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Community and Q&A

Installing leak barrier over existing leak barrier for roof replacement

FlappinPat | Posted in GBA Pro Help on

We plan to replace an existing roof with GAF Timberline Ultra shingles.  We will have the entire roof deck cleared of all previous materials, except the Grace Ice and Water Shield Leak Barrier.  The Grace brand is said to be so adhesive that it is nearly impossible to remove it from the roof deck without damaging the roof deck.  We consulted with GAF’s Technical Specs Department, and they verified we can install one layer onto the existing leak barrier.  GAF sent me a technical bulletin (see Attachment) that, although allows the one layer to be installed over the existing, it recommends the contractor try to remove the existing.  The existing Grace Leak Barrier extends 6 feet from the roof edge around the roof’s perimeter, and 3 feet in valleys and below ridge vents and other openings.  The bulletin recommends feathering by extending the new leak barrier 8 inches past the existing.  This is possible, because the previous install did not extend leak barrier over entire roof deck or over the fascia board.  The bulletin says there is a chance that non-compatible leak barrier asphalt compound can soften, flow, and drip, and potentially cause staining to the fascia, gutters, and building exterior.  Have you experienced this?  Is the risk less with GAF’s Stormguard Leak Barrier (film surface) rather than with GAF’s WeatherWatch Leak Barrier (granulated surface)?  I appreciate your advice.

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  1. GBA Editor
    MIKE GUERTIN | | #1

    I've investigated this same issue with several self-adhering / self-sealing Ice Barrier roof underlayments. The best course of action to take is always install the same brand / model of Ice Barrier as the original one. Almost every manufacturer has a warning not to install their product over another brand and vs. vs. And there may even be incompatibility between different types of Ice Barrier from a manufacturer (granular / film capped - rubberized asphalt/butyl....)

    So to be safe, if GCP Grace Ice and Water Shield is currently down on the roof, install the same brand.

    This question is going to come up more and more as roofs installed in the 80's, 90's and early 2000s are replaced and the first layer of Ice Barriers gets covered over.

    And in the next few years we'll be seeing more second reroofs where there are already two layers of Ice Barrier installed. In those cases the IRC requires that we follow manufacturers' instructions for reapplication. And all the manufacturers I examined only permitted 2 layers of Ice Barrier after which either the sheets have to be removed off the sheathing OR the roof sheathing the membranes are bonded to will have to be removed.

    I think FL code may have a sensible installation requirement that supersedes MFG install instructions - they require non-adhesive underlayment be cap nailed to the roof at 12 in. oc before installing self-adhering membrane. That way it can be completely removed without removing the roof sheathing.

  2. Expert Member

    " they require non-adhesive underlayment be cap nailed to the roof at 12 in. oc before installing self-adhering membrane"

    I'd never thought of that. A really good idea.

  3. Jon_R | | #3

    It would seem that this is adding a layer just to negate the adhesive. Why not just use a single layer without adhesive? Shingles create lots of nail holes, so some more to attach the underlayment shouldn't matter.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      That's a good point. As long as the membrane has the same fastener sealing properties as I&W Shield, securing it with fasteners rather than adhesive, makes no difference. I suppose you could also use I&W Shield and only peel a narrow strip of the backing paper.

  4. Peter Yost | | #4

    If the ice and water shield membrane was installed using a primer, yes, impossible to remove without damaging the roof sheathing. If not, the adhesion of modified bitumen is very temperature dependent. Don't know your climate or season of work but that could change quite a bit how difficult and damaging removal of the membrane is.


  5. FlappinPat | | #5

    Thank you all for your advice. Good points to consider.

  6. Colin63 | | #7

    Asphalt to asphalt is no problem, assuming you are using GAF brand ice and water to maintain the warranty as they require 3 or more GAF brand components to complete a warranted roof system it is fine. Transition from EPDM to asphalt requires a buffer, aluminum usually. Unless you are using an off brand ice n water you're good

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