Fire Protection - Fire Resistance Rated Truss Assemblies

Fire Retardant Treated Wood (FRTW) products are permitted by code in certain situations where a fire resistance rating is required. Per the IBC the majority of buildings will fall into one of five different construction types, I, II, III, IV, or V depending on level of fire resistance of the building’s exterior structural frame. Each type of construction, except IV, also has an A and B designation based on the level of fire-resistance rating required for the building’s interior elements.

In Type III buildings, exterior walls are required to be built of noncombustible materials and interior may be any material allowed by code. Although FRTW framing is not considered a noncombustible material, the 2015 IBC Section 602.3 states that as long as the FRTW complies with IBC Section 2303.2, it shall be permitted within exterior wall assemblies of a 2 hour rating or less. 

One thing to note is that when horizontal assemblies (such as trusses that are a part of floor/ceiling and roof/ceiling assemblies) are bearing on exterior walls that use FRTW to comply with Type III requirements, these horizontal assemblies are NOT required to be made of FRT lumber. 

Top Resources

This report focuses on Type III construction, specifically requirements for the use of Fire Retardant Treated (FRT) Lumber in floor/ceiling and roof/ceiling assemblies bearing on exterior walls of Type IIIA buildings. Available as a sealed code compliance report. 

Provides guidance on the use of FRTW products in trusses and wall panels in above ground applications where allowed by code. Available as a sealed code compliance report. 

Where walls and ceiling assemblies meet, joints need particular attention in terms of fire protection. This report steps through the details of these requirements.

Best Practices

Design Detail for an exterior wall assembly with top chord bearing truss intersecting a 2-hour rated FRT wall assembly where the gypsum extends up to and below the bottom of the floor sheathing and is cut around the truss top chord. Available as a sealed code compliance detail. 

Design Detail for an exterior wall assembly with top chord bearing truss intersecting a 2-hour rated FRT wall assembly where the gypsum extends up to and below the bottom of the floor sheathing and blocking between trusses is provided for fire endurance. Available as a sealed code compliance detail. 

This guide provides an overview of the provisions a designer should make when using FRT wood in building designs. 

This guide points to resources for Type III-A building construction using fire retardant treated wood (FRTW).