APA’s Data File APA Rim Board in Fire Rated Assemblies, Form D350, provides recommendations for the use of APA Rim Board^® in conjunction with fire-rated wall and/or floor assemblies, based on tests conducted by the USDA Forest Service at the Forest Products Laboratory in Madison, Wisconsin. But can one use Rim Boards as a two-hour element without using gypsum drywall?

 

That’s a question I received recently. In theory, I think it can be done, but it would require at least three layers of Rim Board, as the overall rate of char advancement should slow as the system becomes thicker. The wall may also have to be thicker to provide adequate bearing length for the ceiling joists.

 

APA’s Rim Board assemblies were based on tests explained in the USDA/Forest Service FPL-RP-610, Fire Resistance of Engineered Wood Rim Board Products. Those tests included double Rim Boards, but the double layer assemblies did not meet the two-hour requirement. Perhaps three layers of Rim Board would suffice.

 

If you’ve successfully constructed such a system, let us know by adding a comment.