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Community and Q&A

Spray polyurethane foam on flat roof

FrankFulton | Posted in General Questions on

What are the pros/cons of SPF on top side of a flat roof, relative to exterior rigid foam insulation? We have a large (600sqft) flat roof in CZ4, and need to 1) increase R value, and 2) increase slope of the overly flat roof that is beginning to mildly pond. Of course, we’d like things to look aesthetically pleasing when complete. Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    For a full discussion of the use of spray polyurethane foam as roofing, see this article: "Roofing with Foam."

  2. BrianPontolilo | | #2


    What the plan for increasing the roof pitch and how much space with there be for the spray foam insulation?

  3. FrankFulton | | #3

    Martin great article, thank you. Do you have any first or second hand experience?

    Brian, the roof has several dormers and a fairly complex shape (12ft in some places, 18ft in others). One potential appeal of SPF is no carpentry - the slope could be made w the foam.

    To increase R and add slope, our options seem to be:
    1) tear down ceilings and foam underside of roof deck, which would not address roof slope;
    2) external foam, which would require carpentry; or
    3) SPF, which we’ve just learned about - which would be least attractive but perhaps “easiest” to execute and most effective solution. We would still need to address air sealing from beneath - perhaps by dense packing the rafter bays?
    4) Because of a breezeway, we could also do one large section from the inside, and another large section from the outside, but this seems aesthetically least appealing and perhaps overly complex.

  4. FrankFulton | | #4

    As a quick follow-up:
    Could we also "dense pack" cellulose in the 2x8 rafters, to air seal and add R value? Then, we could sum that interior R value (7.25in * 3.5 = 25) with the R value provided by the SPF roof (6.5/inch - say either 6>3 inch taper, or 4>1 inch taper)?

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