Section BK105 — COB WALLS—GENERAL — 2025 California Residential Code (Title 24, Part 2.5)

BK105.1 General. ¶

Cob walls shall be designed and constructed in accordance with this section and Figure BK101.4 or an approved alternative design. In addition to the general requirements for cob walls in this section, cob structural walls shall comply with Section BK106.

BK105.2 Building limitations and requirements for cob wall construction. ¶

Cob walls shall be subject to the following limitations and requirements:

Number of stories: not more than one.
Building height: not more than 20 feet (6096 mm).
Seismic design categories: limited to use in Seismic Design Categories A, B and C, except with an approved engineered design.
Wall height: in accordance with Table BK105.3, and with Table BK106.11(1) for braced wall panels.
Wall thickness, excluding finish, shall be not less than 10 inches (254 mm), not greater than 24 inches (610 mm) at the top two-thirds, not limited at the bottom third and, for structural walls, shall comply with Section BK106.2, Item 2. Wall taper is permitted in accordance with Section BK106.5, Item 1.
Interior cob walls shall require an approved engineered design that accounts for the seismic load of the interior cob walls, except in Seismic Design Category A for walls with a height to thickness ratio less than or equal to 6.

BK105.3 Out-of-plane resistance methods and unrestrained wall height limits. ¶

Cob walls shall employ a method of out-of-plane load resistance in accordance with Table BK105.3, and comply with its associated height limits and requirements.

TABLE BK105.3—OUT-OF-PLANE RESISTANCE METHODS AND UNRESTRAINED WALL HEIGHT LIMITS

WALL TYPEa, g, h AND METHOD OF OUT-OF-PLANE LOAD RESISTANCE	FOR ULTIMATE DESIGN WIND SPEEDS (mph)	FOR SEISMIC DESIGN CATEGORIES	UNRESTRAINED COB WALL HEIGHT_H_b, c, h	UNRESTRAINED COB WALL HEIGHT_H_b, c, h	TOP ANCHORe SPACING (inches)	TENSION TIEf SPACING (inches)
WALL TYPEa, g, h AND METHOD OF OUT-OF-PLANE LOAD RESISTANCE	FOR ULTIMATE DESIGN WIND SPEEDS (mph)	FOR SEISMIC DESIGN CATEGORIES	Absolute Limit (feet)	Limit Based on Wall ThicknessT d (feet)	Limit Based on Wall ThicknessT d (feet)	Limit Based on Wall ThicknessT d (feet)
Wall 1i: no anchors, no steel wall reinforcing	≤ 110	A	H ≤ 8	H ≤ 6_T_	None	48
Wall 2: top anchorsj, continuous vertical 6-inch × 6-inch × 6- gage steel mesh in center of wall embedded in foundation 12 inches	≤ 140	A, B, C	H ≤ 8	H ≤ 8_T_	12	24
Wall Ai: top anchors, no vertical steel reinforcing	≤ 120	A, B	H ≤ 8	H ≤ 6_T_	12	48
Wall Bi: top and bottom anchors, no vertical steel reinforcing	≤ 130	A, B	H ≤ 8	H ≤ 6_T_	12	48
Wall C: top and bottom anchors, continuous vertical threaded rod at 4 feet on center embedded in foundation and connected to bond beam	≤ 140	A, B, C	H ≤ 8	H ≤ 8_T_	12	24
Wall D: continuous vertical threaded rod at 1 foot on center embedded in foundation and connected to bond beam	≤ 140	A, B, C	H ≤ 8	H ≤ 8_T_	N/A	24
Wall E: top anchors, continuous vertical 6-inch × 6-inch × 6- gage steel mesh 2 inches from each face of wall embedded in foundation	≤ 140	A, B, C	H ≤ 8	H ≤ 8_T_	12	24
Wall F: top anchors, continuous vertical 6-inch × 6-inch × 10-gage steel mesh 2 inches from each face of wall embed- ded in foundation	≤ 140	A, B, C	H ≤ 8	H ≤ 8T	12	24

TABLE BK105.3—OUT-OF-PLANE RESISTANCE METHODS AND UNRESTRAINED WALL HEIGHT LIMITS

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s. N/A = Not Applicable a. See Table BK106.11(1) for reinforcing and anchorage specifications for wall Types A, B, C, D and E. b._ H_ = height of the cob portion of the wall only. See Figure BK101.4. The maximum_H_ is the absolute limit or the limit based on wall thickness, whichever is more restrictive. c. Bond beams or other horizontal restraints are permitted to divide a wall into more than one unrestrained wall height with an approved engineered design. d._ T_= Cob wall thickness (in feet) at its minimum, without plaster. e. 5/8-inch threaded rod anchors at prescribed spacing with 12-inch embedment in cob, full embedment in concrete bond beams or full penetration in wood bond beam with a nut and washer. f. Attach rafters to bond beam with 4-inch by 3-inch by 3-inch by 18-gage tension tie angles at prescribed spacing. See Figure BK106.9.5. Where rafters are attached to tension ties, roof sheathing shall be edge nailed. g. All walls shall be tested for compressive strength in accordance with Section BK106.6. h. For curved walls with an arc length (ARCc) to radius (Rc) ratio of 1.5:1 or greater, the_H_/ T factor shall be increased by 1, and the absolute height limit by 1 foot. See Section BK106.11.3. i. Wall type requires a modulus of rupture test in accordance with Section BK106.7. j. See wall Type A in Table BK106.11(1) for top anchor requirements.	For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s. N/A = Not Applicable a. See Table BK106.11(1) for reinforcing and anchorage specifications for wall Types A, B, C, D and E. b._ H_ = height of the cob portion of the wall only. See Figure BK101.4. The maximum_H_ is the absolute limit or the limit based on wall thickness, whichever is more restrictive. c. Bond beams or other horizontal restraints are permitted to divide a wall into more than one unrestrained wall height with an approved engineered design. d._ T_= Cob wall thickness (in feet) at its minimum, without plaster. e. 5/8-inch threaded rod anchors at prescribed spacing with 12-inch embedment in cob, full embedment in concrete bond beams or full penetration in wood bond beam with a nut and washer. f. Attach rafters to bond beam with 4-inch by 3-inch by 3-inch by 18-gage tension tie angles at prescribed spacing. See Figure BK106.9.5. Where rafters are attached to tension ties, roof sheathing shall be edge nailed. g. All walls shall be tested for compressive strength in accordance with Section BK106.6. h. For curved walls with an arc length (ARCc) to radius (Rc) ratio of 1.5:1 or greater, the_H_/ T factor shall be increased by 1, and the absolute height limit by 1 foot. See Section BK106.11.3. i. Wall type requires a modulus of rupture test in accordance with Section BK106.7. j. See wall Type A in Table BK106.11(1) for top anchor requirements.	For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s. N/A = Not Applicable a. See Table BK106.11(1) for reinforcing and anchorage specifications for wall Types A, B, C, D and E. b._ H_ = height of the cob portion of the wall only. See Figure BK101.4. The maximum_H_ is the absolute limit or the limit based on wall thickness, whichever is more restrictive. c. Bond beams or other horizontal restraints are permitted to divide a wall into more than one unrestrained wall height with an approved engineered design. d._ T_= Cob wall thickness (in feet) at its minimum, without plaster. e. 5/8-inch threaded rod anchors at prescribed spacing with 12-inch embedment in cob, full embedment in concrete bond beams or full penetration in wood bond beam with a nut and washer. f. Attach rafters to bond beam with 4-inch by 3-inch by 3-inch by 18-gage tension tie angles at prescribed spacing. See Figure BK106.9.5. Where rafters are attached to tension ties, roof sheathing shall be edge nailed. g. All walls shall be tested for compressive strength in accordance with Section BK106.6. h. For curved walls with an arc length (ARCc) to radius (Rc) ratio of 1.5:1 or greater, the_H_/ T factor shall be increased by 1, and the absolute height limit by 1 foot. See Section BK106.11.3. i. Wall type requires a modulus of rupture test in accordance with Section BK106.7. j. See wall Type A in Table BK106.11(1) for top anchor requirements.	For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s. N/A = Not Applicable a. See Table BK106.11(1) for reinforcing and anchorage specifications for wall Types A, B, C, D and E. b._ H_ = height of the cob portion of the wall only. See Figure BK101.4. The maximum_H_ is the absolute limit or the limit based on wall thickness, whichever is more restrictive. c. Bond beams or other horizontal restraints are permitted to divide a wall into more than one unrestrained wall height with an approved engineered design. d._ T_= Cob wall thickness (in feet) at its minimum, without plaster. e. 5/8-inch threaded rod anchors at prescribed spacing with 12-inch embedment in cob, full embedment in concrete bond beams or full penetration in wood bond beam with a nut and washer. f. Attach rafters to bond beam with 4-inch by 3-inch by 3-inch by 18-gage tension tie angles at prescribed spacing. See Figure BK106.9.5. Where rafters are attached to tension ties, roof sheathing shall be edge nailed. g. All walls shall be tested for compressive strength in accordance with Section BK106.6.

TABLE BK105.3—OUT-OF-PLANE RESISTANCE METHODS AND UNRESTRAINED WALL HEIGHT LIMITS h. For curved walls with an arc length (ARCc) to radius (Rc) ratio of 1.5:1 or greater, the_H_/ T factor shall be increased by 1, and the absolute height limit by 1 foot. See Section
BK106.11.3.
i.
Wall type requires a modulus of rupture test in accordance with Section BK106.7.
j.
See wall Type A in Table BK106.11(1) for top anchor requirements.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s.
N/A = Not Applicable
a. See Table BK106.11(1) for reinforcing and anchorage specifications for wall Types A, B, C, D and E.
b._ H_ = height of the cob portion of the wall only. See Figure BK101.4. The maximum_H_ is the absolute limit or the limit based on wall thickness, whichever is more restrictive.
c. Bond beams or other horizontal restraints are permitted to divide a wall into more than one unrestrained wall height with an approved engineered design.
d._ T_= Cob wall thickness (in feet) at its minimum, without plaster.
e.
5/8-inch threaded rod anchors at prescribed spacing with 12-inch embedment in cob, full embedment in concrete bond beams or full penetration in wood bond beam with a
nut and washer.
f. Attach rafters to bond beam with 4-inch by 3-inch by 3-inch by 18-gage tension tie angles at prescribed spacing. See Figure BK106.9.5. Where rafters are attached to tension
ties, roof sheathing shall be edge nailed.
g. All walls shall be tested for compressive strength in accordance with Section BK106.6.
h. For curved walls with an arc length (ARCc) to radius (Rc) ratio of 1.5:1 or greater, the_H_/ T factor shall be increased by 1, and the absolute height limit by 1 foot. See Section
BK106.11.3.
i.
Wall type requires a modulus of rupture test in accordance with Section BK106.7.
j.
See wall Type A in Table BK106.11(1) for top anchor requirements.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s.
N/A = Not Applicable
a. See Table BK106.11(1) for reinforcing and anchorage specifications for wall Types A, B, C, D and E.
b._ H_ = height of the cob portion of the wall only. See Figure BK101.4. The maximum_H_ is the absolute limit or the limit based on wall thickness, whichever is more restrictive.
c. Bond beams or other horizontal restraints are permitted to divide a wall into more than one unrestrained wall height with an approved engineered design.
d._ T_= Cob wall thickness (in feet) at its minimum, without plaster.
e.
5/8-inch threaded rod anchors at prescribed spacing with 12-inch embedment in cob, full embedment in concrete bond beams or full penetration in wood bond beam with a
nut and washer.
f. Attach rafters to bond beam with 4-inch by 3-inch by 3-inch by 18-gage tension tie angles at prescribed spacing. See Figure BK106.9.5. Where rafters are attached to tension
ties, roof sheathing shall be edge nailed.
g. All walls shall be tested for compressive strength in accordance with Section BK106.6.
h. For curved walls with an arc length (ARCc) to radius (Rc) ratio of 1.5:1 or greater, the_H_/ T factor shall be increased by 1, and the absolute height limit by 1 foot. See Section
BK106.11.3.
i.
Wall type requires a modulus of rupture test in accordance with Section BK106.7.
j.
See wall Type A in Table BK106.11(1) for top anchor requirements.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s.
N/A = Not Applicable
a. See Table BK106.11(1) for reinforcing and anchorage specifications for wall Types A, B, C, D and E.
b._ H_ = height of the cob portion of the wall only. See Figure BK101.4. The maximum_H_ is the absolute limit or the limit based on wall thickness, whichever is more restrictive.
c. Bond beams or other horizontal restraints are permitted to divide a wall into more than one unrestrained wall height with an approved engineered design.
d._ T_= Cob wall thickness (in feet) at its minimum, without plaster.
e.
5/8-inch threaded rod anchors at prescribed spacing with 12-inch embedment in cob, full embedment in concrete bond beams or full penetration in wood bond beam with a
nut and washer.
f. Attach rafters to bond beam with 4-inch by 3-inch by 3-inch by 18-gage tension tie angles at prescribed spacing. See Figure BK106.9.5. Where rafters are attached to tension
ties, roof sheathing shall be edge nailed.
g. All walls shall be tested for compressive strength in accordance with Section BK106.6.
h. For curved walls with an arc length (ARCc) to radius (Rc) ratio of 1.5:1 or greater, the_H_/ T factor shall be increased by 1, and the absolute height limit by 1 foot. See Section
BK106.11.3.
i.
Wall type requires a modulus of rupture test in accordance with Section BK106.7.
j.
See wall Type A in Table BK106.11(1) for top anchor requirements.|

APPENDIX BK-6 2025 CALIFORNIA RESIDENTIAL CODE

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

APPENDIX BK — COB CONSTRUCTION (MONOLITHIC ADOBE)

BK105.3.1 Determination of out-of-plane loading. Out-of-plane loading for the use of Table BK105.3 shall be in accordance with the ultimate design wind speed and seismic design category requirements of Sections R301.2.1 and R301.2.2, respectively. An approved engineered design shall be required where the building is located in a special wind region or where wind design is required in accordance with Figure R301.2.1.1.

BK105.3.2 Bond beams for nonstructural walls. Nonstructural cob walls shall be provided with a bond beam at the top of the wall that complies with Section BK106.9, except for requirements relating to roof and/or ceiling loads or braced wall panels.

BK105.3.3 Lintels in nonstructural walls. Door, window and other openings in nonstructural cob walls shall require a lintel in accordance with Section BK106.10, except for requirements relating to roof and/or ceiling loads or braced wall panels.

BK105.3.4 Reinforcing at wall openings. Reinforcing shall be installed at window, door, and similar wall openings and penetrations greater than 2 feet (610 mm) in width in accordance with Sections BK105.3.4.1 through BK105.3.4.3. Surface voids deeper than 25 percent of the wall thickness shall be considered an opening.

BK105.3.4.1 Opening size limit. Openings shall not exceed 6 feet (1829 mm) in width, and the height of the cob wall below openings shall not exceed 6 feet (1829 mm) above the top of the foundation.

BK105.3.4.2 Horizontal reinforcing. Two-inch by 2-inch (51 mm by 51 mm) 14-gage galvanized steel mesh shall be embedded 4 inches (102 mm) in the cob below the rough opening for windows and shall extend 12 inches (305 mm) beyond the sides of the opening. Walls below rough window openings greater than 4 feet 6 inches (1372 mm) in height shall be provided with additional horizontal reinforcing at midheight. BK105.3.4.3 Vertical reinforcing. Full-height [5] / 8 -inch (16 mm) threaded rod shall be installed 4 inches (102 mm) from each side of the opening, centered in the thickness of the cob wall. The threaded rods shall be embedded 7 inches (178 mm) in the foundation and 4 inches (102 mm) in concrete bond beams or shall penetrate through wood bond beams and be secured with a nut and washer. The threaded rods shall be embedded in concrete lintels or pass through a drilled hole in wood lintels.

BK105.3.5 Minimum length of cob walls. Sections of cob walls between openings shall be not less than 2 feet 6 inches (762 mm) in length. Wall sections less than 4 feet (1219 mm) and not less than 2 feet 6 inches (762 mm) in length shall contain vertical reinforcing in accordance with Section BK105.3.4.3.

BK105.4 Moisture control. ¶

Cob walls shall be protected from moisture intrusion and damage in accordance with Sections BK105.3.4.1 through BK105.4.5.

BK105.4.1 Water-resistive barriers and vapor permeance. Cob walls shall be constructed without a membrane barrier, except as otherwise required elsewhere in this appendix. Where a water-resistive barrier is placed behind an exterior finish, it shall be considered part of the finish system and shall comply with Item 2 of Section BK104.1.2 for the combined vapor permeance rating.

BK105.4.2 Horizontal surfaces. Cob walls and other cob elements shall be provided with a water-resistive barrier at weatherexposed horizontal surfaces. The water-resistive barrier shall be of a material and installation that will prevent erosion and prevent water from entering the wall system. Horizontal surfaces, including exterior windowsills, sills at exterior niches and exterior buttresses, shall be sloped not less than 1 unit vertical in 12 units horizontal to drain away from cob walls or other cob elements.

BK105.4.3 Separation of cob and foundation. A liquid-applied or bituminous Class II vapor retarder shall be installed between cob and supporting concrete or masonry.

Exception: Where local climate, site conditions and foundation design limit ground moisture migration into the base of the cob wall, including but not limited to the use of a moisture barrier or capillary break between the supporting concrete or masonry and the earth.

BK105.4.4 Separation of cob and finished grade. Cob shall be not less than 8 inches (203 mm) above finished grade.

Exception: The minimum separation shall be 4 inches (102 mm) in dry climate zones, as defined in Section R702.7.3 and Table R702.7.3, and shall be 2 inches (51mm) on walls that are not weather exposed.

BK105.4.5 Installation of windows and doors. Windows and doors shall be installed in accordance with the manufacturer’s instructions to a wooden buck of not less than nominal 2-inch by 4-inch (51 mm by 102 mm) wood members. The installation of windows and doors and their bucks shall prevent the passage of air and water into or through the wall system. Windowsills shall comply with Section BK105.4.2.

5.4.5 Installation of windows and doors.** Windows and doors shall be installed in accordance with the manufacturer’s instructions to a wooden buck of not less than nominal 2-inch by 4-inch (51 mm by 102 mm) wood members. The installation of windows and doors and their bucks shall prevent the passage of air and water into or through the wall system. Windowsills shall comply with Section BK105.4.2.

Window and door bucks shall be installed in accordance with Figure BK105.4.5 and one of the following methods:

Side members of the bucks shall be anchored into the cob wall by embedding the protruding end of half-driven 16d galvanized nails at a maximum 6-inch (152 mm) spacing. The buck shall be embedded into the cob not less than 1½ inches (38 mm) and set in from each face of the wall not less than 3 inches (76 mm).
Wood stiffeners not less than nominal 2-inch by 4-inch (51 mm by 102 mm) shall be attached on-edge to the sides of the buck and embedded in the cob wall a minimum of 3½ inches (89 mm). Stiffeners shall anchor into the cob wall with the protruding end of half-driven 16d galvanized nails at a maximum 6-inch (152 mm) spacing. Stiffeners shall be set back not less than 3 inches (76 mm) from each wall face. Bucks are permitted to be exposed and do not require anchoring nails where stiffeners are used with this method.
Other approved methods satisfying the performance requirements of Section BK105.4.5.

Exception: Windows and unframed glass shall be permitted to be embedded directly into a cob wall with an approved design.

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

APPENDIX BK — COB CONSTRUCTION (MONOLITHIC ADOBE)

FIGURE BK105.4.5—WINDOW INSTALLATION (DOOR INSTALLATION SIMILAR)

NOTE:

SIDES OF BUCK

SECTION BK105.3.4.3

SECTION BK105.3.4.2

WITHOUT STIFFENER

COB WALL

STEM WALL

OF FOUNDATION PER

SECTIONS BK106.4

AND BK105.4.4

NOTE:

BK105.5 Inspections. ¶

In addition to ensuring compliance with Section R109.1, the building official shall inspect the following aspects of cob construction:

Anchors and vertical and horizontal reinforcing in cob walls, where required in accordance with Tables BK105.3 and BK106.11(1) and Sections BK105.3.4 through BK105.3.5.
Reinforcing in any concrete bond beams or lintels, in accordance with Section BK106.9.2 and Table BK106.10.