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Thoughts on detailing a window with sill against roof

nhbean | Posted in Energy Efficiency and Durability on

I am in the process of residing/re-roofing our 100-year-old home in climate zone 4, and I have a window whose sill is pretty much right on top of the roofing (see the pic below).  It looks like there is a long galvanized flashing running along the roofline just under the sill, then the siding and step flashings.  It also seems the previous roofers made a dam of roofing tar against the left side of the sill to direct water away from the wall.

My question is as we move forward, how should I detail this problem area?  We are wrapping the walls in a peel-and-stick housewrap and I was going to do Grace Ice & Water as the roofing underlayment.  We’re adding two inches of foil-faced polyiso with a 3/4″ rainscreen detail.  The siding is Boral TruExterior beveled siding, and is rated for ground contact.  I’ve been extending the  jambs by 2 & 3/4″ with cedar and replacing the sills with new 2x cedar material as well (as most had some decay).

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  1. Expert Member
    Akos | | #1


    You can try to flash your way out but it will still be a very iffy install.

    This is one of those where the only good solution is to raise the window sill up a couple of inches so you have proper drainage underneath. This will mean a slightly smaller window, but it is worth it for the peace of mind down the road.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Nathan.

    I agree with Akos. That's too vulnerable of an area to take a belt-and-suspenders approach. I suggest raising the sill height to give yourself the space you need to flash and detail both the window and the sidewall-to-roof transition properly.

  3. Expert Member
    Peter Engle | | #3

    Another option is to cut down the roof. There is a relatively common treatment for this that consists of cutting a notch in the roof about 6"-12" around the bottom of the window. Vertical cut uphill and a (nearly) flat bottom. See sketch. Traditionally the notch would have been lined with soldered sheet metal, but modern rubber membranes work OK, too. The wall side of the notch then gets fully wrapped in peel & stick membrane to resist the inevitable ice damming, with the peel & stick wrapped up into the window jamb and sill, and the tallest backdam on the sill you can manage.

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