A.08 Masonry sill detail

Window sills can be tricky.
First of all, the air barrier applied in the field of the wall needs to terminate properly at the sill. This is typically done with blocking all around the rough opening to provide a termination point. The blocking raises the profile of the CMU so water needs to work harder to get into the wall, and it also provides a backing surface to anchor a flanged or non-flanged window. In this detail, the blocking is wrapped with sheet membrane compatible with the fluid air barrier, with a generous overlap per manufacturer’s requirement.
Then, our beautiful cast stone sill offers a horizontal (let’s hope not) or nearly horizontal ledge where the rainwater and snow can accumulate, so we need to understand how to flash and weep below the masonry sills. This includes returning the flashing vertically at the ends. Watch the video to learn more.
Window sills may be complex, but when they’re built by skilled BAC bricklayers and mason contractors, we can be confident they’ll perform as designed.
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