Achieving a 2 Hour Floor Ceiling Assembly From the Underside Only

March 21, 2009


One of our current projects is a change of occupancy where the change will require a 2 hour rating between the main level A-2 occupancy and the R-2 occupancy above. The challenge in these situations is that you do not want to tear into the floor or change out the floor framing members.

In new construction the entire floor assembly is often use to achieve the rating. However the Gypsum Association manual has a new note for certain assemblies. “Ceiling provides two hour fire resistance protection for wood framing.” Where that note is included the entire rating is provided by the gypsum and you don’t need a particular floor.

On a related note, the woman who answers the phone at the Gysum Association is very knowledgable about fire ratings.

Calculating Fire Resistance Ratings of Wood Assemblies Using the IBC

February 20, 2009

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.