Mixing Cob on Tarps for Your Hand-Built Home

Mixing cob is a “dance” best done by two people. Learn how to mix cob, what types of tarps work best and some tips to make cob mixing more efficient.

| January 18, 2013

Cob cottages are made of the oldest and most natural resources on earth — earth, clay, sand, straw and water. In The Hand-Sculpted House (Chelsea Green Publishing, 2002), authors Ianto Evans, Michael G. Smith and Linda Smiley cover step-by-step how to build one of these whimsical, yet practical structures and explore the deeper meaning of owning a home made of materials from its natural surroundings. Cob houses have a long tradition in Europe and are set to redefine the future of building in America. In this excerpt from chapter 11, “Making the Best Cob,” learn how mixing cob is a “dance” that can be done more efficiently by two people and learn about the types of tarps that work best for this endeavor. Also, pick up some tips for speed and efficiency. 

You can purchase this book from the MOTHER EARTH NEWS store: The Hand-Sculpted House.

A great breakthrough in manual cob mixing occurred in 1994, when Becky Bee developed a system for mixing on tarps. Before that, we had been mixing with shovels on a level platform made of tamped earth, concrete, or plywood. The tarp method is quicker, easier on the lower back, and requires fewer tools. It has now diversified into several quite different techniques—“different folks, different strokes!”—and it pays to change technique as circumstances demand. Experiment!

You will need a squarish piece of durable, slick, and water-resistant material, six to eight feet on a side, larger than your armspread by about a foot. Some people prefer a tarp a little longer in one dimension, 7 × 8 feet for instance, or 8 × 10.

Lay the tarp out on clean and level ground, close to your cob ingredients and your building site. We have found it saves work to dig out a shallow dish for the tarp to lie on: about 8 feet in diameter and 6 inches deep in the center works well. Spread the ingredients out on the tarp, alternating buckets of sand and clay to accelerate mixing. We normally use from three to five 5-gallon buckets of ingredients for a single batch of cob. This is the largest amount most people can handle easily and repeatedly.

When mixing by yourself, you can simply grasp one or two corners of the tarp and walk backward over the tarp until the mix is folded back upon itself. Do this repeatedly, rotating to a different corner each time, until the dry materials are mixed.

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