Nailing Rigid Foam Boards to Plywood
Live in SF Bay Area (CA) in an Eichler-style house built with a low-sloped, hot roof and no insulation. The existing roof system is composed of (top to bottom) tar & gravel roofing, celotex fiber board, and 3”x 6” exposed ceiling beams placed 24” OC. Plan to re-sheath the beams, add insulation and re-roof.
Would like to install 3/4" structural plywood above the beams, then rigid polyurethane boards/panels (1”-2” thick), followed by external roof (most likely a spray-applied foam). Using closed-cell foam for higher R-values.
My question is how to attach the rigid foam panels to the plywood. Since the plywood’s lower surface is exposed to the interior of the house, the nails can’t penetrate the plywood or else they would be visible. Full nailing depth is limited to areas where the nails are located immediately above the beams.
How do I secure the rigid foam panels to the substrate and stay within building code for fasteners and nailing patterns to plywood sheathing. Are nails allowed to "not" penetrate the structural plywood and is there a minimum nailing depth?