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What is the best underlayment for a 5/8:12 sloped metal roof?

Damon Beck | Posted in Green Building Techniques on

I live in south eastern South Dakota and I am building a tiny house on wheels. My slope is very low (5/8:12) and I am going to use a mechanically seamed metal roof or a high seam snap together metal roof that is good down to a 1/2″ slope. The rafters are 2×6 with closed cell spray foam, and all the roofing companies I talk to are telling me to use and ice and water membrane over the entire roof and that if the sheathing is dry when I install and can dry to the inside before the spray foam is applied then the assembly should be fine.

What would the best thing to do be in this case? Is there a breathable synthetic roof underlayment that would work for such a low slope?

Any help would greatly be appreciated.

Thank you
Damon

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Replies

  1. Dillon Vautrin | | #1

    If you use ice and water remember to use a high temp ice and water if you use a metal roof. Especially since you have no ventillation.

  2. Expert Member
    Malcolm Taylor | | #2

    Damon,
    A flat metal roof is a very problematic assembly. That's why you don't see them that often. I agree with your roofers that you are going to need a membrane underneath and cannot rely on synthetic underlayment, permeable or not. Flat, or as near as flat roofs, generally rely on a parapet at the roof/wall intersection because the standard metal trim details allow water to make its way back under the roof by capillary action. You will need to figure a way of stopping this.
    I know this isn't what you want to hear, but I'd be inclined to re-design the roof to have at least a 2/12 slope, or forgo the metal altogether and use a membrane as your finish roofing.

  3. C. B. | | #3

    Hi Damon,

    It will need to dry outwards since you are insulating with low-permeable insulation on the inside. I'd look at using:
    http://www.roofaquaguard.com/synthetic-underlayments/dry-tech/
    or:
    http://www.cosella-dorken.com/bvf-ca-en/products/roof/underlayments_metal/products/trela.php

    These will provide an airspace under the standing seam roofing since solid, continuous metal doesn't allow moisture to escape. If I was installing my roof today I'd get the Delta Trela from:
    http://www.spycor.com/DELTA_TRELA_Roofing_Membrane_by_Cosella_Dorken_p/dltr.htm
    but I'd price compare using the Dry-Tech product over Brea-II:
    http://www.roofaquaguard.com/synthetic-underlayments/brea-ii/

    All of these are designed for metal rooves, and the Trela is exactly what you need under standing seam metal roofing.

  4. Damon Beck | | #4

    Thanks for all the helpful advice. My original plan was to use double lapped 30# roofing felt with a 1/4" to 1/2" rain screen mesh or venting strips on top to allow for air movement between the roof sheathing and the metal panels.

  5. David Meiland | | #5

    I think you can do the roof you want, but pick a metal roof supplier, give them your plans, and get their written specs for the install, including the deck, ventilation, underlayment, etc. There are a lot of airplane hangars here with nearly flat metal roofs, and they are all high-rib types, not real attractive but they work.

  6. GBA Editor
    Martin Holladay | | #6

    Damon,
    Dillon's suggestion is the only way to get a little bit of outward drying -- but even with Dillon's Dry-Tech, you still won't have ventilation or air movement -- just a little bit of a gap.

    Most roofs that are insulated on the underside with closed-cell spray foam are unable to dry to the exterior, and they still work. The advice you've heard -- "if the sheathing is dry when you install it and can dry to the inside before the spray foam is applied then the assembly should be fine" -- is basically correct. Assuming that you have accepted the pluses and minuses of closed-cell spray foam, and that you have decided to go forward with that insulation, then I agree with the advice.

    The type of roofing underlayment you choose doesn't matter very much (assuming that the metal roofing is watertight). If there is some worry about whether the metal roofing can be made watertight, then certainly Ice & Water Shield makes sense.

  7. Damon Beck | | #7

    Would something like this assembly work? I would have the furring strips vertical on the roof and walls to allow for a 1/2" gap for the rain screen. The primary roof sheathing would have felt paper or a breathable underlayment and the secondary roof sheathing would be covered with EDPM.

  8. GBA Editor
    Martin Holladay | | #8

    Damon,
    It should work. You should be aware, however, that most vinyl siding manufacturers don't want their siding to be installed over furring strips. For more information, see Can Vinyl Siding be Applied Over Furring Strips?

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