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Ridge transition detail from EPDM to asphalt shingles?

jonny_h | Posted in GBA Pro Help on

Hi experts,
  I’ve got a roofing question for y’all.  The front half of my house is an 8/12 pitch roof, that will be covered in regular asphalt shingles.  Most of the back half is something like 1.5/12 low-slope, which is covered with Carlisle SAT EPDM.  We did the EPDM ourselves, but I’m contracting out the shingle installation in the interest of time & effort.  However, I’m not sure how to handle the ridge detail where the EPDM on one side meets shingles on the other side — this is an unvented assembly with rigid foam above the original sheathing, and new sheathing above the foam.  Currently I’ve just left a flap of EPDM hanging over the front about a foot, and the rest of the front is covered with synthetic underlayment.

  When I discussed the project with the roofer I plan on using, he said they’d just shingle up to the peak, and then apply regular cap shingles over the ridge, with some extra goop on the EPDM side to keep water from getting under the cap shingles.  I asked if there were any material compatibility issues, and he said no.  I’m semi-inclined to trust him, because, well, he’s a professional and his company does both residential and light commercial, so I assume they have some degree of experience with EPDM….

HOWEVER, I also know that EPDM is generally incompatible with asphalt and petroleum-based products.  I haven’t found specific guidelines for asphalt shingles, but I don’t know why they’d be different than other asphalt products.  I was careful to use Grace Ultra ice & water shield in the areas that come close to or contact the EPDM (vs. the less expensive regular Grace), per specific instructions from Grace on material compatibility.  I used Chemlink polyether sealants specifically listed as compatible with EPDM for roof penetrations.  I really don’t want my EPDM to prematurely fail at the ridge due to a chemical incompatibility that my roofer wasn’t aware of, because admittedly this detail is probably not a very common one.

Has anyone seen this kind of detail before (low-slope EPDM transitioning to asphalt shingles at a ridge)?  Does anyone have evidence of either the compatibility of incompatibility of asphalt shingles with EPDM?  If incompatible, anyone have an alternate detail that I could propose to my roofer?  Anything that could be used as an isolation layer?


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  1. Expert Member
    Akos | | #1

    These are always done with metal, doesn't matter if EPDM or torch-down. The metal ridge cap is glued to the EPD and hangs over the shingled side. The shingles are tucked under the metal cap. If you are in high wind area the ridge cap might need some Z bars to anchor it on the shingle side.

    The fact that they don't know this means they are not used to flat roofs, so be careful.

    You have to be very careful with EPDM and anything asphalt. I was surprised how quickly the EPDM comes apart, I had a piece touching some I&W and within a couple of months it swelled up and become soft. Lot of the standard roofing adhesives your roofer will have will also be an issue, so make sure they don't caulk anything on the EPDM side unless you are supplying the proper adhesive.

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