Fastening Schedule for Rigid Foam Above Roof Sheathing
In the GBA article titled “installing rigid foam above roof sheathing”, Martin says this about fastening the upper layer of sheathing:
“When fastening the upper layer of plywood, OSB, or 2x4s above the rigid foam, use the same number of screws that you would use 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. That fastening schedule is more than enough, with one exception: climates with lots of snow. If you live somewhere with lots of snow, you should consult an engineer.”
I have two questions:
1. What constitutes “a lot of snow”. I’m about 50 miles north of NYC and the snow load design criteria in my town is 30 p g. I don’t think my location qualifies as “lots of snow”, but looking to confirm.
2). A portion of my roof has rafters @ 24″ oc. Does the fastening schedule of 24″ along the rafters still hold true ?
This portion of roof is 2×6 scissor truss. On top of the rafters is 1/2″ plywood, then i’m adding 5 1/2″ of polyiso and another layer of 1/2″ plywood which will be fastened with 8″ fastenmaster headlok screws every 24″ (hopefully) along the rafters.
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