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Air sealing flat roof assembly

user-6610113 | Posted in General Questions on

Remodeling a flat roof in climate zone 3.  1/2″ plywood sheathing is currently in place over previous 3/4″ sheathing. The plan is to add 2″ of polyiso and another 1/2″ plywood then either modified bitumen vs TPO 

Question set 1: In terms of air sealing on top of the sheathing, I’ve seen both tapes and peel and stick membranes recommended.  If I go with peel and stick what brands are good but cost effective?  Do you recommend that I find something permeable?  Is taping the seams alone ok?  What are the tradeoffs?

Question 2 : Ay reason to use plywood on the top of this sandwich vs a coverboard?  The roof will have moderate foot traffic and will house solar panels on skids.

Thanks in advance.  Your site is amazing!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Matt.

    As far as your first question goes, I can't recommend a specific peel and stick membrane. But if you go that route, permeability is not a concern. With 2 inches of polyiso and an impermeable roofing membrane above, your roof assembly (rafters framing, etc.) will have to dry to the interior should it get wet. Also, it is fine to tape or use a fluid applied air sealing on the sheathing, and probably less expensive than a full membrane.

    As for question #2, what is the "coverboard" you are considering?

    You may find this helpful: Insulating Low-Slope Residential Roofs

    1. user-6610113 | | #3

      Thanks Brian! I see pictures of people putting tape and then a peel and stick. Can you comment on why one would just tape the seams only vs peel and stick vs both?

      Coverboard options plywood vs HD polyiso. The roofer is saying he can put the roof membrane right over the regular polysio, but that gives me pause. Thoughts?

      1. GBA Editor
        Brian Pontolilo | | #4

        Hi Matt.

        Perhaps some people use a membrane for extra insurance against water intrusion, I'm not sure.

        Hopefully someone with more experience with low-sloped roofing will come along to correct me if I am wrong here, but I think it is common for roofing membranes to be installed directly over the rigid foam insulation on low sloped roofs.

        However, if the roof is going to see foot traffic and you want to install an additional layer of sheathing, it will be important to make sure there is no risk of fasteners popping and damaging the membrane, which means screws should be used and no nails. If your roofer is experienced, they should know what is best here.

  2. Deleted | | #2


  3. user-6610113 | | #5

    A side question regarding this article

    Once the plywood seams are taped what is the benefit of the felt underlayment?


  4. Expert Member
    Peter Engle | | #6

    Flat roof systems are a different animal than typical sloped residential roofs. Also, much of the information on this site is more directed to northern climates, and zone 3 is warmer than most of us are used to.

    Typical commercial flat roofs have vapor barriers on both sides. This is considered a no-no in residential construction, but it is not a problem, so long as no moisture gets into the roof assembly, or so long as the assembly is not damaged by moisture. in commercial roofs, this is (mostly) achieved using an impermeable steel deck, moisture-tolerant insulation materials, and the rubber roofing membrane. You can make the equivalent by applying sheet or fluid-applied membrane to the decking prior to insulation. The insulation layers do not have to be taped. I would not use plywood sheathing on top of the insulation, though it is acceptable by most membrane manufacturers. I would fasten the insulation boards down using large washers and screws (standard practice), and go with a fully bonded membrane, bonded directly to the fleece-faced insulation. You can use a cover board if you want, but it is probably not necessary. Areas of foot traffic for maintenance should be indentified and protected with either walking boards or another layer (or two) of roofing membrane. If you are going to have regular traffic (like an occupied roof deck), then you should have some sort of paving system (pavers, wood decking, etc.) placed on pedestals above the membrane. Skids for solar equipment should also be padded with another layer or two of membrane.

    Once you select the membrane, read the manufacturer's installation instructions for any special details or requirements.

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