Roof SIPs and 2012 IRC R806
WIth an 8″ polyurethane SIP (OSB skins), would the 2012 IRC R806 code apply for roof venting or is that only applicable for ceiling with roof rafters?
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The 2012 IRC permits unvented roof assemblies, including unvented SIPs. With an unvented SIP roof, the argument would be that the exterior OSB facing constitutes the "structural sheathing" referred to in the code.
Therefore, an unvented SIP roof is permitted under IRC R806.5, "Unvented attic and unvented enclosed rafter assemblies":
"Unvented attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) and unvented enclosed rafter assemblies (spaces between ceilings that are applied directly to the underside of roof framing members/rafters and the structural roof sheathing at the top of the roof framing members/rafters) shall be permitted if all the following conditions are met: .... 5.1. Air-impermeable insulation only. Insulation shall be applied in direct contact with the underside of the structural roof sheathing."
What would you recommend for the screw spacing on the 2x4 furring studs? I assume a 12" spacing would be adequate.
Is the same 12" screw attachment spacing good for the OSB sheathing?
I'm not an engineer. If you have any doubts, talk to an engineer.
When this type of 2x4 is installed above a rafter, and screwed through rigid foam to the rafter below, Joe Lstiburek advises one screw every 24 inches along the 2x4. (For more information, see How to Install Rigid Foam On Top of Roof Sheathing) -- unless you live in a snowy climate, in which case you should talk to an engineer.
Your case is different, because you are screwing into SIPs (presumably). If you are simply screwing into the OSB facing, you should certainly consult an engineer, because I don't know the answer to that one.
When it comes to the upper layer of roof sheathing -- the OSB above the 2x4s -- you should comply with code requirements, which can be found in Table R602.3(1). According to that table, 1/2" roof sheathing requires 6d or 8d common nails every 6 inches on the edges of the sheet and every 12 inches along intermediary framing. If the sheathing is thicker that 1/2", you can't use 6d nails, only 8d nails.