In Type IV-C construction, combustible construction forming concealed spaces must receive noncombustible protection providing an assigned time of at least 40 minutes. Consequently, option D is correct.
Type IV-C permits substantial portions of mass timber to remain exposed within occupied interior spaces, subject to the applicable construction and fire-resistance requirements. That permission does not extend unchanged into concealed spaces. Concealed combustible surfaces create a different hazard because fire can spread above ceilings, inside shafts, or within service cavities without immediate detection. Section 602.4.3.5 therefore imposes a specific 40-minute noncombustible protection requirement.
An NFPA 13 sprinkler system is generally required for tall mass timber buildings, but sprinkler protection alone is not the prescribed substitute for this concealed-space protection. Fire-retardant-treated wood is still combustible and does not constitute the required noncombustible protective membrane. Option C is also incorrect because the concealed-space rule is more specific than the provisions governing exposed surfaces in occupied spaces.
Inspection should occur before closure so that membrane continuity, board orientation, fastener spacing, joint treatment, penetrations, and protection around connection hardware can be fully observed.
References/Topics: 2021 IBC Sections 602.4.3.2 and 602.4.3.5; Table 722.7.1(1); concealed combustible construction in Type IV-C buildings.
===============
