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Community and Q&A

Underlayment in Roof Assembly with Zip Sheathing

jbracey | Posted in General Questions on

I have roof condition that’s it appreciate any wisdom on.

From the inside out: cathedral ceiling with R30 Rockwool batts, zip sheathing, 2in of XPS, and horizontal 1×4 battens, waiting on exposed fastener metal roof.  In climate zone 4, GA.

In short, I am rethinking my choice on zip only for the weather barrier, and wondering if I put a synthetic underlayment over the battens, would that mess with the air gap/ ventilation under the shingles, but above the deck?

I have had a couple drops inside from screw holes where the rafter was missed in securing the batten.  I would imagine once the roof is on it should be a non-issue, but thinking if a simple synthetic underlayment would be cheap insurance, if it didn’t cause any other issues.

Appreciate any thoughts, or something I may be missing.


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  1. Expert Member
    Akos | | #1

    Most metal roof manufactures require an underlayment to manage night time condensation on the underside of the panels. In your case, if you can tape the seams of the XPS it would probably do the same job.

    For metal roofs, I put the underlyment under the 1x4 strapping directly over the rigid, this is pretty much weather tight. I can't see why underlayment over the 1x4 would also not work. I wouldn't worry about blocking any venting, the ribs and striations of the metal panel tend to provide more than enough space to clear any moisture that makes it in there.

    P.S. I know exposed fastener is cheaper, a clean install with neat rows of screws takes a lot of time, works out to about the same as snap lock. With snap lock, you won't have to deal with loose screws down the road.

  2. jbracey | | #2

    Got it, thanks for the advice, I'll put down the added underlayment for some added protection.

    For the snaplock, worth pricing the difference, I had sort of assumed it was the same as standing seam, and not priced that. I can see how the screws could get out of line during install without care.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      The issue with the screws isn't just cosmetic. Each one of them represents a potential leak, and a roof can have several thousand of them.

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