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Appendix BK — COB CONSTRUCTION (MONOLITHIC ADOBE)

Section BK104 — FINISHES

2025 California Residential Code (Title 24, Part 2.5) · 2025 edition · updated 2026-07-27 · California

Italicized text is a California amendment to the model code, as printed in the official publication.

BK104.1 General.

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.

BK104.2 Where required.

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.

BK104.3 Vapor retarders.

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.

BK104.4 Plaster.

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.

on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

APPENDIX BK — COB CONSTRUCTION (MONOLITHIC ADOBE)

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.

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▸ Contents — 2025 California Residential Code (Title 24, Part 2.5)
2025 California Residential Code (Title 24, Part 2.5)
  1. Chapter 1 — ADMINISTRATION
  2. Chapter 2 — DEFINITIONS
  3. Chapter 3 — BUILDING PLANNING
  4. Chapter 4 — FOUNDATIONS
  5. Chapter 5 — FLOORS
  6. Chapter 6 — WALL CONSTRUCTION
  7. Chapter 7 — WALL COVERING
  8. Chapter 8 — ROOF-CEILING CONSTRUCTION
  9. Chapter 9 — ROOF ASSEMBLIES
  10. Chapter 10 — CHIMNEYS AND FIREPLACES
  11. Chapter 44 — REFERENCED STANDARDS
  12. Appendix AA — BOARD OF APPEALS
  13. Appendix AB — PERMIT FEES
  14. Appendix AC — RESERVED
  15. Appendix BA — RESERVED
  16. Appendix BB — TINY HOUSES
  17. Appendix BC — RESERVED
  18. Appendix BD — RESERVED
  19. Appendix BE — RADON CONTROL METHODS
  20. Appendix BF — PATIO COVERS
  21. Appendix BG — SOUND TRANSMISSION
  22. Appendix BH — AUTOMATIC VEHICULAR GATES
  23. Appendix BI — LIGHT STRAW-CLAY CONSTRUCTION
  24. Appendix BJ — STRAWBALE CONSTRUCTION
  25. ▸ Appendix BK — COB CONSTRUCTION (MONOLITHIC ADOBE)
    Overview
    1. Section BK101 — GENERAL
    2. Section BK102 — DEFINITIONS
    3. Section BK103 — MATERIALS, MIXING AND INSTALLATION
    4. Section BK104 — FINISHES
    5. Section BK105 — COB WALLS—GENERAL
    6. Section BK106 — COB WALLS—STRUCTURAL
    7. Section BK107 — COB FLOORS
    8. Section BK108 — FIRE RESISTANCE
    9. Section BK109 — THERMAL PERFORMANCE
    10. Section BK110 — REFERENCED STANDARDS
  26. Appendix BL — HEMP-LIME (HEMPCRETE) CONSTRUCTION
  27. Appendix BM — 3D-PRINTED BUILDING CONSTRUCTION
  28. Appendix BN — EXTENDED PLATE WALL CONSTRUCTION
  29. Appendix BO — EXISTING BUILDINGS AND STRUCTURES
  30. Appendix CA — RESERVED
  31. Appendix CB — RESERVED
  32. Appendix CC — RESERVED
  33. Appendix CD — RESERVED
  34. Appendix CE — RESERVED
  35. Appendix CF — RESERVED
  36. Appendix CG — NONSEWERED SANITATION SYSTEMS
  37. Appendix CH — PRIVATE SEWAGE DISPOSAL
  38. Appendix CI — SWIMMING POOL SAFETY ACT
  39. Appendix CJ — EMERGENCY HOUSING
  40. Appendix CK — AREAS PROTECTED BY THE FACILITIES OF THE CENTRAL…

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