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Housewrap under metal roofing

vigilant1 | Posted in Construction Techniques on October 26, 2010 02:45am

I’m putting steel roofing (raised seam “snap down” concealed fasterner type) on a small patio cover.  The decking is PT plywood.  I’ve covered the deck with 30LB tarpaper, which is what the metal roofing directions recommend.

I also covered the tar paper with housewrap and am ready to screw down the metal and wanted to get opinions here.  The pitch is very shalow (1:12), and I figured the housewrap would help keep any water leakage from backing up through the tarpaper overlaps.  Now I’m having second thoughts–I want any trapped water to be able to evaporate.  Yes, the housewrap has a high perm rating, but it is the Lowes-brand stuff with tiny holes (not TYVEK), and obviousy meant for vertical installation, not to be trapped between metal and tarpaper.  I don’t want water to be trapped against the metal sheeting unable to evaporate for long periods.

Thoughts?

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  1. User avater
    popawheelie | Oct 27, 2010 02:21pm | #1

    If it's permeable, it's permeable. It won't trap moisture.

    I built a low pitched steel roof and used ice and water shield under the steel.

    How much did you overlap the tar paper?

    I'm not sure what the housewrap would do on a roof.

    Adding layers of material that don't work doesn't do anything except maybe cause proplems.

    My metal roofing had a different instal than roofs with more pitch.

    I had to put down a seal on all the seams.

    I live in snow country so snow causing ice dams was my biggest concern.

    1. davidmeiland | Oct 27, 2010 08:50pm | #6

      That would be

       a low pitched steel roof and used ice and water shield under the steel

      my recommendation. The felt might work fine by itself, and with the plywood being treated you have little to lose. I have a customer with a couple of carports... metal roofing over skip sheathing, 1:12 pitch, no felt or other. Condensation formed on the bottom of the metal and dripped down. We pulled the metal and the skip off, put down ply and I&WS, and reinstalled the metal. So far so good.

  2. Piffin | Oct 27, 2010 05:09pm | #2

    It was a waste of money to use it, but it won't do any harm

  3. DanH | Oct 27, 2010 07:38pm | #3

    Like Piffin said, it won't do any good, but won't do any harm either.  The stuff is discouraged on roofs mostly because it's a fall hazard -- it's more slippery than felt and can tear and slide suddenly.

    1. vigilant1 | Oct 27, 2010 08:32pm | #4

      Popawheelie, - The paper

      Popawheelie,

      - The paper is overlapped approx 10" on the horizontal seams - I'm not too concerned about ice dams (this is an outdoor, uninsulated patio cover, so everything should freeze or thaw at the same time.

      - Like the installation you described, I'm using mastic tape under the seams of the metal.

      - Thanks for the pictures.

      Piffin and DanH,

      - I figured the "don't put on roof" warning on the building wrap was due to potential slipping/falling.

      I'll rip off the housewrap before putting on the metal. It doesn't sound like it will be needed and is just another "thing." It won't go to waste, I can use it on another project I've got working. Anyway, we had some terrific winds yesterday in southern OH, and the housewrap stapled tight across the roof deck kept everything dry and did a good job of keeping my felt paper from blowing away, so it probably worked out fine after all. Thanks again.

      1. DanH | Oct 27, 2010 08:48pm | #5

        Yeah, housewrap that's halfway decently fastened will stay in place in the wind much better than felt.  A good argument for using it in your case -- with this wind any exposed roofing felt would be long gone.

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