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Fastening Schedule for Rigid Foam Above Roof Sheathing

4khz | Posted in General Questions on

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.

thanks

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Replies

  1. Expert Member
    KYLE WINSTON BENTLEY | | #1

    There's a building science report out there that has information on this. It's reduced spaced (compared to walls with heavy vertical loads) is because roofs are much more horizontal surfaces compared to walls. At typical roof pitches, this equates to about half the load per square ft, compared to the same weight hanging on the wall.

    It was found that most rigid foams, even mineral wool, didn't really deflect that much (1/16 max or something like that) under 25 psf load (vertically) using #9 or #10 screws, 16" OC, through some thickness of foam. If you double that, for the roof, you could handle 50 psf at 16" OC, which is why the recommendation exists for 24" OC spacing. Most places in the US (as a % of total land area) do not have these kinds of loads. So, if you do, just crank that spacing down until you own a significant share of fastenmaster.

    --------------------
    Unrelated note: make a test setup out of a piece of your rafter stock, sheathings, insulation, and all that and practice getting into those rafters on the first try. I'm not going to say it's impossible, but you need to be prepared for something like a 10% first-miss rate. Those are holes all the way through the assembly and need to be sealed from underneath.

  2. 4khz | | #2

    Kyle,

    thank you for the response and info. I'm not quite certain with 24" rafter spacing and 5 1/2" of insulation. I just don't have the knowledge to come to a comfort level to proceed with the 24" fastener spacing. On my addition, I have 16" rafter spacing and 4" of rigid . I feel comfortable with 24" fastener spacing here.

  3. Expert Member
    AKOS TOTH | | #3

    I think the only person that can answer your question is a structural engineer.

    For a personal piece of mind you can look at the fastener schedule for something like Hunter panels:

    https://www.hunterpanels.com/docman-categories/product-documents/hpanels/speciality-products/1388-h-shield-nb-application-guide/file

    Note they specifically do not fasten into rafters.

    1. 4khz | | #9

      AKOS - Hunter Panel Tech support was pretty helpful. He stated that it's ok to hit the framing as well as sheathing. He also confirmed the fastening schedule would be the same for multiple layers of rigid foam with plywood sheathing (versus the one piece panel).

      It would be much easier to go to the sheathing, but there must be drawbacks. I'm not quite sure how my roofer is going to make out with 8" screws to framing. I'm just glad to have finally found a roofer willing to do it. Are there any concerns with fastening to the sheathing only ?

  4. 4khz | | #4

    Akos - thanks for the reference. That does help with my comfort level. Yes, you're right, the hunter panels specifically state to NOT fasten to framing. I'll call Hunter and see if I can find out. I don't see much difference between a couple layers of polyiso w/ plywood sheathing versus their panel. Additionally, is it viable to seal 2,000 holes from underneath ?

    1. Expert Member
      KYLE WINSTON BENTLEY | | #5

      You only need to seal a hole if there's nothing in it, IE a nail that has been removed, a screw that had to be moved, etc. Otherwise it's not important.

      1. 4khz | | #7

        Thanks Kyle. Hunter Panel Tech support was pretty helpful. I believe his name was Kyle also. He stated that it's ok to hit the framing as well as sheathing. He also confirmed the fastening schedule would be the same for multiple layers of rigid foam with plywood sheathing (versus the one piece panel).

        There was a passive house project I read here on GBA where they were sealing the missed screws. good to know it's not necessary.

  5. user-6184358 | | #6

    Look at the Simpson Fastening Systems technical guide on pg 53 has values for attaching foam to walls. It provides the footnotes and basis to design a solution for a roof. Add the ICC report on the screws and you can do the math to justify the answer.

    https://www.strongtie.com/resources/literature/fastening-systems-technical-supplement

    1. 4khz | | #8

      user-6184358 thanks for the info. very helpful, particularly with reference to the various rigid foam thickness up to 6". In reviewing multiple technical/installation guides, it appears that I could go with 15 fasteners per sheet (20 on the perimeter) either fastened to the sheathing or the framing, or both. Obviously, it would be much easier to fasten to the sheathing. So now I'm wondering what the advantages/disadvantages are for sheathing versus framing

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