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Community and Q&A

Low Slope Roofing – Foam vs IB PVC

Anon48858483 | Posted in General Questions on

I am very confused in trying to compare my options for replacement of a leaky tar and gravel roof. I am deciding between foam and IB PVC (but am open to suggestions if I’m on the wrong track). 

What are the pros and cons of each in terms of indoor air quality and off gassing? The decking is tongue and groove with rigid foam on top so I believe some off gassing to the inside would occur? With wider concerns about PVC and a movement toward PVC-free products, do I want to be covering my roof in PVC? On the foam side, it seems like there are risks also, mostly during application but also in the case of curing problems. The contractor for the foam told me there is no SDS because it’s completely inert, which doesn’t sound correct and seems like a red flag. 

I don’t want to be too paranoid, but also want to do my due diligence. Any help/insights/opinions would be much appreciated!

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  1. Anon48858483 | | #1

    No one has any opinions to share? Any advice about where I can look for answers?

  2. Expert Member
    Michael Maines | | #2

    Flexible PVC has plasticizers which have various problems associated with them, including being endocrine disruptors. But with roofing, any offgassing will be into the great outdoors, not the interior of your house, unless you have an egregiously leaky building.

    Are you suggesting foam alone as another roofing option? I've never heard of that; I only know about rigid foam as an underlayment.

    When forced to work with low-slope roofs, I specify EPDM, as it's the product I have the most experience with.

  3. Expert Member

    Anon ...483,

    I don't know if this is an option for you, but when faced with replacing low-slope roofs I have frequently instead over-framed a structure to increase the pitch so that better performing materials can be used. Here is a cabin I did that with about a decade ago:

  4. Anon48858483 | | #4

    Thanks for the reply, I will take a look at EPDM.

    Yes, that is my concern about the PVC. I hope you are correct that most offgassing would be to the outside, but am unsure especially since they are cathedral ceilings with tongue and groove decking.

    The foam is spray polyurethane foam and recommended by one of the roofers I got a bid from. They seem to get good reviews, but I assume the risks are similar to spray foam insulation.

  5. aaron_p | | #5

    I wouldn't do spray foam roofing - I have seen it done on some commercial jobs and it felt like a temporary measure. The foam application is hard to get smooth and thus creates lots of potential ponding. These systems are waterproofed with a coating that is also sprayed over the foam. This coating would not have anywhere near the durability of a true single ply membrane roofing material such as EPDM, TPO, or PVC. EPDM and TPO would both be sufficient and more cost effective - I have typically only specified PVC in applications requiring chemical resistance (due to lab exhausts or near a helipad for example).

    Edited to add: I would compare warranty terms between these quoted systems, especially against the spray foam if you consider that route.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      Imagine trying to remove or remediate it?

      1. aaron_p | | #8

        The one client that used it would just have it covered at its end of life with another layer - every 5-10 years. On one hand it probably does help with insulation value, but yeah overall has to be terrible for the environment as a whole.

  6. Expert Member
    PETER Engle | | #7

    Spray foam roofing is somewhat popular in the southwest. I've only rarely worked with it in the northeast, and those were all failure investigations. Finding leaks in the stuff is really hard. Plus, end-of-life removal and replacement is a nightmare. I wouldn't specify it for anything but a commercial building in a desert climate.

    For those of us who deal with rain and snow, I generally prefer EPDM or modified bitumen. EPDM is a bit newer and easier to get done right than mod.bit., but both can produce long-lasting and leak-free roofing systems. Like anything else, it is important to follow manufacturer's installation recommendations. They all have entire books of flashing and other details that should be considered mandatory.

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