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Community and Q&A

Metal Roofing Over Purlins

user-917907 | Posted in General Questions on

What is the proper roof assembly for a metal roof (such as Fabral Grandrib 3, which is screwed to 2×4 purlins 2′ O.C., and covers a vented house attic in a cold climate? In particular I’m wondering if any sort of secondary moisture barrier, like felt paper, is needed between the metal and the purlins? I understand felt paper is recommended when metal roofing is applied over plywood or OSB.

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  1. GBA Editor
    Martin Holladay | | #1

    A layer of #30 asphalt felt or rubberized membrane (Ice & Water Shield) is installed between the roof sheathing and the purlins. You don't need an additional layer of felt on top of the purlins (fortunately, since it would get ripped immediately).

  2. user-917907 | | #2

    I guess I didn't make it clear that I was thinking of not using any sheathing, just the purlins nailed directly to the rafters. I know this type of construction is used in unheated utility buildings, but I'm wondering if there would be a problem with this type of construction in a house?

    If I did use sheathing with no purlins is it okay to use 1/2" OSB on 2' o.c. rafters, with the asphalt felt, or is a thicker OSB or plywood recommended? I expect 3' snow loads most winters, though with even a 3:12 pitch a metal roof will slowly shed snow.

    Thanks for your suggestions.

  3. GBA Editor
    Martin Holladay | | #3

    I don't recommend your technique on anything but sheds and barns. Although you could save some money with your technique compared to using a full layer of sheathing, you'd be better off doing it the right way.

    If you go ahead with your method, it's clear that the sloped sections of your roof can't be insulated.


    In the old days we used to do 5-V metal roofs on 2x4 purlins with lead-headed roofing nails and no sheathing. I'll never forget seeing wind-blown snow a quarter to a half inch thick on the attic insulation after one storm.

    I would recommend a minimum 5/8" plywood with 30 lb felt under the steel to provide a good base for the screws.

    The best system, as Martin alluded to above, is 1/2" OSB with 30 lb felt and then 1x4 purlins to allow an air gap between the felt and the bottom of the steel and to hold the roofing screws to attach the steel. 1/2" OSB alone won't hold the screws securely in the long run.

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