For the next 8 months I will be participating in a natural building practicum at Aprovecho in Cottage Grove, Oregon. One of our first projects was the second phase of the garden wall to learn cob basics, work as a group, and continue the design around the entrance. It was started a few months ago but the weather took its toll on the exposed cob so, to begin we gave it a good brushing to get any loose excess clay off. The clay was collected and later reused with some amendments (additional materials as needed). Cob is made using clay, sand, straw, and water in various proportions relating to the type of soil you are working with. For instance, if your soil has alot of clay in it but not alot of sand you may need to add some more sand. The ratio is 2.5 buckets of sand to 1 bucket of clay. Mix the dry materials together. Next, add water in increments so that the materials are wet enough to “ball” together but not too wet because then the cob wall will start to slump on the wall.
The foundation and shape of the wall was already constructed and left with a “spine and rib” so that the new cob would have something to attach to. To apply the cob, use your thumb or a stick to give downward pressure working in the straw for tensile strength. In order to keep the wall plumb, we used our hand as a guide.
We built the wall in a continuing flowing pattern. The roof rafters were cut in conjunction with an introduction to the shop and all its tools. The shapes were jigsawed and the attached by bolting them to the long wooden poles in the cob wall.