How to Make a Mud Straw Wall

Make test batches of cob for sample bricks. Scrape away topsoil at various sites and use only this soil in some batches, following the general cob-making process. Add sand or clay in different proportions to other batches. Label each test brick with the source and recipe. Sand creates the basic cob structure and clay holds the mix together. How much of each is needed depends on the qualities and characteristics of both. Too much clay causes shrinkage and cracking, while too much sand causes crumbling. Add straw last for tensile strength. Cob mixes with more clay accept more straw.

Mix cob in a wheelbarrow. Start with dirt, measuring the quantities and breaking up clods with the shovel. Then add sand if needed, blending it into the dirt. If your soil is sandy, instead add the clay soup, which is heavy clay soil dissolved in water. Stir the mixture well using the trowel. Add water a little at a time, until the mix is stiff like cookie dough. Pour the cob onto a tarp. Work it with your bare feet until it holds together like an unbaked loaf of bread. Flatten the cob and sprinkle it with straw, working it again with your feet. Add more straw and work that in. Continue until you reach the right consistency.

Drag the tarp to the work area. Place handfuls of pliable, moist cob onto the wall’s foundation and massage it into place creating a firm bond. Slowly build your wall this way, carefully marrying each new section of cob to the others using your thumbs, palms, knuckles, sticks and stones as cobbing tools. Create a wide wall at the base gradually tapering as you go up.

Cob only 2 to 3 feet of wall height each day because wet walls are heavy and need to dry overnight to carry more weight. Make sure the cob wall top is level when you stop. Make finger-sized, rewetting holes 3 to 4 inches apart all along the top so you can squirt in water before you start working in the fresh cob for next 2 feet of wall. It’s important that the wall and the fresh cob be equally moist, so that the wall is all of one piece. Cover the wall top with damp burlap in between work sessions.