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Water-resistant materials for use under EPDM roof?

FixitAgainSam | Posted in General Questions on

Hello, I am planning to install an EPDM membrane on an unvented low slope roof insulated from below. The structural sheathing will be new. I don’t like the idea of adhering the EPDM directly to the sheathing, but the EPDM seems to be incompatible with most ice/water barriers that one usually puts under shingles. I am worried that something will poke a hole in the rubber at some point, and until discovered, this would expose the sheathing to rain, rot, and potentially a complete re-do. So I am trying to figure out a solution.
So far, my idea is to cover the sheathing with the usual ice & water barrier, such as Grace, then add a thin layer of something that the EPDM can adhere to. The candidates seem to be fiberboard or Dens-Deck, or perhaps another thin layer of plywood or OSB. But I am not sure any of those can really tolerate exposure — I’m presuming it could be months between inspections of the roof. Installation guides for fiberboard and Dens-Deck both say to avoid exposure, get it covered the same day. Perhaps another possibility is Huber’s Zip System, but their techs told me it was rated for EPDM if applied over an OSB structural sheathing, and I am intending to go with plywood there. EPDM can be glued to Polyiso sheets, but I am having trouble figuring out exactly what kind — what kind of facing, and would 1/2 inch of it have sufficient compressive strength, etc.
Also, this layer of whatever, sandwiched between the EPDM and the ice/water barrier, can’t really dry. I can construct an eave detail to allow a weep channel between the layers, but that’s about it. Yes, this extra layer under the EPDM is intended to be sacrificial, in the worst case scenario, but it is easy enough to patch EPDM, so I would prefer a substrate here that could stand up well to the weather.
Any suggestions?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    Here's the way a traditional roof works: The roofing layer is designed to be waterproof. When this layer wears out, or when someone accidentally penetrates the roofing layer, the roof leaks. Once the roof leaks, it's time for a roof repair or a roof replacement.

    Grace Ice & Water Shield was invented to prevent ceiling damage due to ice dams at eaves. It works, sort of. Roofers who believe in covering 100% of the roof sheathing with Ice & Water Shielf are misguided, in my opinion, unless there are unusual circumstances that need to be addressed.

    Trying to install two vapor-impermeable layers on your roof, with something like fiberboard sandwiched in between, is a recipe for a mess. Once this soggy sandwich finally fails, the unfortunate roofer who has to clean it up will curse you.

    All roofing eventually fails -- whether you are talking about asphalt shingles or EPDM. That's normal. When it fails, we repair it or replace it. Don't try to invent a double roof. Trust the developers of details appropriate for EPDM, and follow the manufacturer's advice on installation details.

  2. jackofalltrades777 | | #2


    What's the longest you have seen a metal roof go without needing to be fixed?

    I was told by a roofer that metal roofs are 50+ years.

  3. GBA Editor
    Martin Holladay | | #3

    An unpainted galvanized metal roof can begin rusting after only 15 years, but a copper standing-seam roof or a stainless-steel standing-seam roof could last 80 years or more.

    Reportedly, acid rain in some regions is shortening the life of copper roofs.

  4. Expert Member

    The most common metal roofs here have a factory painted finish. Aesthetically you might have some discolouration issues after 30 years or so, but the integrity of the roof should last 50+.

    Roughly in order of longevity:
    - Galvanized
    - Galvalum
    - Factory painted
    - Kynar finish
    - Copper

  5. jackofalltrades777 | | #5


    My roof is both Galvalum and Factory Painted. MBCI makes it. Trapezoidal standing seam with hidden fasteners.

  6. Expert Member


    I think most factory painted roofs start with a gavalum panel. That makes them doubly resistant to wear. Something strange i've noticed when installing those roofs is if I inadvertently scratch the finish, when I'm done and go back to touch it up, they seem to self-heal, making it hard to find the mark.

  7. FixitAgainSam | | #7

    Hi Folks,
    I don't see where to check or reset my username, it was Fix It Again Sam last time I logged in. But thanks for your reply, Martin. I guess I *am* trying to implement some kind of double roof. When you go through the trouble and cost of insulating under the roof sheathing, it would sure be nice if next time a re-roof is needed, you don't have to tear off the sheathing and redo the insulation and the ceiling. I agree that it's predictable that a roof will fail, but I'm looking for a secondary line of defense.
    I've re-read the discussion in this forum titled "Rainscreen Roof?". Maybe that would be a better approach. What would you think of a build-up of structural sheathing, breathable membrane like GAF Deck Armor, purlins (just 3/8 inch, after reading Lstiburek's "Mind the Gap" article), 1/2 inch ply or OSB with EPDM adhered? The tiny air gap would be open (screened) at the eave only, so it's only a drain channel, not much of a vent, yet it does avoid trapping either layer of sheathing between impermeable layers. I'm hoping this would confine roof failure to the top, outer layers, and if one saw any dripping from the air gap, that would be a clue to inspect the roof. But I don't know whether allowing the two layers of sheathing to "breathe" would actually be better than trying to seal them.
    For those of you discussing metal, has anyone ever tried Lock Dry Decking as a flat roof metal cover? Looks like it would do what I'm after, but it is pricey.

  8. GBA Editor
    Martin Holladay | | #8

    Fix it again Sam,
    You don't want a sandwich of peel-and-stick, fiberboard, and EPDM.

    If your roof was not a low-slope roof -- if it was, say, a roof with a 4-in-12 pitch -- you could install metal roofing over purlins, with roofing underlayment (asphalt felt) under the purlins and above the roof sheathing. This type of roof has a secondary layer (the roofing underlayment) to catch drips, and an air space to allow the drips to evaporate.

    That approach isn't possible with a low-slope (flat) roof. So you should just proceed with the standard details for an EPDM roof, as provided by the EPDM manufacturer.

  9. Dalton07 | | #9

    If you are confused about choosing a coating for your RV, I would recommend looking at a TPO vs EPDM RV roof. In terms of cost, EPDM will be more economical than TPO. Speaking of durability, EPDM's rubber compound is well known; it's UV resistant and known to stand up to cold weather conditions. As for TPO, its seam strength can outperform both tapes and adhesives in EPDM settings. In terms of maintenance, a properly installed EPDM roof won't require much in the first place; you just need to apply the adhesive occasionally. But since the TPO membrane is not as easy to maintain as the EPDM membrane, it is necessary to use an activator to seal the repair.

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