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How to flash a flangeless wood casement window in a 100-year-old house?

jpe52 | Posted in GBA Pro Help on

The house has cedar shingles nailed to horizontal wooden strips. the strips are nailed to the wall studs. There is no insulation, tar paper, or anything else. I am planning to install a new flangeless wooden casement window. Don’t know how to flash in this situation….

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  1. GBA Editor
    Martin Holladay | | #1

    First, can you tell us your name?

    Ideally, your retrofit work would include more than just replacing the window. At some point, you'll need to face the fact that your walls need a water-resistive barrier (WRB), an air barrier, and insulation.

    But I'll assume you can't afford to fix all of these problems right now.

    To install a new window, the first step is to flash the rough opening. Here is a link to a GBA video series explaining how to flash a window rough opening: Window Sills That Won’t Rot.

    Ordinarily, the sill pan flashing sheds water on to the WRB. Your wall doesn't have a WRB, so you need to flash the sill pan so that it drains to the exterior of your siding. Because the lip of the flashing will be visible, you might want to use an attractive product like copper (with a bend to form a vertical leg that drips over the siding) to bridge the gap between the sill pan and the siding.

    At the head, you can install the shingles so that the shingles lap over the head flashing.

    Once your rough opening is prepared, you can install the window using masonry clips.

  2. severaltypesofnerd | | #2

    You can detail it like the other windows in the house, no wrb required. Your house works by leaking less rapidly than it can dry out.

    Your new flangless window can simply sit in the historic sill, with some new trim and new caulk, but with everything open at the bottom. Everything drains down to the historic sill, which is sloped to the outside.

    If you're cutting a new window frame, get the best quality wood sill you can and make sure it extends well past your siding. Make sure it has a drip edge, to avoid water wicking back behind the siding.

  3. ABradford_1 | | #3

    Greetings, I am reviving this old question with a similar situation where the old house with no insulation, wood sheathing, rosin paper, then cedar shingles is not in a position to be resided, but the new window is flanged and the opening is being altered (reduced). It sounds like a traditional wood sill with overhang is necessary, but in this case would it be appropriate to break off the bottom flange?

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