BJ104 Finishes — 2025 California Residential Code

Finishes applied to strawbale walls shall comply with this section and with Chapters 3 and 7 unless stated otherwise in this section.

BJ104.1.1 Exterior wall finishes. Exterior wall finishes shall be plasters in accordance with Section BJ104.4, or nonplaster exterior wall coverings in accordance with Section R703 and other finish systems complying with all of the following:

1. With approved specifications and details showing the finish system’s means of attachment to the wall or its independent support, and a means of draining or evaporating water that penetrates the exterior finish to the exterior.
2. The vapor permeance of the combination of finish materials shall be 5 perms or greater to allow the transpiration of water vapor through the wall.
3. Finish systems with weights greater than 10 or less than or equal to 20 pounds per square foot (> 48.9 and ≤ 97.8 kg/m) of wall area require a factor of 1.2 for minimum total length of braced wall panels in Table BJ106.13(3).
4. Finish systems with weights greater than 20 pounds per square foot (97.8 kg/m) of wall area require an engineered design.

Strawbale walls shall be finished so as to provide mechanical protection, fire resistance and protection from weather and to restrict the passage of air through the bales, in accordance with this appendix and this code. Vertical strawbale wall surfaces shall receive a coat of plaster not less than 3/8 inch (10 mm) thick, or greater where required elsewhere in this appendix, or shall fit tightly against a solid wall panel or dense-packed cellulose insulation with a density of not less than 3.5 pounds per cubic foot (56 kg/m [3] ) blown into an adjacent framed wall. The tops of strawbale walls shall receive a coat of plaster not less than 3 / 8 inch (10 mm) thick or be tightly covered by gypsum board or a roof-bearing assembly.

Exception: Truth windows shall be permitted where a fire-resistance rating is not required. Weather-exposed truth windows shall be fitted with a weather-tight cover. Interior truth windows in Climate Zones 5, 6, 7, 8 and Marine 4 shall be fitted with an airtight

cover.

Class I and II vapor retarders shall not be used on a strawbale wall, nor shall any other material be used that has a vapor permeance rating of less than 5 perms, except as permitted or required elsewhere in this appendix.

Plaster applied to bales shall be any type described in this section, and as required or limited in this appendix. Plaster thickness shall not exceed 2 inches (51 mm).

BJ104.4.1 Plaster and membranes. Plaster shall be applied directly to strawbale walls to facilitate transpiration of moisture from the bales, and to secure a mechanical bond between the skin and the bales, except where a membrane is allowed or required elsewhere in this appendix.

BJ104.4.2 Lath and mesh for plaster. The surface of the straw bales functions as lath, and other lath or mesh shall not be required, except as required for out-of-plane load resistance by Table BJ105.4 or for structural walls by Tables BJ106.12 and BJ106.13(1).

BJ104.4.3 Clay plaster. Clay plaster shall comply with Sections BJ104.4.3.1 through BJ104.4.3.6.

BJ104.4.3.1 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 shall be permitted to contain reinforcing fibers. Acceptable reinforcing fibers include chopped straw, sisal and animal hair.

BJ104.4.3.2 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.

BJ104.4.3.3 Thickness and coats. Clay plaster shall be not less than 1 inch (25 mm) thick, except where required to be thicker for structural walls as described elsewhere in this appendix, and shall be applied in not less than two coats.

BJ104.4.3.4 Rain-exposed. Clay plaster, where exposed to rain, shall be finished with lime wash, lime plaster, linseed oil or other approved erosion-resistant finish.

BJ104.4.3.5 Prohibited finish coat. Plaster containing Portland cement shall not be permitted as a finish coat over clay plasters.

BJ104.4.3.6 Plaster additives. Additives shall be permitted to increase plaster workability, durability, strength or water resistance.

BJ104.4.4 Soil-cement plaster. Soil-cement plaster shall comply with Sections BJ104.4.4.1 through BJ104.4.4.3.

BJ104.4.4.1 General. Soil-cement plaster shall be composed of clay subsoil, sand and not less than 10 percent and not more than 20 percent Portland cement by volume, and shall be permitted to contain reinforcing fibers.

BJ104.4.4.2 Lath and mesh. Soil-cement plaster shall use any corrosion-resistant lath or mesh permitted by this code, or as required in Section BJ106 where used on structural walls.

BJ104.4.4.3 Thickness. Soil-cement plaster shall be not less than 1 inch (25 mm) thick.

BJ104.4.5 Gypsum plaster. Gypsum plaster shall comply with Section R702.2.1. Gypsum plaster shall be limited to use on interior surfaces of nonstructural walls, and as an interior finish coat over a structural plaster that complies with this appendix.

BJ104.4.6 Lime plaster. Lime plaster shall comply with Sections BJ104.4.6.1 through BJ104.4.6.3.

BJ104.4.6.1 General. Lime plaster is any plaster with a binder that is composed of calcium hydroxide [Ca(OH) 2 ], 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 CEN EN 459. Quicklime shall comply with ASTM C5. BJ104.4.6.2 Thickness and coats. Lime plaster shall be not less than 7/8 inch (22 mm) thick, and shall be applied in not less than three coats.

BJ104.4.6.3 On structural walls. Lime plaster on strawbale structural walls in accordance with Table BJ106.12 or BJ106.13(1) shall use hydraulic or natural hydraulic lime.

Exception: A nonhydraulic lime plaster demonstrating the minimum compressive strength in accordance with Section BJ106.6.1 and Table BJ106.6.1.

BJ104.4.7 Clay-lime plaster. Clay-lime plaster shall be composed of refined clay or clay subsoil, sand and lime, and shall be permitted to contain reinforcing fibers.

BJ104.4.8 Cement-lime plaster. Cement-lime plaster shall be plaster mixes CL, F or FL, as described in ASTM C926.

BJ104.4.9 Cement plaster. Cement plaster shall conform to ASTM C926 and shall comply with Sections R703.7.4 and R703.7.5, except that the amount of lime in plaster coats shall be not less than 1 part lime to 4 parts cement to allow a minimum acceptable vapor permeability. The combined thickness of plaster coats shall be not more than 1 1/2 inches (38 mm).