Are mass timber elements permitted to serve as a fire barrier to an adjacent occupancy...

Yes. The allowance applies to both Type IV-B and Type IV-C construction , making option D correct.

IBC Section 508.4.4.1 regulates the construction of required separations between adjacent occupancies. Where the required separation is formed by a fire barrier or horizontal assembly, mass timber elements are specifically permitted to serve that function in Type IV-B or Type IV-C construction provided the mass timber is separated from the building interior by an approved thermal barrier.

The prescribed thermal barrier is gypsum board not less than 1/2 inch thick , or another material that has been tested and meets both the Temperature Transmission Fire Test and Integrity Fire Test acceptance criteria of NFPA 275.

The requirement is supplemental to the actual fire-resistance rating required for the occupancy separation. In other words, simply installing 1/2-inch gypsum board does not independently create the required fire barrier; the underlying mass timber assembly must also provide the applicable rated performance.

Type IV-HT is not included in this specific Section 508.4.4.1 mass timber allowance.

References/Topics: 2021 IBC Section 508.4.4.1; Type IV-B and IV-C occupancy separations; NFPA 275 thermal barriers.

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