Plywood Panels in UL Fire Resistant Wall Assemblies
Q. Can plywood panels be added to a UL fire-resistance rated wall assembly to increase structural stability?
A. Guide Information for the Fire-resistance Ratings (BXUV) product category describes, among other details, acceptable variations to individual fire-resistance rated assemblies.
Section II, item 13 of this guide includes a description of wood structural panels and the requirements they must meet. Section VI, item 6 of the guide on walls and partitions indicates that the addition of wood structural panels in fire-rated gypsum board wall assemblies is permitted provided:
- The panels are 4 feet wide, minimum 7/16-inch thick oriented strand board (OSB) or 15/32-inch thick structural sheathing (plywood) complying with DOC PS1 or PS2, or APA Standard PRP-108, and manufactured with exterior glue.
- The panels may be applied horizontally or vertically to the framing members, provided vertical joints are centered on studs, and staggered one stud space from the gypsum board joints.
- The wood structural panels can be applied as:
- A base layer (directly to the wall framing and under the gypsum board)
- Between gypsum board layers or
- Over the top of the completed gypsum board layers
- When wood structural panels are used over gypsum board layers of exterior wall assemblies, the panels should be protected from the exterior environment as specified in the design or as specified in the exterior walls requirements in the guide information.
- When wood structural panels are added to wall assemblies that include furring channels, there should be no more than two layers (either gypsum board or wood structural panel or combination thereof) attached to the furring channel.
- When wood structural panels are added to the wall assembly, the length of the fastener used for the outermost layer (either gypsum board or wood structural panel) should be sized appropriately to accommodate the additional thickness of the wall panel.