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Screw size for attaching second layer of sheathing through 3.1” polyiso

andykahn | Posted in General Questions on

I am building a 2/12 sloped roof with an unvented cathedral ceiling. I am screwing 1/2” plywood through 3.1” polyiso, 5/8” zip sheathing and into I joists.  GRK makes a 5 5/8” construction screw which they call 12/14 gauge. It’s the perfect length but I’m unable to figure exactly what gauge the screws should be and how many to use per sheet. I’m in Ashland Oregon (4C) with minor snow load and modest wind speed. There will be framing around the perimeter of the roof to nail into but no wood in the field. 

does anyone have a source for that information or a recommendation of the right screw and spacing?


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  1. Patrick_OSullivan | | #1

    This is exactly the sort of thing you need to consult the appropriate design professional for in your particular jurisdiction.

  2. Expert Member
    Akos | | #2

    You can start with the screw schedule for something like Hunter roof panels, but like Patrick pointed out the proper answer is get it designed by a professional.

    I find roofing deck screws used for attaching rigid to flat roofs (sold in big pales at commercial roofing suppliers) come in a variety of lengths and are much cheaper.

  3. kbentley57 | | #3


    You can find the design values from their technical docs website. Here's one PDF that has some of the info you probably need.

    Here is another resource from the american wood council, that has the general theory on the lateral load calculations for dowel type fasteners

    My personal thoughts are this -

    A 2:12 pitch is only 10 degrees, so the lateral or shear load is only a small fraction of the total load that needs resisted by the screws, and the frictional forces between all the components. The top flange of most I joist is 1.5". That's the side width thickness in those documents. Without doing the load calculations myself, but with experience building roofs like this, I would think 24" OC would be sufficient.

  4. Expert Member
    Michael Maines | | #4

    I can't imagine getting a flat surface with 1/2" plywood over 3" foam; it will compress at every screw unless you use a lot of screws and you're very careful about how far you drive them in. I always spec solid furring, either 5/4 or 2X for roofs, which spreads out the load. You can fill in between the furring with more foam or leave the bays open for ventilation.

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