12 Nov Cob House: Made for the Earth from Earth

Last Sunday afternoon, I took a short ride in the country out to Manchaca to see a cob house by Clay Sand Straw under construction. The house's interior and roof were still unfinished so you could see how it was put together. Here's an overview of what I saw and learned:

What is cob and how do you build with it?

Cob is a combination of clay, sand and straw mixed with water and then used to form earthen walls. The mixture is usually one part clay, two or three parts sand and one part chopped straw. Walls are formed by piling up the wet mixture on a concrete or stone slab about one foot thick and 16-18 inches high. After the first 16-18 inch layer dries for about four hours, another layer can be added until you achieve the wall height desired. The final rough, uneven wall structure can then be given a more uniform surface by cutting it with a hand saw.

The house I visited had outer walls made of cob with a 1:2:1 clay to sand to straw mix. The clay was high-quality caliche dug from the Texas Disposal Systems property not too far from the site. The house sits on a double thick concrete slab to prevent washout from flash-flooding.

What are some advantages of cob walls?

• Long history – This type of earthen wall construction technique has been around for around 6,000 years, according to project manager, John Curry. There's a lot of general knowledge and a long track record for cob. It's been around much longer than our modern c
• onstruction engineering techniques.
• Forgiving and durable – Cob walls are less prone to catastrophic failure than our current conventional approaches. Engineers who've studied this have noted that the more rigid, permanent and waterproof you try to make something the more potential for failure you introduce. Cob is also long-lasting even in tough environments. Earthen wall structures in San Francisco for example, have outlasted many major earthquakes.
• Simplicity – Cob is relatively simple to make and easy to handle. Besides who doesn't love to play with mud?

What's the basic design plan for this project?

The Manchaca house is a 2,200 square-foot 3-2 bed-bath with a high open living room and a loft over the kitchen. The outer walls and a couple of the interior walls are cob while the interior is standard stick framing. The second story loft and roof are plywood and stick framing. The interior side of the cob walls will be finished with a clay-lime plaster. Bed and bath floors are concrete or wood planking. In the kitchen-dining-living areas there will be packed earth sealed with linseed oil that will polymerize and feel like linoleum. Future plans also include the installation of a metal roof, solar PV and solar thermal water heating.

Look for project updates on the Clay Sand Straw Facebook page.

Texas-sourced cedar timbers provide bracing and anchor points for door frames.