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Community and Q&A

Plywood seams: tape or liquid flashing?

pbout | Posted in Green Building Techniques on

Building up our roof, adding polyiso on top of the roof deck, then another layer of plywood. I plan to tape the seams of the lower plywood deck to air seal. Any comments on using liquid flashing instead of tape to seal the seams of the plywood?

Are there any additional benefits from also taping/liquid flashing the polyiso, and/or the top layer of plywood deck above it? I will have a standing seam metal roof with Delta Trella underlay. 2×8 Rockwool in the rafter bays below.

CZ 03 coastal.

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  1. GBA Editor
    Martin Holladay | | #1

    The vast majority of roofers would rather use a high-quality tape than a liquid flashing for this purpose. You have enough hassles up on a roof without messing with liquids.

    1. doughpat | | #3

      Sorry to resurrect an old thread, but can you elaborate, Martin? I prefer liquid flashing when I can use it. Whats so tough about liquids on the roof? I can imagine:

      -The product sloughing/running down as it dries (though its pretty dang thick)
      -Spillage (not really an issue as its in a sausage)

      But other than that....why do you advise against?

  2. ohioandy | | #2

    I've used the 4" wide Zip tape for a similar airseal application. Can others with experience weigh in on whether this is a good idea? It's tenaciously grippy, especially once rolled and baked for a while in the sun. It seems to score well in Martin's ongoing Backyard Tape Tests, and it's readily available where I am. Obviously, it's designed to pair to the Zip panels, with their proprietary coating, but I'm betting that it's a good choice--for an air barrier--even on unprimed plywood or OSB. I stand to be corrected.

  3. doughpat | | #4

    I like this roof 'assembly'.....the 'foam sandwich' approach. I don't have much room to work with (2 x 12 shed roof), and this idea of adding some insulation ABOVE the roof deck seems like it would be relatively efficient and simple. Is there a term for this kind of roof assembly? I'd like to do more research.

  4. GBA Editor
    Brian Pontolilo | | #5

    Hi Ryan.

    I don't know that there is a name for it, other than exterior insulation. It is an excellent way to insulate a roofline, but there are a lot of details to get right including the ratio of exterior to interior insulation R-values. If you are considering this route, I suggest you read this: How to Build an Insulated Cathedral Ceiling

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