Calculating Fire Resistance Ratings of Wood Assemblies Using the IBC

Architects and Contractors can spend a lot of time hunting for miscellaneous fire rated assemblies in wood construction. What you’ll eventually learn is that the easiest place to design your own 1-hour assembly is Section 721.6 of the International Building Code. The section details the procedures for calculating fire resistance of walls, floor/ceiling and roof/ceiling assemblies.

The total fire rating is determined by adding each component to reach a minimum total of 60 minutes. Fire protection rating is calculated from the flame side of the assembly and of course some assemblies have multiple flame sides.

Your wood framing members are required to be at a minimum of 16″ on center. When they are you get 20 minutes in a wall and 10 minutes with floor and roof joists.

Wall Finishes and their ratings:
3/8″ wood structural panel with exterior glue – 5 minutes
15/32″ wood structural panel with exterior glue – 10 minutes
19/32″ wood structural panel with exterior glue – 15 minutes

3/8″ gypsum wallboard – 10 minutes
1/2″ gypsum wallboard – 15 minutes
5/8″ gypsum wallboard – 30 minutes

1/2″ Type X gypsum wallboard – 25 minutes
5/8″ Type X gypsum wallboard – 40 minutes

3/8″ + 3/8″ gypsum wallboard – 25 minutes
1/2″ + 3/8″ gypsum wallboard – 35 minutes
1/2″ + 1/2″ gypsum wallboard – 40 minutes

Where that isn’t quite enough in wall assemblies you can add insulation for another 15 minutes. The spaces between studs must be completely filled with glass fiber mineral batts a minimum of 2 lb/cf or rockwool or slag material batts a minimum of 3.3 lb/cf or cellulose a minimum of 2.6 lb/cf.

Floors must have a subfloor of at least 15/32″ wood structural panels or 11/16″ T&G. Roofs must have a deck of at least 15/32″ wood structural panels or 11/16″ T&G and finished roofing.

There are quite a few additional rules for fire resistance so please ask questions.

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