Water-resistant materials for use under EPDM roof?
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.
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