One Hour Fire Rated Assemblies
Question:
The Gypsum Association “Fire Resistance Design Manual” (GA-600-12) lists file RC2601 with two layers of 5/8 in. gypsum wallboard (GWB) on wood joists. The last sentence in the report says, “this provides 1-hr protection for wood framing, including trusses.” It seems that the primary difference between this and UL P522 is the additional layer of gypsum wall board vs. metal furring. Is RC2601 an acceptable system when wood trusses span an exit corridor with attic storage above? How about the top of an elevator shaft wall?
Answer:
Generally, there are two types of 1-hour wood truss assemblies: the first is one layer GWB with metal furring strips or resilient channel, and the second is two layers GWB board. An additional difference between the two assemblies is that P522 allows insulation in the cavity and RC2601 does not.
Section 17a and section 17b of SBCA’s Metal Plate Connected Wood Truss Handbook contains detailed information on the 1-hour fire assemblies for trusses. L534, L528, and FC5515 are all one–GWB-layer wood truss assemblies. L542 and FC5512 are both two-GWB-layer assemblies. For additional information on fire-resistance rated assemblies see SBCA Research Report 1509-01: Fire Resistance Rated Truss Assemblies.
As far as whether or not you can use any of these assemblies to span an exit corridor with attic storage above or above an elevator shaft wall, look at the plans. Do the plans call for a 1-hour assembly or a 2-hour assembly? If trusses are specified, then these assemblies can be used to achieve the desired fire rating, as long as all the details of the assembly requirements are met.
Keep in mind that most of the wood truss fire-rated assemblies call for a minimum depth of 12 in. Therefore, make sure that parallel chord trusses are at least this depth and pitched top chord trusses have at least a 12 in. high heel, unless specifically permitted as in the case of UL P522.