Fastener schedule for plywood / OSB roof sheathing over rigid foam
I am installing a new unvented cathedral ceiling in a desert. Roof is low-slope and has a structural plywood deck over 2x rafters, with 1″ polyiso on top of the deck, then another layer of plywood, followed by a built-up reflective membrane roof. I cannot find any information anywhere about how many screws to use on that second layer of plywood.
The way I figure it, I have three choices:
1. Fasten the first layer of plywood normally with 8d nails on a code-compliant nailing schedule. Tack the foam, then fasten the second layer of plywood with 4″ screws on the same schedule as the first layer. Seems ridiculous to drive 1200 nails into the roof and then follow with 1200 screws.
2. Fasten the first layer of plywood normally, tack the foam, and install the second layer of plywood with screws more sparingly. The second layer is just a nailbase for the built-up roofing, after all, and 4″ screws have a heck of a lot more holding power than 8d ring shanks. So why use so many? But if this is the way to go, how many screws do we need? Seems like we would still need quite a few to properly secure that second layer and keep it flat/level.
3. Fasten the first layer of plywood loosely with just enough nails to hold the panels in place, tack the foam on, then install the second layer of plywood with 4″ screws on a code-compliant schedule. This plan makes the most sense to me.
I intend to consult with my inspector and will do what he suggests, but I am curious what you all think. What is best practice?
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