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Recommended build up for unvented metal roof

jeffesonm | Posted in General Questions on

Hi – Building a new house in Austin TX.  Exposed trusses 4′ on center with 2×6 center match v groove decking, also exposed to inside.

Original plan was peel and stick WRB on top of T&G decking, then 4.5″ of poly iso above that, screwed down with the plates.  Fully adhered 60 mil EPDM.

Seems like nobody does EPDM down here, and everyone does metal roofing, so I’m considering sticking with what’s locally popular.  What would this setup look like with metal on top?

WRB still on top of decking, and below insulation?  Or on top of insulation?

Maybe 1×4 or 2×4 sleepers on top of insulation, on the diagonal, and then metal roofing screwed to that?

Appreciate any advice.  Thanks!

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  1. onslow | | #1


    You might want to rummage about on Matt Risinger's site and see how he handles the metal roofing. I think he is even based in or near Austin. I noted he used diagonally set sleepers to encourage any underside condensation to drain rather than be trapped by horizontal sleepers. I don't recall how he allowed the edge to drain without becoming a beekeeper.

    Your wind conditions might suggest particular screwing protocols to keep things on the roof. 2x4's will clearly offer a better bite than 1x4. I know when the winds kick up toward 40mph my own roof panels do a little dance that one can hear in the room below and my material bite is a bit tenuous. My own PVC roofing on the flat parts is not adhered, but screwed down with large washered fasteners. The wind lift can be quite disturbing.

    FWIW, my metal unvented roof layup is sheathing, synthetic roofing membrane as primary air barrier, 8" nailbase, full high temperture Grace watershield and then clip style standing seam panels. My top edge flashing and other details are done to prevent wildfire ember problems. In Austin that probably isn't a concern, but driven rain might well be. Your polyiso should be protected from getting wet, so mind the proposed top edge flashing details. There should be no openings. I would avoid face fastened panels for multiple reasons, but mostly the screws work loose over time and the little neoprene washer eventually die in the heat and sun. Both things you have an abundance of.

    1. jeffesonm | | #2

      Thanks for the feedback Roger.

      I have indeed watched Risinger's videos on the same with the diagonal sleepers. Comments suggest he used a Cor-a-vent product to let the water out but not let the bugs in (maybe SV-3?)

      I had considered nailbase but it is a good bit more expensive than the used polyiso I'm looking at.

      At 1/2:12 slope I'm definitely looking at a mechanically seamed double lock concealed fastener system. Getting some airflow between the metal and the insulation below seems like a good idea, but no roof overhangs so would need to figure out where the air comes in. I could have the metal extend few inches into the gutters but don't love the idea of a vent in my gutters.

  2. jeffesonm | | #3

    So far I've got 2x6 T&G laid down and peel and stick underlayment over the whole thing. No overhangs on the eaves or gable ends so the peel and stick laps onto the housewrap.

    Next will be the polyiso followed by some type of water protection, notched horizontal battens and then the standing seam metal roof, clips screwed to the battens.

    Trying to decide what material to go on top of the polyiso. Another layer of peel and stick seems overkill. My primary water barrier will be the standing seam metal roof of course, and then the peel and stick under the polyiso is the backup. I really just want something to keep incidental condensation or moisture off the polyiso. Also seems like perhaps I want something waterproof, but vapor permeable to allow any moisture from the polyiso to escape.

    Any reason I can't just use Tyvek? Cheap, comes in big rolls, etc. It's a little slippery I suppose but the roof is 1/2:12 so pretty flat.

    Or I guess alternative is some synthetic roofing underlayment?

  3. onslow | | #4


    You will definitely want to keep moisture/water out of your polyiso. In my locality which is high altitude mountainous, the code calls for complete ice and water shield from drip to ridge under metal roofing. Two good reasons here relating to deep snow cover and ice dam potentials. I don't think Austin is going to see much snow, but then we still have a few weeks of 2020 left.

    Using the gummy stick on ice and water shield has an one advantage over non-adhering types, in that it will tend to seal the penetrations. Your clips will be going into the sleepers, but the sleeper's fasteners will be going through your top water guard and foam, as well as the lower water shield. If both are self sealing you will be more likely to keep the water out of the polyiso.

    Keeping the water out of the insulation layer is far more critical than counting on it "breathing" after getting wet. There will not be sufficient air flow under the metal to count on removing errant moisture. In Austin, my understanding based on Matt's guidance, is that condensation cycling in spring and fall is why he moved toward drainable sleepers. You can see why counting on drying out wet insulation in a relatively humid environment is not going to go real well. At least you are not on the coast.

    Grooved sleepers will drain, although perhaps a bit less quickly. Capillary forces will tend to keep a wet film under the 2x which would be more slowly redistributed by the wood. If you still have the option, pressure treated sleepers might be a worthwhile upgrade. If you have stock already, consider treating them with a deck type water repellent.

    Whichever path you take, be sure to get high temperature rated material. Metal roofs radiate heat equally on both faces, so it gets quite roasty underneath.

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