayish surface that clay plasters bond to easily.

To make flour paste, mix 1 part white flour (or "white whole wheat" or white rice flour) into 2 parts cold water, beating with a wire whisk to eliminate lumps. Then pour that mixture into 4 parts water which is already boiling on the stove. Bring back to a boil, stirring with wire whisk all the time. If it's lumpy, you can strain through a wire colander or window screen.

One thing to keep in mind is that if you put a thick layer of earthen plaster right up to your window, you will end up with a shrinkage crack where the plaster pulls away as it dries. One way to reduce this crack is to apply the plaster in several thinner layers, waiting for each to dry before applying the next. Even so, you will probably end up with a fine crack around the window where water running down the wall could get in behind the plaster. Use flashing appropriately to prepare for this, and I'd also recommend caulking the seam between plaster and window with silicone or other flexible caulk.

Q: I have built a cob bench with a good overhanging roof structure. The bench does not get wet even in driving rain. However, the bottom of it does absorb a fair amount of moisture from the concrete & urbanite foundation when we have heavy rains. I have a beautiful earthen plaster finish on it and am wondering what my finish options are. It is at a children's museum so I want to put something on it to protect it from high use and kids spilling drinks. I think linseed oil will mildew due to the water absorption and I am likely to need to repair it for scratches or damage from kids. I tested milk paint with an "outdoor additive" which I am guessing is additional lime and have considered a lime wash but these are opaque and I am sad to see the straw flecks and beautiful finish disappear. I read that a simple mixture of casein and water on an earthen plaster finish will provide a translucent glaze allowing the plaster to shine through but I have concern using it in an outdoor environment. The bench is in a sunny location in the San Francisco Bay Area so it completely dries out in the summer months. What do you recommend?

A: In similar situations I have used linseed oil. But, as you say, it has a tendency to mildew, especially on any surfaces where the sun doesn't strike it. Lime, casein, and the like will not penetrate deeply into the plaster and harden, so they won't really protect the surface in the way you want them to. The only other thing I would consider trying is waterglass. This is a solution of potassium silicate, much like dissolved glass. It is available from pharmacies or online. Dilute the waterglass 1:5 with water and paint it on; it should absorb quickly into the earthen plaster and you can do many coats. Be careful as it will etch glass or metal. I have much less experience with waterglass than linseed oil, but it will reduce the water absorption of the plaster and it should not discolor or mold because it has no organic component.

Q: We want to use cob to face a concrete block chimney in the interior of our house. The height is about 16 feet. The concrete block courses have wire cloth extending from the seams to help anchor the cob. My concern is the cob will be quite thin - 5" in most places but only 1" in some. Are there particular problems we should watch out for? change in the clay/sand/straw ratio? Or anything else you think we should know about this project.

A: There are a few issues that concern me about your project. One is how to get a good bond between the cob and the block chimney. Especially where the cob is quite thin there is danger of it pulling away and separating from the blocks, either during the initial drying or later on. Also with a lot of variation in the thickness of the cob you can end up with big cracks between the fast-drying thin areas and the slow-drying thick areas. To address the potential for cracking and separating, I would create some sort of armature for the cob that is fastened to the blocks. The first thing that occurs to me is to use metal fencing - stiff deer fencing rather than floppy chicken wire - bent into the shape you want the cob to take and fastened to the blocks here and there with concrete screws. Try to keep the wire about an inch below the surface of the cob. You will want an excellent cob mix for this project with a good proportion of both sand and straw to reduce shrinkage and cracking. You might consider using short straw - chopped if necessary - because cob with long straw will be harder to push through the metal fencing and also harder to sculpt into a pleasing shape. Most likely you will want to apply a coat or 2 of clay-based plaster after the cob is completely dry, in order to get a nice shape, a smooth surface, and to cover up any cracks that show up in the cob as it dries.

Q: I we live in a grade 2 listed cob wall thatch roof cottage. We are having quite bad mold issues and are having trouble removing from the cob walls. Do you have any advice on the best way to remove the mold and treat? We have central heating which cones on twice a day and stays off at night, so the house cools down then warms up again. Would it be best to have the heating set to a consistent temp and just turn the thermostat up when a bit colder?

A: My guess is that you have moisture problems in the house that need to be addressed. This could be due to a lack of drainage, earth built up against the outside of the foundation, leaks in the roof, or some other way that moisture is finding its way into your home. Ideally you would find the source of that problem and address it before taking any other steps. British cob homes were traditionally rendered with lime plaster inside and out. Lime plaster is not only vapor-permeable (allowing the escape of humidity that might otherwise build up inside the wall) but also very alkaline, creating an inhospitable environment for mold to grow. It may be that someone at some point either replaced the lime render with cement-based stucco or painted over it with oil- or latex-based paint. If any of that has happened, the coatings need to be stripped off and the original lime finishes restored.

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