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Community and Q&A

Moisture between underlayment, plywood, and metal roof

Benjamin von Cramon | Posted in General Questions on

We’re building a sunroom addition, nuts to do this in winter, especially this winter in the Atlanta area, but so it is. I’m doing a lot of the work myself, help from a metal roofer with other skills. This contractor wasn’t finished framing a 1.5/12 shed roof dying into a 5/12 roof, waiting to sort out a particular issue, but went on to place 1/2 plywood, Rhino underlayment, then 24 gauge 1.5″ standing mechanical seam metal roof panels. The underlayment running out the top runs a couple feet, was tucked under asphalt shingles from garage roof, a couple screws thrown in to keep it down. It wasn’t enough, underlayment blew loose, it rained, moisture showed up in several places, inside adjoining garage, pools also formed in addition. The contractor disappeared for various (lame) reasons and then the legitimate reason of this wintry mix that’s put a big comma in life around here as of recent.

I tucked the underlayment back under the shingles, secured it adequately, a subsequent rain shows we’ve stopped water from flowing into either structure, garage and addition. My concern is what all this portends for moisture that I have to assume might be between metal panels and underlayment, underlayment and plywood, as well as in and through whatever layers belonging to the garage roof that are under the new shed roof. I’m disinclined to cover anything up, restricting airflow in any way to layers of materials with moisture sitting in between. How big an issue is this? Must I have the panels removed, underlayment removed, let it all dry before reinstalling? Or, will a couple days in the sun be enough to cook the moisture free? I appreciate your time.

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  1. davidmeiland | | #1

    I wouldn't remove materials to dry it. Things like that won't stay wet unless more water is added. I will say that I think it's fairly hard to get flashing into the upper roof correctly unless you strip some of the shingles to do it. "Tucking" sometimes doesn't work. With a shingle roof above and a metal roof below, you need felt and a pitch change flashing well up under the shingles (the felt is under the metal, the flashing on top). You might want to strip a few courses to get that part done right.

  2. GBA Editor
    Martin Holladay | | #2

    It's too bad that your roofer chose the wrong roofing underlayment. According to the manufacturer of RhinoRoof synthetic roofing underlayment, "RhinoRoof is a 15 lb synthetic roofing felt replacement for use under Asphalt Shingles only." As far as I know, you're not supposed to use it under metal roofing.

    Moreover, this synthetic roofing underlayment is vapor-impermeable (0.05 perm), unlike #15 asphalt felt, which is a smart vapor retarder with variable permeance. While it's possible for moisture to evaporate through asphalt felt, the same cannot be said for RhinoRoof.

    The concern at the moment is that there may be water between the impermeable underlayment and the metal roofing. In the long run, this moisture will probably dry out, once you get some sunny days that make the metal roofing very hot. But the situation isn't ideal.

    For more information on this topic, see Synthetic Roofing Underlayments.

  3. davidmeiland | | #3

    In my experience, metal roofing rarely lays perfectly flat. He is using mechanical seam roofing whereas we usually use snap-lock, but there always seems to be a bit of "oil-canning" and I would expect moisture to dry out. With a low slope metal roof tying into a steeper roof above, there is always the chance that wind blown rain will enter under the metal roof at the top, and it needs to be able to get out.

  4. Expert Member
    ARMANDO COBO | | #4

    " The underlayment running out the top runs a couple feet, was tucked under asphalt shingles from garage" and "I tucked the underlayment back under the shingles:..
    I don't know if I understand this correctly, but any new the underlayment should be tucked under the old underlayment, not under the shingles, in a shingled manner to prevent moisture running under the new underlayment.

  5. Benjamin von Cramon | | #5

    Each these responses are much appreciated. Not the best news that my contractor used RhinoRoof, which is only rated for asphalt shingles and impermeable. Assuming I see to it that he properly manage the underlayment and flashing at the top, if no more water is allowed to flow in, and if some hot days cook off any existing moisture, what is the problem with the underlayment being impermeable? Is it that preventing any moisture driving in, like the one comment suggested will happen at the top with wind blown rain, that this impermeable underlayment won't allow moisture to cook out very easily?

    Is there general agreement that I shouldn't try to force the issue, pointing out that he used a material not suited for metal roofing and insisting he take down what's there and start over with #15 asphalt felt?

    If we move forward as is, I'm hearing that new underlayment has to go under old underlayment and old shingles, which might require stripping a few courses, and that flashing should extend up under these several courses, correct?

    Many thanks.

  6. davidmeiland | | #6

    My suggestion that you not remove material to dry it was based on your description of mechanical seams. If that's what my roofer calls mechanical seam, it can't be undone easily at all. If this were snap lock roofing, I would say unscrew and unsnap it, set it aside, and start over with correct underlayment and correct tie-in to the existing upper roof.

    What should have happened is that the entire lower roof should have been completed with some of the shingles removed out of the way. Tuck the lower underlayment a foot or so under the upper, then install the panels, then install the pitch change metal and tuck again under another, deeper lap of the upper underlayment.... then install shingles. You can probably get a "closed" profile of pitch change metal to reduce water blow-in at the top.

    I had to fix one exactly like you are describing because the first guy did not want to demo about $30 worth of shingles, result being that they only tucked a few inches and ripped their new stuff in the process.

  7. GBA Editor
    Martin Holladay | | #7

    I'm not sure why the manufacturers of RhinoRoof specify that their underlayment can only be used under asphalt shingles. You can always call them and ask.

    One possibility is that the underlayment includes texture particles to make it easier for roofers to walk on, and that these texture particles could abrade the underside of the metal roofing as the roofing expands and contracts with temperature changes. But I'm just speculating.

    All building codes include a provision to the effect that building materials must be installed according to manufacturers' instructions. This blanket provision means that your roofer's choice of RhinoRoof appears to be a violation of the building code. If you take this stance, you may be able to negotiate with your roofer and require the roofing to be removed and the proper underlayment installed -- if you want that to happen.

    The fact that the underlayment is impermeable to water vapor probably wouldn't be a problem if everything was buttoned up and made watertight before rain got between the underlayment and the roofing. But your situation is a (possible rare) example of a situation in which old-fashioned asphalt felt would perform better than a synthetic underlayment, because asphalt felt allows inward drying.

  8. Benjamin von Cramon | | #8

    Again, excellent advice all around. I have, in fact, called RhinoRoof, was referred to their guru, will learn today what all this portends. Correction, think I had my language confused, thinking now that mechanical seam is the type involving a specialized tool for creating the mechanical seam, this not being the case with us. He has a box of clips, is screwing down, so I like the idea of calling foul on breaking code, requesting he pull off and start over, which should be 1/2 a day lost for him.

    I'll get informed by the RhinoRoof rep and share what I've learned. Again, many thanks for taking time to set me straight.

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