Can I add structural wood panels like plywood to a wall assembly?

Yes. The addition of wood structural panels in fire rated gypsum board wall assemblies is permitted as follows:

• Wood structural panels that are 4 ft. wide, minimum 7/16 in. thick oriented strand board (OSB) or 15/32 in. thick structural sheathing (plywood) complying with DOC PS1 or PS2, or APA Standard PRP-108, manufactured with exterior glue, may be applied horizontally or vertically to the framing members.
• Vertical joints should be centered on studs, and staggered one stud space from the gypsum board joints.
• The wood structural panels are permitted to be applied either as (1) a base layer (directly to the wall framing and under the gypsum board), (2) in between gypsum board layers, or (3) over the top of the completed gypsum board wall.
• When wood structural panels are used on top of the gypsum board layers of exterior wall assemblies, the wood structural panel should be protected from the exterior environment either as specified in the design or as specified here.
• 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.