GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Standing-Seam Metal for Low-Slope Roof

George_O | Posted in GBA Pro Help on

We have a Prairie style house in East Tennessee (Mixed-Humid, Climate Zone 4A), built in the 1950s, with a low slope roof pitch of 1.5 per 12. The roof (~50 squares) has some complexity because of a 45 degree house section and a chimney. It is a planks-on-rafters roof that was originally tar and gravel. Today, the roof has traditional asphalt shingles and underlayment with tighter than usual spacing, which was done about 15 years ago before we bought the house. This seems to go against shingle manufacturer recommendations (minimum 4 per 12) but we have not noticed any issues in the last 12 years.

I like the longevity and looks of a standing seam metal roof and need some advice. I got a couple of roofing estimates for standing seam with concealed fasteners. But when I read the manufacturer’s recommendations, they do not recommend installation on slopes below 3 per 12. In lower slope situations, they recommend standing seam that is mechanically sealed.

Is mechanical seal overkill in our climate zone? The roofers don’t seem to be concerned.

Thanks for your advice.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member


    A few snap-lock roofing manufacturers allow use down to 2/12, so you aren't that far off. I don't think it's good idea to use materials right up to their limits, so it isn't something I'd do - but if shingles worked, it may well too.

    It's also worth remembering, that the manufacturer's installation instructions aren't just recommendations. Failing t0 follow them both voids the warranty, and is a building code violation.

  2. Expert Member
    Michael Maines | | #2

    George, to add to Malcolm's advice, there is metal roofing available with taller ribs that come with a warranty for installation on lower slopes. I don't know of any that will warrantee a 1.5:12 slope, but they may exist. When I've done metal roofing on slopes that low it's been soldered copper.

    1. brendanalbano | | #5

      There are some more commercially focused products for low slope metal roofs. I've never used it, but the McElroy 238T is good for 1/2:12

      The symmetrical seams allow individual panels to be easily replaced, which seems cool as well. Or at least, so the product rep told us at the lunch and learn I attended a while back ;)

  3. George_O | | #3

    A snap-lock metal roof installer (using a Rollformer to make panels on site from metal stock) recommends a high temp ice and water shield under the low-slope (1.5/12) snap-lock. Does that make sense from a longevity viewpoint?

    If you put a rubberized material under a metal roof that has a potential life of 40+ years, it seems the rubberized material would fail before that, negating its benefit and the metal roof longevity. Would this potentially mean removing the metal and reinstalling new rubberized material after, say, 30 years?

    1. GBA Editor
      Brian Pontolilo | | #6

      Hi George.

      Two things that your roofer told you are pretty commonly done. First, installing underlayment beneath metal roofs that can stand up to high tems, even if it is a more common synthetic roofing underlayment, and second installing a fully-adhered underlayment as a good option for low slope roofs where it is easier for water to back up than on a steeper roof.

      Also, if a roof lasts 40+ years, that would imply that it is doing its job of keeping water out. It's also protecting the underlayment from UV, which is what kills these products. So, how is it possible that the underlayment would fail and need to be replaced? Maybe I'm missing something, but I have never heard of a roof that is in good shape, but the underlayment needs to be replaced.

      1. creativedestruction | | #13

        I have only once, but it was under 100-year-old slate roofing. The roofers pulled it up to inspect, installed new underlayment and put the slate right back on. Good for another hundred years.

  4. carl_f | | #4

    George - take a look at the Vertical Seam roofing offered by Best Buy Metals there in your state ( which is rated to be used down to 1:12. I'm just north of you in central KY and plan on using it on a 1:12 and 2:12 pitched shed style roof planes. While we won't be installing for a few months, I've already worked with them on my roof plan and they have been very responsive and helpful and have given me a pick list of materials, cost and even a recommendation of a roofer in my area familiar with their product. I'm also getting my gutters from them as well.

    In re-reading your question, I have found your experience correct in that the snap lock types seem ok down to just 3:12 (Best buy offers some of these as well). I'm personally ok with using their mechanical fastening method...


  5. vashonz | | #7

    Locally (PNW) Bridger Steel has specs for their snap lock and mechanically fastened that are designed to 1:12. Some of their profiles discuss using butyl mastic below 3:12.

    We're doing a 2.5:12 roof, working with the roofing company to get a metal profile that will work.

  6. George_O | | #8

    Thank you all for your helpful input! I now find there are several manufacturers who have a 1.75" vertical snap-lock panel rated down to 1:12 slope on solid substrate.

    To make things more concrete, I attach a drawing of the roof. All roof pitches are 1.5:12 and there are vaulted sections (unvented), vented attic sections, and an open carport and porch section.

    Are there different treatments that you would recommend under the snap-lock panel for each of the different roof sections?

    Thanks again,

  7. JakeBramante | | #9

    Probably not as important for Zone 4 folks, but for snowy Zone 6 areas like mine, you need to have a mechanical lock with a sealant. We're building a house with a 1.5/12 pitch shed roof and my gutter guy (and metal roof provider) said, "Don't let them talk you into a snaplock!" He's seen too many of them not work properly and had to replace them. Also, just make sure you read the specs for the roof. I wouldn't push the envelope.

  8. joenorm | | #10

    I just went back and forth on this issue for weeks. I have slightly greater than a 2/12 and could not decide what to do. I live 4C, very little snow. Our local roofing manufacture had the same specs, not to be used below 3/12. They did make a taller seem that goes to 2/12 but all the flashings are different and everything is more money.

    I finally just called the manufacturer and they said "yeah, those are recommendations, but pretty much every roofing contractor will go down to a 2/12 with our standard roofing(snap lock) and occasionally a 1/12. We've never heard of it being a problem".

    They basically gave it their blessing. I think it really depends where you are and if you get cold. snowy conditions. I do not.

    Anyway, hope that is helpful.

    1. Expert Member
      MALCOLM TAYLOR | | #11


      One of my reservations about using metal on low-sloped roofs is that many of the standard details, ridge caps, eaves, sidewall-flashing, etc. rely on lapping and gravity to work. When you don't have the slope, you are much more likely to get water making it's way into the roof system through capillary action, wind, or dams caused by debris build-up. They may well work, but they are at the limits of the assembly or material. It seems like a problem which could be easily avoided at the design stage by not including a low-slope roof without thinking through the consequences.

      1. jason_v | | #12

        wow, nice passive aggressive reply. Many many options exist for low slope metal roofs, even as low as 0.25/12. here is one

        1. Expert Member
          MALCOLM TAYLOR | | #15


          Try dealing with the issues and leave out the commentary. Questions about how to build low-sloped roofs occur pretty frequently here on GBA. They are a problem that is best avoided at the design stage.

          As my post said, the main issue isn't the panels, it's the rest of the details. Perhaps you could post some for us so we understand how it would all work?

  9. qofmiwok | | #14

    I'm told the same thing here in 6B, snowy climate, that nobody will warrant a standing seam below 3:12. Mine is 2.5:12.

  10. jackofalltrades777 | | #16

    MBCI engineered, makes and warranties a metal roof down to 1/4" : 12. No, that is not a typo. Down to 1/4" : 12 It's a tall trapezoidal snap standing seam metal roof with hidden fasteners.

    1. Expert Member
      MALCOLM TAYLOR | | #17


      If you want to use a metal roof that relies for it's integrity on double-sided tape, sealant, and closure strips, fly at it. It may well work in most climates, but it is in no way comparable in its robustness to a conventional standing-seam roof on a pitch of 3/12 and over.

  11. jackofalltrades777 | | #18


    That is not a correct representation of how that metal roof is attached and sealed. It uses a mechanical metal folded seam that is folded & locked under to prevent water from entering.

    I have no stock in MBCI, so I have no dog in this fight. Just stating that the roof design was engineered and is in place in thousands of homes and businesses and is working properly as required. Local roofer that has been installing these roofs in my area for 15+ years, has never seen a failure, even on a 1:12 pitch.

    I agree that having a steeper sloped roof helps with water shedding as it uses gravity to its advantage. No arguing on that but to say the MBCI roof design, "relies for it's integrity on double-sided tape, sealing and closure strips" is not an accurate representation of that engineered roof design.

    Per the online documents:

    1. ASTM E 1680-95:Standard Test Method for Rate of Air Leakage Through Exterior Metal Roof Panel Systems
    2. ASTME 1646-95:Standard Test Method Water Penetration of Exterior Metal Roof Panel Systems by Uniform Static Air Pressure Difference

    Air Leakage was conducted with a uniform static air pressure differential of +/-6.24 PSF & +/-12.00 psf. Water Penetration was conducted with a uniform static air pressure differential of 6.24PSF. & 12.00 psf.

    Water Penetration Test Results: No uncontrollable water leakage at 6.24 psf or 12 PSF when five gallons per hour of water were sprayed per square foot of roof area.

    The roof panels are both Florida and Texas approved for severe storm applications.

    In the BELOW PHOTO. The roof panel ends lock into each other. The 2 metal pieces are folded and designed so when they lock, they provide a mechanical seam that has no gap and no way for water to enter.

    1. Expert Member
      MALCOLM TAYLOR | | #19


      Did you look at the installation manual you linked to? Again, as I've now said repeatedly, it isn't how the panels themselves join that is the problem, it is the ways they interact with other components - at eaves, peak, end-walls, penetrations etc. A 0.5 / 12 (for all intents and purposes flat) roof made with this system relies on closure strips at the eaves and ridge, sealed with tape, to stop water movement. Similar details also occur at the transition between roof slopes, valleys, hips, etc.

      But that's a secondary argument. The point I've been trying to make is that all this can usually be avoided at the design stage. I can't think of a house I've designed that would have been particularly affected by going from say 2/12 to 3/12. Discussions about low slope roof construction on GBA invariable occur once the design has been finalized, and the owners or builders are then looking for a solution to a problem the didn't have to happen..

  12. jackofalltrades777 | | #20


    On small houses (under 900 sqft), doing a 3:12 or steeper roof pitch is not always possible, especially if looking for a modern (non-gable) roof line like a skillion or shed roof. There are solutions and companies like MBCI offer such a solution. The ridges & peaks I have seen are actually mechanically seamed during install. Tapes and sealants are just additional secondary methods as a belt & suspender approach. Speaking of which. Modern building science is using tapes for window, roof & wall penetrations and it has been holding up quite well. SIGA developed house tapes and they have been in use for 50+ years.

    I agree that steeper roofs are naturally better in being watertight as gravity is always at play. 3:12 is good and 5:12 or higher is better in snowy areas. Although in small house designs that can't run a 3:12 or higher pitch because the footprint of the house is so small, then MBCI offers a solution in metal for such a roof slope. It's engineered and designed for even 1/4":12 which seems insane as that is basically a flat roof. If their roof product leaked and failed, they would not be certified and allowed in code.

    1. noddon | | #21

      Peter and Malcolm:
      I believe you are both correct, but it just depends on snowfall and ice buildup weather in your climate zone, to limit the roof slope to a 3:12 pitch for standing seam. I believe a lower slope such as 2:12 will work, or even 1.5:12 but you need to place peel and stick underlayment on the roof, or another similar product. Some roofers recommend strapping the roof above the underlayment to allow condensation to run between the panels. A torch on roof would be more economical in that case, and would give you a longer life span than an asphalt shingle roof.
      I have a recently completed snap lock standing seam metal roof on my new home. I used Gold Synthetic underlayment. I didn't use strapping above the underlayment. I has survived one winter with typical snowfall. The roof pitch is 3:12, the minimum recommended by Pro Lock.

      1. Expert Member
        MALCOLM TAYLOR | | #22


        At the risk of flogging a dead horse in this discussion... Isn't the necessity 0f adding waterproof underlayment under the metal roofing an admission it can't be relied upon in low slope situations, and needs a back-up because of the likelihood it will leak?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |