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I couldn't find any information if one can put EPDM rubber (ethylene propylene diene monomer) directly over XPS (extruded polystyrene)? The materials are used for hydro and thermo isolation (consequently), thus it's reasonable to cover XPS with EPDM when building a roof. I'm interested mostly if anyone tried to combine them. Were there any problems with their chemical interaction or adhesion? How long such type of covering could exist? The links will be useful too. Thanks in advance.

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    The biggest problem I can think of is that the foam is so soft that foot traffic and hail will leave dents.
    – isherwood
    Jun 23 '20 at 20:15
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    eXtruded PolyStyene is XPS, not EPS, which is Expanded PolyStyrene, and it's not so soft - indeed, the standard grade is 25PSI and you can get 100PSI or more - don't wear your stiletto heels on the roof and it will be just fine - though you can also get roofing panels consisting of a sheet of cement board bonded to a sheet of XPS. Those are commonly laid above the waterproof membrane.
    – Ecnerwal
    Jun 23 '20 at 21:21
  • @Ecnerwal thanks for the clarification Jun 24 '20 at 7:31
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Always follow the manufacturer’s installation instructions. One of the largest roofing companies is Johns Mansfield (JM).

There’s many considerations including attachment to roof structure, splicing, fitting around vents, conduit, etc. If you use their accessories, fasteners, etc. it’s more likely that the system will be weathertight.

Here’s their product description:

https://www.jm.com/en/commercial-roofing/epdm-roofing-systems/membranes/

Here’s their insulation and cover board options. I don’t see XPS, but I do see a lot of polyisosanurate options.

https://www.jm.com/en/commercial-roofing/roofing-insulation-and-cover--boards/

As you can see, the insulation comes with an isolation board to isolate it from the membrane. In addition, they recommend Fesco Board to install on the roof deck to isolate the insulation and membrane from the roof structure.

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