Which of the following is true regarding Type IV-B unprotected portions of mass timber ceilings...

Type IV-B construction is fundamentally protected mass timber construction, but the 2021 IBC permits specifically limited areas of interior mass timber to remain exposed. Section 602.4.2.2.2 establishes two independent exposure limits. Unprotected portions of mass timber ceilings, including attached beams, are limited to 20 percent of the floor area of the dwelling unit or fire area. Unprotected portions of mass timber walls, including attached columns, are limited to 40 percent of that floor area . Accordingly, option C correctly states the permitted percentages.

These percentages do not operate in isolation. Exposed areas must also comply with Section 602.4.2.2.4, which establishes separation requirements between unprotected mass timber surfaces. Where both walls and ceilings are exposed within the same dwelling unit or fire area, Section 602.4.2.2.3 requires use of the mixed-unprotected-area equation rather than independently maximizing both percentages.

Option A describes neither Type IV-B nor its controlled-exposure concept. Option B incorrectly characterizes Type IV-B as requiring complete encapsulation. Option D reverses the ceiling and wall percentages.

References/Topics: 2021 IBC Sections 602.4.2.2.2, 602.4.2.2.3 and 602.4.2.2.4; Type IV-B permitted exposed mass timber.

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