Extending window sill

Old 03-29-10, 04:59 AM
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Extending window sill

I would like to extend the window sill on an Anderson double hung prepainted window.

The manufactured sill is flush with the wall. I would like to extend the manufactured sill by adding 1-1/2" of trim to act as a sill so that I could trim out the window in a more craftsman or traditional way.

Has anyone done this and are you happy with the result?


Old 03-29-10, 05:04 AM
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Welcome to the forums Mary!

I'm not a carpenter but I've painted more than a few windows trimmed out the way you want. I believe they install the apron [moulding below the sill] first, that gives them a better way to secure the sill. The sill is either butted up to stop at the bottom of the window [caulk the joint] or that stop is removed [looks best imo] and the sill takes it's place.

I'm sure one or more of our carpenters will chime in later with better advice
Old 03-29-10, 05:30 AM
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What marksr said is correct, The window isn't completely flush there should be an inch between the window jam and the window. Get a 1x5 board about 7 inches wider than the window (wide enough so that the trim on both sides of the window will fully sit on the board with about an inch left over on each side). Then put the board up to the bottom of the window and mark where the inside of the window is as well as the distance to the window. Notch both ends off so that the sill sits flush against the window. The sill will actually be higher than the window bottom at this point. Nail the sill in place and install the apron below it. Look at anoter window and what I'm saying will become clearer.

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