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Roofing underlayment

JoeNorm | Posted in General Questions on

Hello, 

I have a pretty long shed roof of just barely steeper than a 2:12 pitch in the pacific northwest. 

I have been advised to lay down peel and stick underlayment at this “low” of a slope. 

The material is very expensive so I want to make sure it’s necessary. I have noticed synthetic underlayment are good to 2:12 pitch without the sticky and much cheeper. 

Thoughts?

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    It depends on what you are using as roofing. Both metal and shingles can be used on low sloped roofs with the appropriate installation and materials, although shingles will always be riskier.

  2. JoeNorm | | #2

    Forgot to mention I am putting on metal over it.

    1. Expert Member
      MALCOLM TAYLOR | | #3

      Check the installation guide of whatever metal supplier you are using. I alternate between two companies. As far as I can see their products are close to identical, but one allows installation down to 3/12, the other 2/12. To be confident their panels work at that pitch, they probably specify a number of details, including what underlayment they require.

      1. JoeNorm | | #4

        Which is the company that allows for 2:12 ? Most snap lock I have seen is as you say, only warranted for 3:12 unless you go with the taller rib

        thanks

        1. Expert Member
  3. JoeNorm | | #6

    Thanks.

    Taylor Metals and ASC are more common and available metal roofs in my area. Both of their basic snap lock style systems are only listed for 3:12. But I know that professional roofers around here go below that with them all the time.

    1. Expert Member
      MALCOLM TAYLOR | | #7

      Joe,

      I wouldn't be comfortable going below 3/12. Too many of the details on metal roofs depend on laps. I would worry about leaks due to capillary action, and their vulnerability to debris building up.

  4. Jon_R | | #8

    It seems all to common to build right to the limit of specifications (in many areas). But being conservative is a good thing. I'm not a roofer, but perhaps in this case, that would be a roof covering that is OK at zero slope.

    1. Expert Member
      MALCOLM TAYLOR | | #9

      Jon,

      I agree.

      Backing this up a bit - at some p0int someone decided to build a roof with a 2/12 slope. That's where the trouble starts. I'm baffled by how many of these low-slope roofs we hear about on GBA, most of which are new builds.

  5. alan72 | | #10

    I can only speak as a customer.

    We have the underpayment down and the roofers just starting our standing seam metal roof. We have a long low slope roof - 1.5:12 (most); 2.5:12 in parts. Basic gable roof, no valleys. They recommended an adhered high temp underpayment.

    https://www.dewittproducts.com/client_images/catalog19954/pages/files/964_LITE_SMOOTH_Tech_Data.pdf

    The metal roof is a mechanically seamed product - ultraseam.

    https://ultraseam.com/panel-profiles/ultra-seam-us-150/

    At this point, I don’t know if the shape and size of our roof was a good idea- there were several compromises along the way. Either way, we are doing it.

    I’ll let you know how it goes.

    1. Jon_R | | #11

      Sounds like lap sealant is required at that slope.

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