BK103 Materials, Mixing and Installation — 2025 California Residential Code

Clay subsoil for a cob mix shall be acceptable if the mix it produces meets the requirements of Section BK103.4.

Sand or other aggregates such as, but not limited to, gravel, pumice and lava rock, when added to cob mixes, shall yield a mix that meets the requirements of Section BK103.4.

Straw for cob mixes shall be from wheat, rice, rye, barley or oat, or similar reinforcing fibers with similar performance. Before mixing, the straw or other reinforcing fibers shall be dry to the touch and free of visible decay.

Cob mixes shall be of any proportions of refined clay or clay subsoil, added sand (if any) and straw that produce a dried mix that passes the shrinkage test in accordance with Section BK103.4.1, complies with the compressive strength requirements of Section BK106.6 and complies with the modulus of rupture requirements of Section BK106.7.

BK103.4.1 Shrinkage test for cob mixes. Each proposed cob mix of different mix proportions shall be placed moist to completely fill a 24-inch by 3 1/2-inch by 3 1/2-inch (610 mm by 89 mm by 89 mm) wooden form on a plastic or paper slip sheet and dried to ambient moisture conditions, or oven dried. The total shrinkage of the length shall not exceed 1 inch (25 mm), as measured from the dried edges of the material to the insides of the form. Cracks in the sample greater than 1/16 inch (1.5 mm) shall first be closed manually. The shrinkage test shall be shown to the building official for approval before placement of the cob mix onto walls

The clay subsoil, sand and straw for cob shall be thoroughly mixed by manual or mechanical means with water sufficient to produce a mix of a plastic consistency capable of bonding of successively placed layers or lifts.

Cob shall be installed on the wall in lifts of a height that supports itself with minimal slumping.

Each layer of cob shall be prevented from drying until the next layer is installed, to ensure bonding of successive layers. The top of each layer shall be kept moist and malleable with one or more of the following methods:

1. Covering with a material that prevents loss of or holds moisture.

2. Covering with a material that shades it from direct sun.

3. Wetting.

Where dry joints are unavoidable, the previous layer shall be wetted prior to application of the next layer.

Where holes to facilitate drying are used, such holes shall be permitted to be of any depth and shall not exceed 3/4 inch (19 mm) in diameter. Drying holes shall not be spaced closer than 10 hole-diameters and shall not be placed in braced wall panels. The design load on load-bearing walls with drying holes shall not exceed 90 percent of the allowable bearing capacity as determined in accordance with Section BK106.8. Drying holes shall be filled with cob before final inspection.

Roof and ceiling loads shall not be added until walls are sufficiently dry to support them without compressing.