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Community and Q&A

Window sill pan flashing

Michael Geoghegan | Posted in General Questions on

I’m using beveled cedar siding to create a slopped sill pan with DuPont flexwrap applied over top of it. Unfortunately we only added 1/4″ to the rough opening height, and with the cedar siding ripped to the depth of the window sill, the thick end is thicker than 1/4″. With the cedar siding cut where it is 1/4″ thick, it’s depth is about an inch or so shy of covering the entire depth of the rough opening. I don’t want to change the rough opening height on all of my windows, so can I just use a piece that isn’t as deep as the opening? I’m thinking if I do that it would be best to install it flush with the inside of the house.

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  1. Expert Member
    Malcolm Taylor | | #1

    The only place the sill benefits from being sloped is from the exterior sheathing to the inside of the window frame. Having the rest of the RO flat also makes casing it easier.

  2. GBA Editor
    Martin Holladay | | #2

    GBA readers,
    Don't be like Michael. Think about your flashing plan when you frame your window rough openings.

    Malcolm gave you good advice. Another option is to leave the rough sill level and install a back dam on the interior side of the rough opening.

  3. David Meiland | | #3

    You could also just cut down the rough sill a bit to either allow for the cedar, or (more likely) to create the slope you want without the cedar. I have done this plenty of times when replacing windows in existing openings. A combination of skilsaw, sawzall, and power plane does the job easily. As Malcolm points out, you only need the slope where the window is, not necessarily through the entire depth of the opening.

  4. Mark Fredericks | | #4

    Michael, given your situation I think David's advice is good, just sink any nails well below the surface, first! Another option for removing the material is with a grinder as seen in this recent Fine Homebuilding Video:

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