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Appendix BI — LIGHT STRAW-CLAY CONSTRUCTION

Section BI103 — NONBEARING LIGHT STRAW-CLAY CONSTRUCTION

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.

BI103.1 General.

Light straw-clay shall be limited to infill between or around structural and nonstructural wall framing members.

BI103.2 Structure.

The structure of buildings using light straw-clay shall be in accordance with this code or shall be in accordance with an approved design by a registered design professional.

BI103.2.1 Number of stories. Use of light straw-clay infill shall be limited to buildings that are not more than one story above grade plane.

Exception: Buildings using light straw-clay infill that are greater than one story above grade plane shall be in accordance with an approved design by a registered design professional.

BI103.2.2 Bracing. Bracing for buildings with light straw-clay infill shall be in accordance with Section R602.10. Walls with light straw-clay infill shall use Method LIB and shall not be sheathed with solid sheathing. Walls without light straw-clay infill shall comply with any bracing method prescribed by this code.

BI103.2.3 Requirements and properties of light straw-clay mixtures. The requirements and properties of light straw-clay mixtures shall be in accordance with Table BI103.2.3.

TABLE BI103.2.3—REQUIREMENTS AND PROPERTIES OF LIGHT STRAW-CLAY MIXTURESa

DENSITY
(pcf)		 STRAW
(pcf)		 SUBSOIL
(pcf)		 WATER
(gal/cf)b		 MIN. % CLAY
IN SUBSOIL		 MINIMUM
CLAY:SILT
RATIO		 SUBSOIL
TESTING
METHODc, d		 MAX. WALL
THICKNESS,
INCHES		 R-VALUE
(hr/F°/cf/BTU/inch)
10 6.7 3.3 1.55 70 3.5:1 A 15 1.80
12 6.7 5.3 1.63 46 1.7:1 A 15 1.72

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

APPENDIX BI — LIGHT STRAW-CLAY CONSTRUCTION

TABLE BI103.2.3—REQUIREMENTS AND PROPERTIES OF LIGHT STRAW-CLAY MIXTURESa—continued

DENSITY
(pcf)		 STRAW
(pcf)		 SUBSOIL
(pcf)
W
(g		 ATER
al/cf)b		 MIN. % CLAY
IN SUBSOIL		 MINIMUM
CLAY:SILT
RATIO		 SUBSOIL
TESTING
METHODc, d		 MAX. WALL
THICKNESS,
INCHES		 R-VALUE
(hr/F°/cf/BTU/inch)
13 6.7 6.3 1.67 40 1.33:1 A 15 1.69
15 6.7 8.3 1.74 35 0.95:1 A 15 1.63
20 6.7 13.3 1.93 30 0.60:1 A 12 1.48
30 6.7 23.3 2.31 NA NA B 12 1.22
40 6.7 33.3 2.70 NA NA B 12 1.01
50 6.7 43.3 3.08 NA NA B 12 0.84
©Douglas Piltingsrud and StrawClay.org. Used by permission.
a. Interpolation permitted. Extrapolation not permitted.
b. Water mixed with subsoil equals clay slip.
c. Subsoil Testing Methods:
1. Lab test for percent of clay, silt and sand via hydrometer method.
2. The Figure 2 Ribbon Test and the Figure 3 Ball Test in the appendix of ASTM E2392/E2392M.
d. Trace amounts of organic materials are acceptable.

BI103.2.4 Stabilization of light straw-clay. Light straw-clay shall be stabilized as follows, or shall be in accordance with an approved design by a registered design professional:

  1. Vertical stabilization shall be of structural or nonstructural wood framing in accordance with Figure BI103.2.4(1), BI103.2.4(2) or BI103.2.4(3). Framing members that are both load-bearing and stabilization members shall meet the requirements of Section R602 and this section. Nonstructural stabilization members shall be not more than 32 inches (813 mm) on center.
  2. Horizontal stabilization shall be installed at not more than 24 inches (610 mm) on center and in accordance with Figure BI103.2.4(1), BI103.2.4(2) or BI103.2.4(3). Horizontal stabilization shall be of any of the following with the stated minimum dimensions: [3] / 4 -inch (19.1 mm) bamboo, [1] / 2 -inch (12.7 mm) fiberglass rod, 1-inch (25 mm) wood dowel or nominal 1-inch by 2-inch (25 mm by 51 mm) wood.

FIGURE BI103.2.4(1)—LIGHT STRAW-CLAY WALL WITH LARSEN TRUSSES

DOUBLE TOP PLATE

VERTICAL SOLID BLOCKING

FLAT SOLID BLOCKING

LADDER (LARSEN) TRUSSES MADE WITH 2x4s AND GUSSETS PER SECTION BI103.2.4, ITEM 1

GUSSETS: [3] / 8 ″ x 3 ″ PLYWOOD OR 1x4 FASTENERS: 4–4d NAILS OR 4–1¾ ″ STAPLES

WOOD OR METAL STRAP BRACING (LIB) PER TABLE 602.10.4 WHERE APPLICABLE PER SECTION R602.10

LIGHT STRAW-CLAY INFILL

HORIZONTAL STABILIZATION OF LIGHT STRAW-CLAY PER SECTION BI103.4, ITEM 2

ANCHORAGE PER SECTION R403.1.6

2X SILL PLATE

CONCRETE OR MASONRY FOUNDATION PER SECTION R401

FRAMING FASTENERS PER TABLE R602.3(1)

For SI: 1 inch = 25.4 mm.

APPENDIX BI-4 2025 CALIFORNIA RESIDENTIAL CODE

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

APPENDIX BI — LIGHT STRAW-CLAY CONSTRUCTION

FIGURE BI103.2.4(2)—LIGHT STRAW-CLAY WALL, SINGLE STUD WIDTH

DOUBLE TOP PLATE

FLAT SOLID BLOCKING

WOOD METAL STRAP BRACING (LIB) PER TABLE R602.10.4 WHERE APPLICABLE PER SECTION R602.10

2X STUDS PER SECTION BI103.2.4, ITEM 1

1x1 CLEAT CENTERED ON EACH STUD FACE BETWEEN HORIZONTAL STABILIZATION

HORIZONTAL STABILIZATION OF LIGHT STRAW-CLAY PER SECTION BI103.2.4, ITEM 2

LIGHT STRAY-CLAY INFILL

ANCHORAGE PER SECTION R403.1.6

2X SILL PLATE

CONCRETE OR MASONRY FOUNDATION PER SECTION R401

FRAMING FASTENERS PER TABLE R602.3(1)

FIGURE BI103.2.4(3)—LIGHT STRAW-CLAY WALL WITH BLIND STUDS

DOUBLE TOP PLATES

2X LET-IN PLATE

WOOD METAL BRACING (LIB) PER TABLE R602.10.4 WHERE APPLICABLE PER SECTION R602.10

2x6 MINIMUM STUDS PER SECTION BI103.2.4, ITEM 1

LIGHT STRAW-CLAY INFILL

HORIZONTAL STABILIZATION PER SECTION BI103.2.4, ITEM 2

2X BOTTOM PLATE

2X SILL PLATE

ANCHORAGE PER SECTION R403.1.6

CONCRETE OR MASONRY FOUNDATION PER SECTION R401

FRAMING FASTENERS PER TABLE R602.3(1)

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

APPENDIX BI — LIGHT STRAW-CLAY CONSTRUCTION

BI103.3 Materials.

The materials used in light straw-clay construction shall be in accordance with Sections BI103.3.1 through BI103.3.3.

BI103.3.1 Straw requirements. Straw shall be stems of wheat, rye, oats, rice or barley, and shall be free of visible decay, insects and green plant material.

BI103.3.2 Clay subsoil requirements. Suitability of clay subsoil shall be determined in accordance with Table BI103.2.3.

BI103.3.3 Light straw-clay mixture. A light straw-clay mixture shall consist of loose straw mixed and coated with clay slip such that there is not more than 5 percent uncoated straw, and shall be in accordance with Table BI103.2.3.

BI103.4 Wall construction.

Light straw-clay wall construction shall be in accordance with the requirements of Sections BI103.4.1 through BI103.4.7.

BI103.4.1 Light straw-clay maximum thickness. The maximum thickness of light straw-clay shall be in accordance with Table BI103.2.3.

BI103.4.2 Distance above grade. Light straw-clay and its exterior finish shall be not less than 8 inches (203 mm) above exterior finished grade.

BI103.4.3 Moisture barrier. An approved moisture barrier shall separate the bottom of light straw-clay walls from any masonry or concrete foundation or slab that directly supports the walls. Penetrations and joints in the barrier shall be sealed with an approved sealant.

BI103.4.4 Contact with wood members. Light straw-clay shall be permitted to be in contact with untreated wood members.

BI103.4.5 Contact with nonwood structural members. Nonwood structural members in contact with light straw-clay shall be resistant to corrosion or shall be coated to prevent corrosion with an approved coating.

BI103.4.6 Installation. Light straw-clay shall be installed in accordance with the following:

  1. Formwork shall be sufficiently strong to resist bowing where the light straw-clay is compacted into the forms.

  2. Light straw-clay shall be uniformly placed into forms and evenly tamped to achieve stable walls free of voids. Light straw-clay shall be placed in lifts of not more than 6 inches (152 mm) and shall be thoroughly tamped before additional material is added.

  3. Temporary formwork shall be removed from walls within 24 hours after tamping, and walls shall remain exposed until moisture content is in accordance with Section BI103.5.1. Visible voids shall be filled with light straw-clay or other insulative material prior to plastering.

BI103.4.7 Openings in walls. Openings in walls shall be in accordance with the following:

  1. Rough framing for doors and windows shall be fastened to structural members in accordance with this code. Windows and doors shall be flashed in accordance with this code.

  2. An approved moisture barrier shall be installed at window sills in light straw-clay walls prior to installation of windows.

BI103.5 Wall finishes.

The interior and exterior surfaces of light straw-clay walls shall be protected with a finish in accordance with Sections BI103.5.1 through BI103.5.5.

BI103.5.1 Dimensional stability of light straw-clay prior to application of plaster finish. Light straw-clay infill having a density of 30 pounds per cubic foot (480.6 kg/m [3] ) or greater shall be dry to a moisture content of not more than 20 percent at a depth of 4 inches (102 mm), as measured from each side of the wall. Light straw-clay infill having a density of less than 30 pounds per cubic foot (480.6 kg/m [3] ) shall be sufficiently dry such that the overall shrinkage of the light straw-clay is dimensionally stable.

BI103.5.2 Plaster finish. Exterior plaster shall be clay plasters or lime plasters. Interior plasters shall be clay plasters, lime plasters or gypsum plasters. Plasters shall be permitted to be applied directly to the surface of the light straw-clay walls without reinforcement, except that the juncture of dissimilar substrates shall be in accordance with Section BI103.5.4. Plasters shall have a thickness of not less than [1] / 2 inch (12.7 mm) and not more than 1 inch (25 mm) and shall be installed in not less than two coats. Rain-exposed clay plasters shall be finished with a lime-based or silicate-mineral coating.

BI103.5.3 Separation of wood and plaster. Where wood framing occurs in light straw-clay walls, such wood surfaces shall be separated from exterior plaster with No.15 asphalt felt, Grade D paper or other approved material except where the wood is preservative treated or naturally durable.

Exception: Exterior clay plasters shall not be required to be separated from wood.

BI103.5.4 Bridging across dissimilar substrates. Bridging shall be installed across dissimilar substrates prior to the application of plaster. Acceptable bridging materials include expanded metal lath, woven wire mesh, welded wire mesh, fiberglass mesh, reed matting or burlap. Bridging shall extend not less than 4 inches (102 mm), on both sides of the juncture. BI103.5.5 Exterior cladding. Exterior cladding shall be spaced not less than [1] / 2 inch (12.7 mm) from the light straw-clay such that a ventilation space is created to allow for moisture diffusion. Furring strips that create this ventilation space shall be securely fastened to the stabilization members or framing. The cladding shall be fastened to the wood furring strips in accordance with the manufacturer’s instructions. Insect screening shall be provided at the top and bottom of the ventilation space.

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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
    Overview
    1. Section BI101 — GENERAL
    2. Section BI102 — DEFINITIONS
    3. Section BI103 — NONBEARING LIGHT STRAW-CLAY CONSTRUCTION
    4. Section BI104 — THERMAL PERFORMANCE
    5. Section BI105 — REFERENCED STANDARDS
  24. Appendix BJ — STRAWBALE CONSTRUCTION
  25. Appendix BK — COB CONSTRUCTION (MONOLITHIC ADOBE)
  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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