Cob Building Permit!

It took about half a year, but in September 2020 the cob octagon was finally approved by the Orange County Building Department for construction. The process was relatively straightforward if time-consuming, and basically entailed the following steps:

1) Working with an architect to produce the needed plans, including elevations from all four sides, foundation and cob wall cross-sections, framing details for interior load-bearing walls, roofing details, and electrical.

2) Consulting a structural engineer to determine, based on the plans, what the strength of the cob samples would need to be (it called for compressive strength of 100psi).

3) Finding another structural engineering firm that would do cob testing; determining how many they would need; making the samples and letting them dry sufficiently (takes a month +); then waiting impatiently for the results.

4) After the first batch came in too low, making multiple samples using different ratios of sand/clay and with a mix of binders - straw, hemp, and straw/hemp mix. I also switched methods of creating the samples. The original batch was made in 8” sections of 4” PVC pipe, but this took forever to dry. For the second batch, I switched to the 4” cubes called for in the IRC cob code made out of wood (2”x6”s ripped down to 4”), making batches of five each. The optimum mix turned out to be a 3:1 sand/clay mix with both wheat straw and hemp fibers (Group C below). Second best was hemp fiber alone (Group B). Third best was traditional straw (Group A). Interestingly, Group D was a batch that I put cement in with the idea that if I had to I could add cement. But it turns out that didn’t help. The concrete seemed to keep the samples moist and may have reduced the effectiveness of the clay binder.

5) Submitting the plans to the Orange County Plans Reviewer and making multiple changes to accommodate their concerns. A big problem was cob not meeting the Energy Code. So we agreed to wrap the exterior of the cob in 3.5” of hempcrete (nominal R-value 7.35 @ R-2.1/inch).

6) Sweating it as they reviewed the changes, which took almost two months.

7) Celebrating! Then stressing again because the hard work hadn’t even started yet…