Interior walls and partitions shall be of solid wood construction formed by not less than two layers of 1-inch (25 mm) matched boards or laminated construction 4 inches (102 mm) thick, or of 1-hour fire-resistance-rated construction.
Exterior walls shall be of one of the following:
- Noncombustible materials.
- Not less than 6 inches (152 mm) in thickness and constructed of one of the following:
- Fire-retardant-treated wood in accordance with Section 2303.2 and complying with Section 602.4.1.
- Cross-laminated timber complying with Section 602.4.2.