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Unvented slate roof: Best underlayment to prevent sheathing rot?

jcrittenden | Posted in Energy Efficiency and Durability on

We are removing and resetting our slate roof in southern Oklahoma.  So, a somewhat hot and humid climate without too much cold weather.  The attic space is unvented and a conditioned space with about 6″ of open-cell spray foam.  So, I guess you would call it a “hot roof” of the slate variety.  The roof sheathing is made of 1 x 12 boards with no gap really.  My question is what is the best underlayment for this situation?  The contractor would like to use the peel & stick ice/water shield, but is also fine with the 30# felt paper.  It is really just a matter of cost to us.  The ice/water shield would give me a little more comfort about any future leaks, but would there be a risk of the sheathing rotting?  Or, would the rot risk be more with the felt paper?  (Please note, that rigid foam on the exterior is not an option.)

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  1. GBA Editor
    Martin Holladay | | #1

    The best roofing underlayment to use in this case would be a vapor-permeable synthetic roofing underlayment, followed by 1x3 or 1x4 skip sheathing (to provide an air space under the slates). The distance between the courses of skip sheathing should equal the slate exposure.

    Here are examples of vapor-permeable synthetic roofing underlayments:

    Perma R Products PermaFelt
    GAF Materials Deck-Armor
    Cosella-Dörken Delta-Maxx Titan
    VaproShield SlopeShield
    Cosella-Dörken Vent-S
    Nemco Industries RoofAquaGuard BREA
    Cosella-Dörken Delta-Foxx

    Note also that the use of open-cell spray foam under attic roof sheathing is risky. For more information on ways to mitigate that risk, see this article: "High Humidity in Unvented Conditioned Attics."

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