How to Build a Cobb House
A cobb house is built with bricks, formed loaves or just "globs" of a mixture of clay, soil, mud, straw and sand. Once you build a foundation and frame the structure, you make the walls with the cob mixture. Once these walls are complete and thoroughly dried, another plaster-like mixture is applied to the walls, which effectively seals them against the elements. Cobb houses are an ancient building method that has been used by civilizations all over the world. Some cobb houses have been occupied for hundreds of years. In many areas of the world cobb houses are built without any wood framing; the walls are simply made of stacked "globs" of the mud mixture. Modern building codes require a framing structure, usually framed with 2 by 6 lumber. Once the frame is in place, any savvy do-it-yourselfer can build the cobb walls. Does this Spark an idea?
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Things You'll Need
- Mud Straw Sand Trowel Additional straw, cattail fiber, or cow manure Piece of rubber or burlap for burnishing
Instructions
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Mix mud/straw/sand mixture. The exact recipe is arbitrary but the consensus is that you start with clay soil, add water until it is mud-like, mix in sand, then add straw until the mixture holds together and is moldable. Depending on the clay content of your soil, start with two parts sand to one part clay soil. Add long pieces of straw to strengthen the mixture. Again, the exact amount of straw is arbitrary, but add enough so the mixture holds together.
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Make the cob blocks, loaves, or "globs." The mud mixture can be molded into bricks using any brick-shaped mold you purchase or make. Some cobb experts advocate making the mud mixture the consistency of bread dough. Then a large handful of mud is kneaded, just like bread, and formed into loaf-shaped bricks. These loaf-shaped bricks are then "globbed" or slapped on top of one another to form the walls. Still other cobb enthusiasts merely take handfuls, or "globs" of the mud mixture and slap them in place on the walls. Whichever method you choose, only mix enough of the mud mixture to apply a 6 inch course to the walls, which must then dry before continuing.
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Build the walls. The walls must be built slowly, over time. Only add about 6 inches to the height of the walls at a time, then allow it to dry completely. If you build the walls too high too fast, they will dry unevenly and crack. Use the cobb building-block method of your choice and stack the loaves, bricks or globs of cobb-mud mixture as though you were building a brick wall. Smooth the areas where the bricks come together using some of the cobb mud mixture so the wall looks like a solid mass. You shouldn't be able to distinguish individual cobb bricks, loaves or globs.
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Mix plaster coating. Once the walls are built to the desired height, a mud-based plaster is applied to both the interior and exterior surface. The plaster is made with similar materials to the cob mud, but it is smoother and thinner. Natural plaster contains sand, clay, short fibers--such as short chopped straw, cattail fluff or even fresh cow manure--and a binder such as wheat paste made from flour and water. Believe it or not, cow manure is the fiber of choice for many builders of cob houses. Once dried, plaster containing cow manure does not smell like manure.
Mix three 5-gallon buckets sifted sand, one 5-gallon bucket wet clay, three-fourths of a 5-gallon bucket cow manure about 8 cups of wheat paste, and enough fiber (chopped straw, cattail fluff or fresh cow manure) so you can see and feel the fiber particles in the mix. -
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Apply the plaster. First, wet down the completed cobb wall (this helps the plaster adhere better). Don't soak the wall; merely wet down the surface. Then apply plaster either by hand or with a trowel. Apply a thin coat of plaster, something between ¼ to ½ inch. Smooth out the plaster until it is aesthetically pleasing. If the plaster is difficult to spread, add a little more water.
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Burnish surface of plaster. When the plaster has dried somewhat but not completely, burnish the surface with a piece of rubber or a wet piece of burlap. This will further smooth the surface and give it a finished look.
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Once the walls are built and plastered, install the doors and windows, and put on the roof.
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Tips & Warnings
Cobb houses are designed to be built slowly. It should take several weeks to erect the walls. Install one 6 inch course of the exterior walls at a time, let dry completely, and then add another course. Failure to let each course dry thoroughly before adding the next can cause the cobb to dry unevenly and crack.
Check your local building codes and ordinances before beginning to build a cobb house.
