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Flat roof — spray polyurethane foam risks

FrankFulton | Posted in General Questions on

We are considering removing the ceiling and applying closed cell foam under the roof deck of a flat EPDM roof. What is the risk that when we reroof, we will have to remove the newly foamed plywood (eg, if there is ever a leak in the roof, or if the plywood ever gets wet, etc)? If the plywood does ghet wet, is there a workaround (ie, is there anything that can be done to minimize this risk)? It’s an expensive proposition to seal the underside of the deck, then have to tear it off in 5-10 years.

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  1. Aedi | | #1


    I don't have much experience with flat roofs, but I think it is unlikely the whole roof will be affected by water damage. More likely, only sections of the plywood would need to be replaced. Assuming damage isn't extensive you can patch it up the replaced plywood yourself with canned spray foam. I don't think anything can be done to minimize the risk, other than preemptively replacing the roof membrane.

    However, there are likely cheaper ways to add insulation to your roof. This article covers some possibilities:
    In general, it probably makes more sense to remove the existing roof and add rigid foam under it and/or add blown-in insulation to your attic. An added benefit to replacing the roofing is that you can use tapered foam sheets, which increase the effective roof slope and reduce the likelihood of leaks.

  2. Jon_R | | #2

    I expect that something that reduced the adhesion between the foam and the plywood would allow the plywood to be removed while leaving the foam intact.

  3. FrankFulton | | #3

    Thanks Aedi and Jon R. The roof is in good shape, so I don't think we want to reroof just yet.

    The worst case is the SPF adheres to the plywood, then has to be removed if plywood is ever replaced. That is highly undesirable. I'd love to hear from someone who has dealt with this issue, and lessons learned.

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