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Community and Q&A

Substituting Roofing Cement for Liquid Flashing

mcdonder | Posted in Green Building Techniques on

If I am using 30lb felt as my WRB under 1.5″ of ComfortBoard 80 with rain screen, can I flash my outie window bucks with asphalt roofing cement, or the acrylic based versions? It seems to have very similar properties as something like Prosoco fast flash, but much much cheaper.

Climate zone 3 central valley CA if that matters.


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  1. Expert Member


    Roofing cement initially is very similar, but then hardens and loses the flexibility that you need in a liquid flashing product.

    Plus, have you ever managed to use it without getting it on your hands, clothes, tools, pickup truck seats, the family dog? That stuff is evil.

  2. Expert Member
    BILL WICHERS | | #2

    Roofing cement hardens like Tar as Malcolm mentioned.

    There is an EPDM type rubber sealant too that I've used before that is much more flexible. I think it's more common in the commerical world for patching membrane roofs. It usually comes in 5 gallon buckets, but I have also seen one gallon cans. The usual commerical roof patching process is somewhere along the lines of "guess at where the leak is, then pour a five gallon bucket of sealer there and hope the leak stops". That's why all the patches on commercial roofs are about the same size :-)

    I think if you used roofing sealant on a wall it would run and give you that "old window glass" look.

    The rubberized sealants are just as icky and sticky and messy as the tar ones. You will also inevitably track some of it inside somewhere too and get yelled at. For us contractors, our customers yell at a us. For DIYers it's the spouse doing the yelling. The end result is the same though, and you'll find out this stuff is very difficult to clean out of carpet and rugs.


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