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Preferred underlayment under metal on unvented roof?

Robert McKee | Posted in General Questions on

Hi Everyone,
Need to make a final decision on which underlayment to use under a metal roof on an unvented roof system. The roof is currently built as follows: Beams on 5′ centers; 1 1/2′ T&G decking; 3/8″ plywood diaphragm – seams taped; with TriBuilt granulated self adhering underlayment on top of the plywood.

Next I’ll be screwing down 5 1/2″RayCore SIPS panels over heated space and framing boxes over the eaves covering everything with 5/8″ CDX. Still looking at metal roof products but am zeroing in on a high seam (1 3/4″) snap lock system. Roof pitch is 2/12.

Question: What would be the preferred type of underlayment be for this application? I’d do a high temp ice & water shield in a minute but am concerned about the need for the RayCore panels and perimeter framing to dry to the exterior. I&W under the metal roof would prevent that is my understanding.

Thank you!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Robert,
    If the manufacturer of your RayCore SIPs requires that the SIPs be able to dry to the exterior, you should certainly follow the manufacturer's installation instructions.

    Certainly, experience shows that SIP roofs are less risky when the roof has a ventilated air gap between the SIPs and the roofing. This ventilated air gap is especially important in cold climates. (For more information on this issue, see Air sealing SIP seams.)

    If you want the SIPs to dry to the exterior, this would be the typical stackup, from the interior to the exterior:

    - Whatever you want on the interior side of the SIPs (as long as the seams between the SIPs are sealed with tape)

    - SIPs

    - Asphalt felt (or vapor-permeable roofing underlayment)

    - A ventilated air gap (usually created with 2x4s, laid soffit-to-ridge, either 16" or 24" on center, installed on the flat to create a 1.5-inch-deep channel)

    - Plywood or OSB sheathing

    - Almost any type of roofing underlayment (if you are in a hot climate, you may need to seek out a high-temperature underlayment)

    - Metal roofing

  2. Robert McKee | | #2

    Thank you for the quick response Martin. I forget to mention in the original post that the project is in Portland, Oregon.

    Am interested in keeping this an unvented roof assembly without adding an air gap as you suggested. Agree with your comment though that an air gap does reduce risk which has given me pause to consider...

    Back to the original question based on the current design that does not include a ventilated air gap:

    How vapor-permeable should the underlayment between the metal roof and the sheathing applied to the SIPs panels be to allow for drying to the exterior?

    Is this even a concern? (Plan to follow-up with RayCore tomorrow for their requirements though their literature shows roofing felts on their unvented roof assembly stackup.)

    My roof stackup as currently designed and half built:

    - 2x6 T&G decking
    - 3/8" CDX - seams taped
    - 100% Granulated self adhering underlayment (TriBuit)

    - 5 1/2" SIPs panels
    - 5/8" CDX
    - High Temp Underlayment
    - Metal Roof

    Thanks again!

  3. GBA Editor
    Martin Holladay | | #3

    Robert,
    With most forms of metal roofing, there is no drying to the exterior.

    If you install your roofing on 1x4 or 2x4 purlins, 24 inches on center, parallel to the ridge, you can have drying to the exterior.

    Ordinary asphalt felt is a smart vapor retarder, although a vapor-permeable synthetic roofing underlayment (for example, Perma R Products PermaFelt, GAF Deck-Armor, Cosella-Dörken Delta-Maxx Titan, VaproShield SlopeShield, Cosella-Dörken Vent S, Nemco Industries RoofAquaGuard BREA, or Cosella-Dörken Delta-Foxx) will allow more outward drying than asphalt felt.

    The vapor permeance of the roofing underlayment is irrelevant, however, unless you include a ventilated air gap between the roofing underlayment and the metal roofing.

  4. Expert Member
    Malcolm Taylor | | #4

    Martin,
    In several recent discussions posters have recommended proprietary underpayments that sound like versions of Cedar Breather or Home Slicker for metal roofs. Have you had any feedback on whether these products actually allow the roof sheathing to dry to any meaningful extent?

  5. GBA Editor
    Martin Holladay | | #5

    Malcolm,
    I haven't used one of those products, nor have I seen any data from researchers. But I think the concept is a good one. Thanks for the reminder of another possible approach.

  6. Expert Member
    Malcolm Taylor | | #6

    Thanks Martin,
    I asked because i'm a bit sceptical. They would provide a drainage plane for any moisture, and a capillary break (although in this context I'm not sure how much use that would be), but on a good sized roof I wonder how much they would actually allow the sheathing to dry outwards.

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