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Properly flashing windows

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  • Properly flashing windows

    I'm going to have some windows replaced in the near future and would like some input from those more knowledgeable than I.

    Specifically, I want to know what I should see as far as flashing and watersealing is concerned. I'm a bit skeptical that nailing the window into place with a bead of caulk, polysealing any gaps around the frame, then taping up the edges and calling it good is adequate protection for the life of the window (which will probably be the life of the house).

    What else can/should be done?

  • #2
    Re: Properly flashing windows

    I totally agree with you. Relying on sealant and tape is a poor approach. Think about all the old tape you've seen... and this is what is going to protect your home's structure from water?? I'm in CA and we have window places that do what we affectionately call "California window replacement" which is just sealant. I would never go for that. Weather here isn't aggressive but even so you need to flash properly.

    What you need will depend on the type of siding and the contractor's preference. I prefer the siding to be cut back (or broken out if stucco) but leave the old felt or housewrap. I like sheet metal flashing installed (from the bottom up, weatherboard style) over the felt or housewrap (ie, the felt or housewrap should be between the flashing and the studs/sill plate/header). I think the vinyl coated sheet is probably best. I like the sill pan to have a bent-up lip on the inside and a bent-down lip on the outside. Then use polyurethane sealant on the sill, nailfin and all the nails or screws.

    Next, use poly on the outside. I've been using the P&L stuff from HD (about $6) which seems to be very tough and sticks well. But it's paint thinner cleanup, not water... so a little inconvenient. Worth it though. Even with the flashing, you want a good sealant to seal up the gaps. Most window places around here recommend that you don't seal up the bottom of the new window where it meets the siding so that there is a path for water to get out.

    Finally, if I can, I like to put an aluminum drip edge above the window. I use a piece of aluminum angle, primed and painted to match the siding and polyurethaned to the siding with a few screws. This helps keep water from running down the siding into the top edge of the window. Every little bit helps.

    In any case, I would get the contractor to describe exactly what he is planning to do to flash your window and make sure that the water path is sound. Anyone that says you don't need flashing..... don't hire.