Understanding Fire Design in Mass (Heavy) Timber Construction: A 10-Minute Read

Mass timber construction is rapidly gaining popularity as a sustainable and efficient alternative to traditional steel and concrete building methods. With this growth, ensuring the fire safety of these wood-based systems has become a critical focus for architects, engineers, and code officials. This article summarizes the fire design principles for mass timber buildings, particularly as defined in the International Building Code (IBC) and the AWC Fire Design Specification (FDS).

The 2021 IBC introduced three new mass timber construction types: Types IV-A, IV-B, and IV-C. These classifications expand on traditional heavy timber (Type IV-HT) and impose stricter fire resistance requirements. Key features include:

• Use of either mass timber or non-combustible materials for structural elements.

• Non-combustible protection for most mass timber surfaces, especially in Types IV-A and IV-B.

• Automatic sprinkler systems required in all three types, with redundant water supplies mandated for buildings over 120 feet.

• Prohibitions on combustibles in concealed spaces, with some exceptions.

Fire resistance ratings for structural elements are outlined in IBC Tables 601 and 705.5. In Type IV-A, primary structural frames and bearing walls must meet a 3-hour rating; floors require 2 hours. Types IV-B and IV-C demand 2-hour ratings for most structural elements and 1-hour ratings for roofs. These ratings are similar to those required in Types I-A and I-B (non-combustible construction), though height and area allowances for mass timber buildings are more limited.

To help designers meet code requirements, several key documents are used:

• AWC Technical Report 10 (TR10) and Chapter 16 of the National Design Specification (NDS) provide mechanics-based methodologies for calculating fire resistance.

• AWC Fire Design Specification (FDS) expands on these resources, offering detailed guidance on char depth, structural fire resistance, and protection strategies.

The FDS supports designs that go beyond the 2-hour limit imposed on exposed members by including provisions for additional wood cover, gypsum board, and insulation as fire protection.

The FDS allows engineers to calculate char depth and effective section properties for exposed wood members using models validated by empirical data. For cross-laminated timber (CLT), a distinct model accounts for edge joint behavior and lamination thickness. Simplified and full mechanics-based methods are both supported.

For assemblies like timber decks, the FDS introduces triangular char profiles to improve accuracy in fire resistance calculations, outperforming traditional NDS methods, especially for thicker decking.

Connections between structural members must achieve the same fire resistance rating as the members themselves. Unlike the wood elements, connections do not have inherent fire resistance and must be protected entirely. Two code-approved methods are:

• Full-scale ASTM E119 fire testing.

• Engineering analysis limiting average temperature rise at the connection to 250°F.

The FDS provides calculation methods for designing wood and gypsum board protection to achieve required thermal separation. Additional wood or gypsum layers can be applied to delay heat transfer to the connection. Importantly, only the material layer directly adjacent to the connection counts toward thermal separation.

Special care is needed at intersections of wood members to account for char penetration, which may extend deeper at abutting edges. The FDS requires added blocking or protective collars to prevent early exposure of connection hardware. Diagrams and examples are provided to assist in proper detailing.

Conclusion

The evolution of mass timber construction has brought new opportunities—and challenges—in fire design. The AWC Fire Design Specification offers comprehensive tools to meet code requirements for fire resistance in both structural members and connections. With continued development and empirical validation, the FDS supports safer, taller, and more sustainable mass timber buildings. For any further inquiries regarding this topic, as well as for code consulting and fire engineering design support related to your project, please don’t hesitate to contact us via email at contact@engineeringfireprotection.com.

Application of any information provided, for any use, is at the reader’s risk and without liability to Engineering Fire Protection (EFP). EFP does not warrant the accuracy of any information contained in this blog as applicable codes and standards change over time. The application, enforcement and interpretation of codes and standards may vary between Authorities Having Jurisdiction and for this reason, registered design professionals should be consulted to determine the appropriate application of codes and standards to a specific scope of work.