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Cob Plaster on Straw Bale wallsBob Bolles bbolles at cts.com
Wed Jul 2 06:47:11 PDT 1997
Speireag Bill Steen was kind enough to teach us how to use cob as plaster. The clay-sand mixture has a very high percentage of straw mixed in. Too little straw will allow excessive cracking of the cob We use 1/2" hardware cloth to screen the dry clay. Bill used a wheelbarrow to mix the cob 1 1/2 - 5 gallon buckets of water in a large wheelbarrow "sprinkle" the clay-sand mix into the water until it is even with the top of the water let it sit for 15 minutes or so - then mix by hand, until there are no lumps. Slowly add straw until you get the right mix. The cob is applied by hand and is forcefully applied to the straw wall. It is also forced into the cracks between the bales with a stick. (Actually, we are using a clay-sand mix between the bales as a thin mortar - about 1 inch works well - the bales are pressed into the mortar, and they become "rock solid" when the clay drys) The thickness is determined by how much of this plaster it takes to make a flat wall. We do apply more to the inside to create a good thermal mass - 2-3 inches. While the cob is still wet, we place 1x2's perpendicular about 2 feet apart - the 1x2's will stick to the wet cob. Use cob to shim them so that they are plumb. Cob is applied between the vertical boards, and a screed is used to create a flat wall surface. Depending on how many people you have to mix and apply, you can do an entire (small) building in a day. The finish plaster is made with very fine fibers mixed into the clay-sand. We have found that we can chop up the straw with a weed eater in a barrel If the cob drys out between applications, mist the wall (the cob, not the straw!) lightly with a hose. Cob is truly an amaizing material. We are beginning to experiment with waterproofing the plaster - more on that later! We have used cob plaster on 9 buildings to date In our climate, (hot and dry) the moisture has created no problems that we are aware of. Good luck! Bob You said: To what extent have you done this? How thick do you make the cob? Can you do more than 8 inches of height per day because the cob is thinner and partially supported by the straw bale wall? Does this create any sort of problem as far as introducing moisture into the straw or not permitting the straw to breathe sufficiently?
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