Window Sill Options

Written by duane craig
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Window Sill Options
Expanded window sills provide new spaces. (Bouquet with roses in a room on a window sill. image by Yuri Bizgaimer from

Window sills bridge the gap between the window and the lower part of the wall. They must blend well with the trim of the window and still fit with the other decor of their surroundings. Sills are an important architectural detail inside. They can also make design statements outside when buildings have thick enough walls and exterior sills have been planned.


Any building can use sills on the exterior but they are most effective when the windows are recessed. This is true for buildings with thick walls like adobe, but also for buildings that have exterior insulation beneath the siding. Use brick or stone sills when those materials are the building's exterior finish. Turn bricks their long way and angle them slightly downward for a brick sill. Use stone slabs cut to the windows' widths for sills where stone is used as siding. Consider low maintenance wood like cedar, pressure treated or redwood for stucco or adobe walls. Use inch-and-a-half-thick lumber to complement brick, stone, stucco and adobe.


Use 3/4-inch lumber for interior sills and cut them to extend an inch or an inch-and-a-half beyond the wall surface. Angle, or round the edges for a softer look when the window is not framed with trim. For new installations, consider letting the sill extend to each side of the window and let side trim pieces die into it.

Make old, narrow sills more functional by adding a new, deeper sill right over top. Cut the new sill to the width of the old sill but a few inches deeper. Then fasten it in place with countersunk screws or finishing nails driven down from the top and into the old sill.

Interior sills can do double duty as plant stands and even cat perches. Make the sill 6 or 8 inches deep to create a place for plants. Add 6-inch shelf brackets below them to increase their load bearing capacity. For a cat perch use 3/4-inch plywood and make it 10 to 12 inches deep. Use one or two shelf brackets below for necessary support.


Exterior sills pose some special challenges. They create a place where snow and moisture can collect and stay. If they are not installed so they have a slight outward slope water can work its way into the wall cavity and cause rotting. Install sills so they fit underneath the window frame, or use flashing that does, and caulk all seams with a high quality silicone caulk.

Finish interior window sills to match the window or room trim. If your sill is for special duty, like a plant stand, then dress it up with contact paper so it is easy to clean and is waterproof. Use a pillow or your pet's favourite bed on a cat perch.

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