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Help sorting through metal roof assemblies.

veeneck | Posted in General Questions on

I’m in CZ4 with a traditional attic (cellulose above ceiling drywall), and I’m trying to weigh a few metal roof options. Roof will be 24 gauge, standing seam with no striations, around 12”-16” per panel. The more I read on these forums and other sites, the more I talk myself in circles. I’m curious what others would do in my shoes. My considerations are longevity, performance and oil canning.

 

Option 1: Synthetic peel and stick with metal on top. Ventilation through attic at soffits and out ridge.

 

Option 2: Same as Option 1, but with 3/8 backer rod under each panel to reduce oil canning? Does this actually work?

 

Option 3: Same as Option 1/2, but with added ventilation above the roof deck. With drying through the attic, this is not necessary for drying — it would just be for performance / longevity. My architect is in favor of the thermal performance. Various products promote this, as do companies / builders online. I’ve also read speculation of longer life. Might come down to just a cost issue. With this route, there are 3 options:

 

 – Mesh underneath the metal. Not enough airflow.

 – Dimple mat. More gap than mesh, but still too small. Also gives backing to the panels, which may help oil canning? Can be used in a system with radiant barrier like Sharkskin. 

 – Notched underside wood battens. 3/4” to 1.5” gap. Wonder if the spacing of the battens affects oil canning at all? 

In all cases, vent through eaves and out ridge. Roof slope must be high enough.

 

Option 4: It looks like some places promote just above deck ventilation. With a synthetic underlayment, I’m not sure how the sheathing would dry in this situation. In a poor design situation (i.e: no overhangs), would this be enough. What about with gable venting thrown in?

Thanks for any thoughts!

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    veeneck,

    With an ventilated attic, I don't think there is any good reason to add a layer of anything between the roof panels and underlayment, either to help cool the roof, or promote drying. The sheathing has more than adequate drying to the underside, neither the metal panels or sheathing care how hot they get, and the attic can be kept cooler with good ventilation.

    There are several reasons not to add a small top-side gap.
    -The gap creates a concealed space for condensation to occur. No gap, no condensation.
    - Most Snap-lock panel roofs still rely on some gasketted fasteners to secure gable trim, roof boots, etc. Those either compress a flexible underlayment (leaving dimples), or are too loose to be relied on.
    - Roof panels directly backed by the sheathing are much less prone to mechanical damage from workers of debris hitting the roof.

    I don't know if the backer rod trick actually works. The panels I use have very small striations that conceal or limit oil-canning. I suspect completely flat panels will exhibit some no matter what you do. https://www.westform.com/products/residential/prolok-12-and-16

  2. veeneck | | #2

    Thanks, Malcolm. I know you're not a fan of ASV. The only data I can find on it is here:

    https://www.researchgate.net/publication/26901052_The_Impact_of_Above-Sheathing_Ventilation_on_the_Thermal_and_Moisture_Performance_of_Steep-Slope_Residential_Roofs_and_Attics

    Unless I'm reading it wrong, it looks like reflective paint was more effective at heat control, but that you can still get another ~15% gains with ventilation. It also looks like the moisture content of the OSB was lower with ASV, so I'm not sure how that works with the introduction of more condensation in the gap. This is all a bit over my head, so I'll spend some more time with the paper to try and get my facts straight.

    As for the backer rod, if anyone has any first hand experience reducing oil canning, please share.

    1. Expert Member
      MALCOLM TAYLOR | | #3

      veeneck,

      My prejudices may be coloured by there I build. Roof and attic heat aren't significant problems here in the PNW. Or perhaps more accurately, they are outweighed by the primary problem which is controlling bulk water intrusion. RDH looked at this in the context of permeable underlayment and concluded "the improved water ingress resistance provided by (an impermeable) membrane as compared to the roofing felt would likely outweigh the potential reduction in drying capacity."

  3. Expert Member
    AKOS TOTH | | #4

    I haven't had good experience with keeping larger flat metal section from oil canning. The backer rod does work but don't expect miracles. If the substrate is not dead flat (which is never the case with older homes), it will oil can.

    If you are like me and that will be bothered by this, get panels with striations. Unlike oil canning, most striations are not visible once the roof is up.

    1. veeneck | | #5

      Good to know, thanks. Follow up questions -- do you have better luck with 12" panels not oil canning? Also, is there specific profile for the striations that is less visible once installed?

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