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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?
 

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Instead of plywood, concider using T & G 2 X 6's or 8''s.
Much stronger, will look far better from the bottom side, added R value and will give you far more thickness for fastening. Most often the foam is screwed down with special screws with phillips head screws and fender washers built right onto the the screws. Any real roofing supply will have them.
 

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Instead of plywood, concider using T & G 2 X 6's or 8''s.
Much stronger, will look far better from the bottom side, added R value and will give you far more thickness for fastening. Most often the foam is screwed down with special screws with phillips head screws and fender washers built right onto the the screws. Any real roofing supply will have them.
+1

Great advice.

You may want to consider covering the T&G board with a peel and stick ice/water shield for proper air seal as well.
 

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First, don't apply the foam under the plywood but on top of the plywood.

That is the accepted practice and really can't figure out how to do the foam first.
 

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Strange to put down foam and then spray. Why not just put down 4" of foam and a membrane (EPDM, PVC, TPO...). These can be adhered with glue.
 
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