BK104 Finishes — 2025 California Residential Code

Cob walls shall not require a finish, except as required by Section BK104.2. Finishes applied to cob walls shall comply with this section and Chapters 3 and 7 unless stated otherwise in this section.

BK104.1.1 Interior wall finishes. Where installed, interior wall finishes and interior fire protection shall comply with the applicable provisions of Section R302, and shall be plasters in accordance with Section BK104.4 or nonplaster wall coverings in accordance with Section R702.

BK104.1.2 Exterior wall finishes. Where installed, exterior wall finishes shall be plasters in accordance with Section BK104.4, nonplaster exterior wall coverings in accordance with Section R703, or other finish systems in accordance with the following:

1. Specifications and details of the finish system’s attachment to the wall or its independent support and of its means of draining or evaporating water that penetrates the exterior finish shall be approved.

2. The vapor permeance of the combination of finish materials shall be 5 perms or greater to allow the transpiration of water vapor from the wall.

3. Finish systems with weights greater than 10 pounds per square foot (48.9 kg/m) and less than or equal to 20 pounds per square foot (97.8 kg/m) of wall area shall require that the minimum total length of cob braced wall panels in Table BK106.11(3) be multiplied by a factor of 1.2.

4. Finish systems with weights greater than 20 pounds per square foot (97.8 kg/m) of wall area shall require an engineered design.

Cob walls exposed to rain due to local climate, building design and wall orientation shall be finished or clad to provide protection from excessive erosion.

Class I and II vapor retarders shall not be used on cob walls, except at cob walls surrounding showers or as required or addressed elsewhere in this appendix.

Plaster applied to cob walls shall be any type described in this section. Plaster thickness shall not exceed 3 inches (76 mm) on each face except with an approved engineered design.

BK104.4.1 Plaster and membranes. Plaster shall be applied directly to cob walls to facilitate transpiration of moisture from the walls and to secure a mechanical bond between the plaster and the cob and shall comply with Section BK105.4.1.

BK104.4.2 Plaster lath. The surface of cob walls shall be permitted to function as lath for plaster, with no other lath required. Metal, plastic and natural fiber lath shall be permitted to be used to limit plaster cracking, increase the plaster bond to the wall or to bridge dissimilar materials.

BK104.4.3 Clay plaster. Clay plaster shall comply with Sections BK104.4.3.1 and BK104.4.3.2.

BK104.4.3.1 General. Clay plaster shall be any plaster having a clay or clay subsoil binder. Such plaster shall contain sufficient clay to fully bind the sand or other aggregate and any reinforcing fibers. Reinforcing fibers shall be chopped straw, sisal, hemp, animal hair or other similar approved fibers.

BK104.4.3.2 Clay subsoil requirements. The suitability of clay subsoil shall be determined in accordance with the Figure 2 Ribbon Test and the Figure 3 Ball Test in the appendix of ASTM E2392/E2392M.

BK104.4.4 Soil-cement plaster. Soil-cement plaster shall be composed of clay subsoil, sand, not more than 7 percent Portland cement by volume and, where provided, reinforcing fibers.

BK104.4.5 Gypsum plaster. Gypsum plaster shall comply with Section R702.2.1 and shall be limited to interior use.

BK104.4.6 Lime plaster. Lime plaster is any plaster with a binder composed of calcium hydroxide including Type N or S hydrated lime, hydraulic lime, natural hydraulic lime or slaked quicklime. Hydrated lime shall comply with ASTM C206. Hydraulic lime shall comply with ASTM C1707. Natural hydraulic lime shall comply with ASTM C141 and EN 459. Quicklime shall comply with ASTM C5.

BK104.4.7 Clay-lime plaster. Clay-lime plaster shall be composed of refined clay or clay subsoil, sand, lime and, where provided, reinforcing fibers.

BK104.4.8 Cement-lime plaster. Cement-lime plaster shall be plaster mix types CL, F or FL, as described in ASTM C926. BK104.4.9 Cement plaster. Cement plaster shall have not less than 1 part lime to 4 parts cement and be not thicker than 1 1/2 inches (38 mm), to ensure minimum acceptable vapor permeability.