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Class A underlayment assembly order for a ZIP deck & metal roof

complication_jones | Posted in General Questions on


Looking for a straightforward way to do a ZIP roof deck and metal roof to meet Class A in wildfire country, when I’m guessing I’m going to want underlayment better than the bare minimum between the two.
1. Does putting gypsum on *top* of a 5/8″ ZIP roof deck make sense for this case (or any case)?
2. Since it’s pulling teeth to get 5/8″ ZIP where I am, can gypsum instead directly below a 7/16″ ZIP deck both meet minimum deck thickness, and, work equally well as underlayment in that order, leaving the top of the ZIP unmolested to be air and bulk water barrier?
3. What’s a better way? Cost, efficacy, buildability.

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  1. canada_deck | | #1

    I don't know a lot about this subject but the proposal doesn't sound great to me. If you want to get a non-combustible layer, I would think that there are other types of cement-board products that are non-combustible but much more appropriate for that application. Drywall is very fragile and susceptible to mold.

    What about just going with 5/8" plywood, a Class A underlayment (e.g. Owens Corning Pro Armor), and then your metal roof?

    What type of internal ceiling/attic do you have? Is this a vaulted or non-ventilated ceiling or just a normal ventilated attic?

  2. complication_jones | | #2

    Hi, thanks. It'll be non-vented (don't want embers) in a hot desert climate with very little rainfall over the course of the year, so I'm not worried about a lack of drying potential.

    Anyway, OK. If I need to run zip to the interior of the entire roof assembly instead, by running it over the top wall plates and below the bottom chord or bottom of the rafters, and then basically exsulating that interior sheathing layer from that point, (really sandwiched beneath uncoated plywood roof sheathing and underlayment) that can make sense to me, too.

    1. canada_deck | | #3

      I have very little knowledge of that type of assembly or climate so don't take too much from my comments.

  3. Expert Member
    Akos | | #4

    Unvented with just a deck under the roof deck means spray foam on the underside. Even if you are in desert country, an unvented roof with fluffy insulation can fail from moisture that escapes from the inside in the winter.

    If you want to avoid SPF, you need rigid insulation above the roof deck. This is not a problem for most standing seam roofs but does mean either another roof deck above the rigid or strapping it out with horizontal 1x4s. Some snap lock roof panels that come with clips can also be mounted onto metal bearing plates which avoids the extra roof deck or 1x4s.

    For any metal roof you'll need an underlayment to deal with night time condensation. The simplest is for this to be your class A layer. Under that it can be any type of solid roof deck, ZIP or not it doesn't matter.

  4. complication_jones | | #5

    Thanks, Akos. As above, in terms of moisture escaping through the ceiling-to-roof-deck assembly, one notion is that I can run ZIP continuously under the bottom truss chords or underside of the rafters, and can strap another layer to the underside of that, to not disturb it. I see a few of the big kids doing that detail with the ZIP coming up and over the wall across the top plate. In that situation, seems I can avoid spray foam, and run rockwool above that control layer, instead.

    In terms of condensation or bulk water under the metal roofing, whether it's ZIP, Pro Armor, Grace Vycor, roofing felt, or whatever: I still am concerned about picking a layer to sandwich below the metal roofing that won't fall prey to a slow-burning ember that finds its way in, nor to a fast-blowing front of superheated air coming ahead of a wildfire.
    So, what is that durable layer?

    1. Expert Member
      Akos | | #6

      Having a solid warm side air barrier tied to the walls is definitely a good thing. A taped sheet good under rafters does make for a decent air barrier.

      This will help with but won't stop moisture movement, there are always leaks plus some vapor diffusion. Once the warm moist air makes it under the sheathing it will condense. With no venting, there is nowhere for this moisture to go so during the heating season the MC of the sheathing will slowly increase. If you are lucky, it won't get to the point where you get mold and rot. Up to you if you want to take that chance.

      I would pick an assembly that is guaranteed to work, fixing damaged roofing is a very expensive proposition. When it comes to roofs, robust is the key word.

      As for ambers under the roof, I can't help you much. I'm not in a fire area so I'm not familiar enough to comment. Generally good metal roofs should be installed in a tight manner with edges either hemmed or capped with flashing so any gaps won't be all that large.

      You can also look at some of the gypsum based underlayments used for flat roofs such as densdeck. The 1/4" stuff is not too much extra weight, easy to lay but can be spendy.

  5. user-6184358 | | #7

    Look at the metal roof mfg website for an approved Class A assembly - They typically offer DensDeck boards or GAF Versashield fiberglass roll underlayment. Here is one companies literature in a link to a Class A assembly
    Also check screw embedment and spacing - screws hold much stronger in plywood vs OSB - so make sure they give you a screw pattern for OSB and for your thickness of OSB- the 7/16 might not work

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