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?
Thanks!
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Replies
Ryan,
You wrote, "I cannot find any information anywhere about how many screws to use on that second layer of plywood."
Well, you should have started here at GBA. The answer is in one of my articles: How to Install Rigid Foam On Top of Roof Sheathing.
In that article, I wrote, "If you don’t expect a lot of snow, you can install the same number of screws that are recommended for furring strips installed on walls — basically, one screw every 24 inches along each rafter, with a minimum penetration into solid wood of 1½ inch — and you’ll have more than enough screws."
You should also read this article: Insulating Low-Slope Residential Roofs.
I hope that you are located in Climate Zone 1, 2, or 3. If that's where you are located, your planned assembly will work. However, if you are building in Climate Zone 4 or anywhere colder, you have a problem, because the polyiso isn't thick enough to meet code requirements.
-- Martin Holladay
Thanks Martin. I did read your article several times and concluded you were referring to fastening 2x purlins, not plywood. Thanks for the clarification. My inspector asked me to do option 3 above, so just tacking the first layer and screwing the snot out of the second.
Ryan,
I'm sorry if my article was unclear. Thanks for letting me know. I've edited it so that the meaning is clearer.
You should nail the first layer per code. It will be the structural diaphragm that distributes the imposed loads on the house to the walls. Roof diaphragms are designed with nails so the nails can be the weak link and deform to disperse energy. The second layer if it is just foam and no framing is similar to nail base panels. The nail base panels are foam bonded to sheathing. You can search for Nail Base SIP and various SIP panel sites will come up where you can look at their technical info to see the attachment schedule.