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Community and Q&A

What roof underlayment can be used on a non-vented metal roof?

Paul May | Posted in GBA Pro Help on

I’ve put up a building that has a 14/12 pitch non vented roof but now the building inspection dept. is hassling me over using Titanium UDL30 underlayment under the metal roof. They say it doesn’t breathe and can rot the roof sheathing. The ceiling is R49 with 4″ of foam against the plywood sheathing followed by 8″ of fiberglass and 2 layers of 5/8″ firesheild drywall.

I’d like to know 2 things – are my building officials wrong and if they’re not what type of underlayment is allowable under a hot metal roof? I’m in the midwest in zone 6a.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Paul,
    Your building official is correct. Most manufacturers of synthetic roofing underlayment do not allow their products to be installed on unvented roof assemblies. It is a code violation to install any building materials in violation of manufacturer's installation instructions.

    From the manufacturer's installation instructions: "Titanium® UDL30 is an air, water and vapor barrier (.06 perms) and therefore should be installed above a properly ventilated space(s)."

    I suggest that you install good old-fashioned asphalt felt.

  2. Paul May | | #2

    Thank you Martin, are there any problems with using asphalt felt under a metal roof? What weight? Paul

  3. Expert Member
    Malcolm Taylor | | #3

    Paul, synthetic underlays were pushed for three reasons:

    They allow you to get the roof quickly weatherproof and stay that way for an extended period of time.

    They provide a walkable roof deck which is harder to damage.

    In some climates the asphalt roofing paper can get hot enough to melt and adhere to the metal roofing, which reduces it's effectiveness. I have no direct experience with the latter. I've only heard about it being a problem from the synthetic underlayment manufacturers. Maybe someone else can chime in.

  4. Peter L | | #4

    Paul,

    Most metal roof manufacturers will NOT allow you to install building/asphalt paper underneath a metal roof. The reason is : 1 - Asphalt paper will deteriorate very quickly under a hot metal roof. 2 - Building paper rips/tears easily.

    My advice is to invest in a proper roofing membrane like SIGA Majcoat which is water/air resistant but vapor open so if water gets underneath, it will still allow the sheathing to dry.

  5. GBA Editor
    Martin Holladay | | #5

    Paul and Peter,
    Metal roofing has been installed over asphalt felt for at least 100 years. If you don't like #15 felt because it tears easily, use #30 felt.

    I'm not aware of any metal roofing manufacturers that forbid the use of asphalt felt. But if they do forbid felt, you would have to use one of the new (expensive) vapor-permeable synthetic roofing underlayments.

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