The highlighted exposed surfaces in this figure are mass timber wall surfaces . Under 2021 IBC Section 602.4.2.2.2, Exception 1.2, unprotected portions of Type IV-B mass timber walls, including attached columns, are permitted up to an area equal to 40 percent of the floor area in the applicable dwelling unit or fire area. Option D is therefore correct.

This should be distinguished from the corresponding ceiling limitation. Unprotected mass timber ceilings, including attached beams, are limited to 20 percent of floor area. The larger 40-percent allowance applies specifically to wall exposure.

Where only wall surfaces are exposed, the allowable wall area is calculated directly from the base floor area. For example, a 1,000-square-foot fire area would permit up to 400 square feet of exposed wall surface under this exception. However, Section 602.4.2.2.4 additionally requires unprotected wall portions to maintain the prescribed 15-foot separation from other applicable exposed wall or ceiling portions.

Where exposed walls and exposed ceilings occur together, the individual 40- and 20-percent limits cannot simply be maximized simultaneously; the mixed-exposure equation in Section 602.4.2.2.3 governs.

References/Topics: 2021 IBC Sections 602.4.2.2.2 through 602.4.2.2.4; Type IV-B wall exposure.

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