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Community and Q&A

Low/flat slope metal roof underlayment

kevinjm4 | Posted in General Questions on

I currently have a snap lock standing seam metal roof and live in climate zone 4c. The attic is vented soffit to ridge, and fiberglass insulation in the ceiling joists.

 I’ve been told by many I need a new roof, and a particular style at that. A metal roof mechanically seamed/crimped, and butyl injected at the seams… designed to handle low slopes, as low as .5:12. My roof at lowest is .75:12. the other side is 2:12.

I have found the proper roof but the underlayment options and guidance has been scattered. Seems to me that it’s best that I cover the entire roof in peel and stick synthetic ice and water shield, rather than just around the eaves, perimeter etc. some brands recommended have been grace, iko, and epilay plystik plus (which is the recommended and/or partnered brand with my roofing manufacturer)

Have I been given good advice?… cover the entire roof in peel and stick? I have read about breathability/permeability being a need in some roofs so I’m just not sure…

ill be doing the roof myself.

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  1. Expert Member


    On low-slope roofs like you have a water-proof membrane like I&WS is good insurance.

    I don't see any point in sealing the roof penetrations to the sheathing. Water that gets below both the roofing and underlayment will cause the same amount of problems whether it permeates the plywood, or follows the pipe down. By all means seal the penetrations to the underlayment.

  2. Stockwell | | #2

    Not just plain Ice and water shield, but you have to use the HIgh Temp version.

  3. Expert Member
    Michael Maines | | #3

    Kevin, is the membrane you plan to install EPDM rubber? If so, it does not play well with bituminous underlayments--they make the EPDM brittle and porous.

    In my opinion, the best way to handle penetrations is with counter-flashing--two layers that work together for a fairly bulletproof installation. Next best is a "pitch block"--basically a ring full of goo that sits at the base of penetrations and maintains a good seal.

    1. kevinjm4 | | #4

      Michael, it is SBS Modified Bitumen, I don’t know what that is but, there it is... and it is high temp.

      Also, I didnt quite understand your second paragraph. The first tip I am not quite sure what you mean. Explain further? Link to a product? The second tip regarding the goo is understandable enough conceptually, just would ask for a product or detail or something...

      Here’s the peel and stick HT ice and water shield I’ll be using:

      1. Expert Member
        MALCOLM TAYLOR | | #5


        Here's how I install plumbing vents in metal roofs.

        - Make sure the vents are located so they fall onto the middle of a roofing panel.

        - After the underlayment has been installed, cut a square of self-adhering membrane about 12" x 12", and make a hole slightly smaller in diameter than the vent-pipe and slide into place.

        - Install the roofing panels and seal between the pipe and panel with a caulking rated for use on metal roofs.

        - Dry-fit the roof boot and mark the location of the top of the gasket on the pipe.

        - Apply a suitable sealant to the underside of the flange of the roof boot, and a ring of sealant around the pipe at the height you marked.

        - Slip the roof boot into place and secure with gasketted screws.

        1. kevinjm4 | | #6

          Thanks for the tips.

          And if you wouldn’t mind (or somebody), I just spoke with the manufacturer, a couple of them actually, of HT peel and stick Ice and water underlayments... they said they don’t recommend their product for any slope below 2:12. I have a portion at .75:12 or so, let’s call it 1:12. What would be a good approach for this new obstacle I’ve come across? Is there really much I can do beyond just covering the whole thing in HT ice and water shield?

          1. Expert Member
            MALCOLM TAYLOR | | #7


            I think what they are getting at is their products are designed as a secondary layer of protection against failure on typical metal roof installations. Their fear is that roofs installed on that low a pitch - that is below that is commonly allowed by manufacturers - will mean the underlayment may effectively become the primary layer stopping water intrusion. Something it isn't designed for.

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