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Roof Underlayment Beneath Metal Panels

MHunt11 | Posted in General Questions on

Our GC used a non high temp, granulated underlayment under our metal roof panels. A roofing specialist has recommended we replace the entire roof deck with new underlayment. We’re trying to avoid replacing the entire roof deck  – could we just take off the metal panels and lay a smooth, high temp underlayment over the granulated one and then put the panels back on? Or would the wrong underlayment underneath continue to cause problems? Any other suggestions?

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Replies

  1. Kyle Bentley | | #1

    This is very much depending on where you are located. Ie southern Texas, or Maine?

    In very hot climates, I'm not certain that would work as expected. The primary effect is that it'll probably melt, and may chemically interact with whatever you place on top of it. It will be at the same temperature regardless, being in contact, via conduction.

    Can you tell us the roof color, gauge, and where you're located?

    1. MHunt11 | | #2

      In zone 3 Charlotte NC. It’s a black 26 gauge snap seam metal roof on 1:12 pitch.

  2. Kyle Bentley | | #3

    I tried my darndest to find a service temperature for GAF's mineral roll roofing, but I couldn't find a written max temperature.

    However, that stuff starts to become slimy fairly quick if you're installing it in the summer.. shingles get up to 165F or so. I can imagine a black metal roof may get warmer, but I honestly don't know how much.

    Shingles are already a pretty good black body, but they have high emissivity. Metal on the other hand has a lower emissivity, and doesn't shed as much heat through radiation, one of the reasons for the higher temperature, even when both are black.

    I'd call GAF, or whoever makes the product you're using and confirm. It may be fine.

    1. MHunt11 | | #5

      Owens Corning makes the underlayment they used and they say it cannot be used under metal. I just need a solution. I guess taking of panels, ripping off underlayment and replacing any osb with holes would be best?

  3. Expert Member
    Malcolm Taylor | | #4

    MHunt11,

    I'm more worried the new underlayment won't be an adequate barrier between the original one's granulations and the backside of the panels than I am about it not being high-temp.

    1. MHunt11 | | #6

      The granulation’s are not nearly as abrasive as say a shingle but more like a fine grit sandpaper.. any suggestions for fixing this or is redoing the whole roof our only option?

      1. Expert Member
        Malcolm Taylor | | #7

        If that's the case I wouldn't have any reservations about just adding the second layer.

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