To build a cob house, make the cob from a mixture of one part straw, one part clay or subsoil, and four parts sand. Start with a well-drained foundation, and form the walls of the house by compacting balls of wet cob.
- Construct the foundation
Mark the foundation with pegs or paint. Remove any plants and the topsoil. Dig a trench around the foundation that directs water away from the house. Fill the trench with drainage rocks. Tamp the ground within the trench's perimeter, and cover the area with a layer of stone chunks. The foundation is ready when the stones feel tight and secure underneath your feet.
- Mix the cob
Soak subsoil with enough water to make a pudding. If you do not have your own soil, use clay instead. Let the soil soak overnight. On a tarp, dump a 5-gallon bucket of sand, followed by an equal amount of pudding and then another bucket of sand. Mix the sand and wet clay with your feet. While mixing, add another two buckets of sand. Once you have a pile, sprinkle it with straw. Use the tarp to flip the pile back and forth, and work the straw through the mud.
- Build the wall
Take a softball-size amount of the cob, and form a ball. Place the wet cob where it is needed, and add more to build the structure. Smash the balls together to create a solid wall.