Fasteners used to attach sheathing through foam to roof
I am looking for suggestions on what fasteners to use to attach sheathing through foam to an 5/8 existing roof deck. We are wrestling with this issue right now on a project. We could easily spent $1500 on Fastenel large diameter head screws. We have considered roofing tins and 6″ long deck screws, but we still are not convinced that is the best way to go.
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I like these http://www.mcfeelys.com/product/TMR-4060/x-6-Extra-Long-Wafer-Head-Screws-Spider-Drive but they may not sink into Advantech if that's your sheathing. Will you be trying to hit the rafters or just going into the sheathing? It would be better to aim for the rafters if you can.
The screws used for installation of SIPs, available in lengths from 3" to 18", would be the appropriate choice, but they should penetrate 1½" into framing.
With 4" or more of foam thickness, I would also recommend applying a foam-compatible adhesive between foam and top sheathing to create a mono-lithic structure less susceptible to shear deformation under load.
Robert, when you say
"With 4" or more of foam thickness, I would also recommend applying a foam-compatible adhesive between foam and top sheathing to create a mono-lithic structure less susceptible to shear deformation under load."
Would that be adhesive between original deck and foam, and adhesive between new deck and foam, and 7" screws to mechanically secure new 5/8" deck through 4" foam old 5/8" deck and at least 1.5" into rafters ?