Boise Cascade Type III Construction Updates for Wall-Floor Intersections
Boise Cascade Company
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Recently, The International Building Code (IBC) updated the fire resistance requirements for the wall-floor intersection in Type III Construction exterior walls. It can be confusing so we'll guide you through the updates.
First, a little background: Type III Construction has allowed for the use of fire-retardant-treated-wood (FRTW) studs and sheathing instead of noncombustible materials within exterior walls requiring a 2-hour fire resistance rating. Type IIIA floor/ceiling assemblies are required to have a 1-hour fire resistance rating.
Up until the latest code cycle, the IBC had not clearly defined the requirements for the wall-floor intersection when utilizing FRTW walls, so code enforcement professionals were left to interpret the requirements with little direction.
The 2024 IBC now provides additional clarity which allows non-fire-retardant-treated wood (FRTW) rim board on wall-floor intersections. Boise Cascade manufactures rim board that you can use for this application. Let's break down the updates.
705.7.l Floor Assemblies in Type III Construction
The Code: In Type III construction where a floor assembly supports gravity loads from an exterior wall, the fire-resistance rating of the portion of the floor assembly that supports the exterior wall shall not be less than the fire-resistance rating required for the exterior wall in Table 601.
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What it means: The rim board needs the same fire-resistance rating as the wall, in the case of Type III, 2 hours.
The Code: The fire-resistance rating provided by the portion of the floor assembly supporting and within the plane of the exterior wall shall be permitted to include the contribution of the ceiling membrane when considering exposure to fire from the inside.
What it means: The floor/ceiling assembly can be counted as part of the rim board fire resistance rating. E.g., a 1-hour floor/ceiling assembly will provide 1 hour of protection to rim board.
The Code:?Where a floor assembly supports gravity loads from an exterior wall, the building elements of the floor construction within the plane of the exterior wall, including but not limited to, rim joists, rim boards, and blocking shall be in accordance with the requirements for interior building elements of Type III Construction.
What it means: The rim board does not need to be FRTW, same as with interior building elements. To achieve the second hour of protection, we have a few options. The American Wood Council (AWC) published DCA-3, which details wall-floor intersections and provides instructions to build up a sacrificial rim to achieve that hour of protection.
Want to learn more? Boise Cascade has also published details based on the methodology originally presented in DCA-3 and adopted into the 2024 IBC code.
For additional clarification, see the following DCA-3 and 2024 IBC code complying details utilizing Boise Cascade EWP for Type III construction.
Do you have additional questions regarding multi-family/light commercial construction? Contact Jeff Bryce ([email protected]) or Zach Shearer ([email protected]) with Boise Cascade EWP.