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Earthen Plaster for Earthbags
Questions answered by Kelly Hart

Q: I am curious as to which stucco topcoat combination you are finding works best for the earthbag type construction?

A: We usually recommend stabilized plasters, especially for the exterior. This could be lime or concrete based earthen plaster, or a standard cement stucco. For the interior, earthen plasters are often very nice, or a lime plaster will render a brighter color.

Q: Can you tell me if an earth plaster can be used on the exterior of an earthbag building in a wet climate, or would I have to use a stucco cover for the sake of waterproofing the walls?

A: An earthen plaster can be used on the exterior of an earthbag building, but it might be advisable to stabilize the plaster some with Portland cement or emulsified asphalt...or plan on repairing the plaster regularly. Unless the walls are exposed to direct rainfall, it should not be necessary to waterproof the walls.

Q: We have had a friend level a driveway and house pad for us with a backhoe and discovered a very sticky substance in the volcanic soil that seems to be clay-like. You can squish it together, bend it and it holds its form very well. We are planning to make a cob mix with it to put on the inside walls of the earthbag dome home and create an earthen floor. Do you think that would work, this clay-like substance mixed with straw and sand and water?

A: The soil that you describe does indeed sound like clay, which is one component of good cob, adobe or rammed earth. But you really only want about 15-30% clay, with the rest sand for a good mix. You can figure out the relative % of your natural soil by doing a jar test. Fill a clear glass jar about 1/3 full of the soil, then add water to nearly the top and shake the whole mixture vigorously. Then let it stand for a day to settle out. Eventually the solids will settle to the bottom and the water will be fairly clear on top, with perhaps any organic material floating on that. On the very bottom will be the heavier aggregate, then above that the courser sand and then finer sand, and finally on the very top will usually be a clearly marked line of clay. Then you can compare the amount of each layer to determine the composition of your soil.

Q: Is a mud mixture or mud/cement/sand an alternative? What mix?

A: I'm sure there are many formulations that would work. I would suggest some cement for stabilization, although I know of people using lime for the same reason. Pure mud plasters would require more maintenance.

Q: I really want a living roof.  What are our options?

A: I know that with their Honey House, Kaki and Doni had that roof planted with grass for a year or two, and talked about this working for them. It is a bit steeper than I would imagine would work. Perhaps embedding some fish net or something in the soil would help keep it up there on the dome.

Q: What is used to finish walls and ceilings in a domed or vaulted structure?

A: Usually it is recommended that a mix of about one third clay with the rest sand is best for an earthen plaster, because the straight clay has a tendency to expand or shrink, depending on its moisture content and the sand tends to stabilize this. A finish coat of a lime plaster will produce a more durable finish. I have also used papercrete to plaster the interior of an earthbag dome.

Q: I LOVE the idea of earthbag housing but I come from a wet country (Southern New Zealand) and also don't like concrete. How do I make an exterior surface that will stand up to wet conditions without using concrete? Do you need chicken wire when you plaster your house or will the baling twine be enough? Would old fishing net do the job?

A: You could use an earthen plaster stabilized with emulsified asphalt or you could use a lime plaster...or both. For a dome that might receive lots of rain, you might want to first cover the bags with plastic sheeting before the plaster. In this case you would definitely need some chicken wire or possibly fish net to hold the plaster in place. Otherwise, baling twine might suffice.

Q: How do you stop the bottom of your plaster on the outside of your walls from getting soggy if you live in a wet climate? What do you put on the bottom of your walls?

It depends on what sort of plaster you are using how necessary this might be; you can use a cement-based stucco for the first foot or two, if needed.

Q: I paid a visit to CalEarth and took the tour. I live only 20 miles away from the center. When I showed them a photo of the progress on my dome, they were impressed. They asked what I was going to coat the structure with and when I told them I was going to make cob from earth, cement, straw and water, they said if I used straw I didn't need the cement or if I used cement, I didn't need straw. With my soil having little or no clay I'm wondering if they are right. What do you think?

A: Traditional cob is a mixture of sand, clay and chopped straw, but earthen plasters can take many forms. Most earthen plasters do not use straw, unless it is wanted for texture or appearance. For a dome, an unstabilized earthen plaster on the outside will certainly require regular maintenance, which you most likely do not want to commit to. That leaves the option of stabilizing the plaster with something, such as cement, lime, emulsified asphalt, or some other industrial product. Without much clay in your soil, straw alone will not hold it together. If it were me, I would go with Portland cement, since it is likely more durable over time.

This brings up a related question about how waterproof the building will be. Because domes do not have roofs with eaves, the wall/roof is completely exposed to the weather. In your arid climate, your soil-packed bags covered with an earthen plaster will probably be fine. In a more humid environment, this system might lead to leaks, so then it might be necessary to put a moisture barrier between the bags and the plaster. In your situation, it is probably better to leave the whole thing breathable.

Q: I am wondering what would be the best material and best way to cover the surface of the earth bags after we finish the construction of the dome. In Africa we used cement for covering the surface. However, I feel that is not very natural. It looks not very pleasant for a dome located at a beautiful and remote location. Would you kindly instruct me about the proportion of the content of the surface, to keep it waterproofed and nice looking?

A: I have also puzzled over this question of plastering earthbag domes. The most common approach does seem to be to use a cement stucco, but as you say this is not so natural. On the other hand, a simple earthen plaster on a dome would require regular maintenance for sure, because it would tend to wash away with the weather. Some degree of stabilization would be needed. Another option would be a lime plaster, which is fairly durable and more natural than cement...or possibly an earthen plaster at first which is then given a final coat of lime plaster. I think that a lime plaster over an earthen plaster would be a reasonable approach to explore. Both of these are naturally breathable, and the lime may shed water sufficiently to keep the interior dry. I haven't tried it on a dome, nor am I aware of anyone actually trying it. But this doesn't mean that it isn't a good idea. I think some experimentation is in order.
Unlike Portland cement, lime reabsorbs the CO2 that is released in its manufacture while it cures, so it is a net zero polluter in this regard. Lime has been traditionally used in Europe for centuries as a plaster over cob and rammed earth buildings.

None of these guarantee that the domes won't leak in a heavy rain, because cracks might occur...even with the cement stucco. The only way to be certain of no leaks might be to place a plastic curtain over the dome before plastering, which then would require the use of some sort of netting or wire mesh to hold it in place.

On the dome house that I built in Colorado, I used papercrete for a plaster, and it worked well in that rather dry climate...but papercrete requires machinery to make, and also uses some cement, so it is not so natural either. And it would not be very good for wetter climates.

Q: We have a small winter home in Baja Mexico and want to construct a workshop/generator room using sand filled bags. I would like to plaster the finished structure (inside and out) and would also appreciate any advice as to what wire or ? would be best to use.

A: Using some form of stucco netting or chicken wire with the plaster will make it more durable over time.

Comment: I found a great solution to render the bags, using a mix of 55% earth, 30% coarse sand, and only 15% (white) cement, plus some red and yellow oxides. The plaster is alive and looks great. Much better than the crazy amount of cements we used to render our first structure!  Also I have oiled the entire house with linseed oil and am very content with that too, it feels so much better than these toxic stains...

Comment: I have some interesting info about earthen plaster. I used sand, cattails, clay and water on my garden bed. What is interesting is how the rocks embedded in the mixture makes the plaster even stronger. I also experimented with a little bit of wheat paste. Each section of the garden bed is an experiment. The section that is doing the best in rain and freezing temperatures is the section with the imbedded rocks, which I sealed with flour paste, I also added the paste to the mix. After the rocks where imbedded, I waited about an 1 hr until the plaster hardened up a bit then put a layer of flour paste on the plaster around the rocks, it did an amazing job to seal it up.

Q: We have read and seen a lot on the internet and some CalEarth books and we started practicing on building with sandbags (read recycled cement bags). We started building a perimeter wall, but we keep having problems with plastering our bags. We don't seem to find the right mixture/balance of clay and sand. When we carry out the suspended jar test, we don't get any layers. It is a big blur. But the soil feels sticky, and pliable. We also use cactus juice in our plastering and limewash, and we finish of with a lime coat. After strong rain last week parts of the wall got washed away. Would you be able to have a look at some pictures taken at our site and give recommendations? We are planning to build a dome, but now we are not too sure about the durability after a rainstorm?

A: (Owen) It sounds like your soil is not suitable for plaster. Marginal soil can work in the bags, but earthen plaster is more critical. An earthen plaster subcoat ruined the domes in the Philippines.

Just a guess, but maybe you have silt for soil. Talk to a local engineer who does construction. They'll be able to tell you. It's probably best to eliminate the earthen plaster and use only lime or cement plaster on the bags.

We warn people all the time that domes are a poor choice in rainy climates. Domes evolved in desert regions. They're very difficult to waterproof, will need regular maintenance, and all the bags need to be stabilized.

(Kelly): The use of mesh or netting (either metal or plastic) attached to the wall before plastering will help keep it all intact.

Q: I have a project coming up where we will be building a wood-fired oven on top of earth bags. I am curious if you have advice about plastering them. I have great success with clay sand mixes but I am curious how they will adhere and hold up over time on the polypropylene. The oven will be roofed and there will be a stem wall. Do you recommend cement... and/or mesh?

A: Earthen plasters adhere surprisingly well to the polypropylene material, partially because it is actually woven and provides some "micro tooth." We do recommend the use of a mesh material (either wire or plastic, like fish net) for a more durable plaster.

Another possibility is to use mesh bags in the first place, and these expose some of the earthen filling at the surface as well as the mesh, so a secondary mesh may not be needed. You can read more about this new development in earthbag building here.

Whether to stabilize the plaster with cement or lime mostly depends on the exposure it will get to the elements or abrasion. It is really the same consideration you would give to any other plaster situation with a strawbale wall.

Q: I am looking for some advice on plastering the inside of an earthbag dome. In the book they advice "fat plaster", but I don't like the list of materials (paper cellulose etc). Is there an easier recipe that you know of for a plaster that will last, based on a 30% clay, 70% sand base mix?

A: A basic 30% clay, 70% sand mix for earthen plaster is all you need. You can add a bit of lime to it for stabilization, but for interiors this is not necessary. In some circumstances a plaster mesh will help keep it intact and on the wall.

Q: We live in Louisiana and one type of clay available in our area is "gumbo" clay. My question to you is, will this be an appropriate clay to use for plastering interior and exterior walls? I've read a little about it retaining moisture and shrinking and expanding.

A: You should be able to use your "gumbo" clay, but will certainly need to add a significant amount of sand to it for plastering. Highly expansive clays need even more sand than others. I suggest running some experiments to see what a good mix would be, and you might start with as little as 10 or 15% and go up from there.

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