How to Install an Exterior Sill

The sill is the bottom piece of a window that supports the glass, trim and molding. A sill is cut to either sit flush against the wall or extend out from the wall. A window sill that extends on the inside of the wall is an interior sill, while a sill that extends on the outside of the wall, out of the building, it is an exterior sill. An exterior window sill not only adds a finished look to the outside of a window, but it can be used as shelf for potted plants and other outside objects as well.

Measuring tape
Wood board
Table saw
Sander
Fine grit sand paper
Wood stain or paint
Paint brush
Level
Wood shims
1 1/2-inch Roofing nails
Hammer

Instructions

- 1
  Measure the width and depth of the window frame with a measuring tape.
- 2
  Cut the length of the wood board to match the width of the window frame with a table saw. Cut the width of the board 2 inches longer than the depth of the window frame. If the window frame is 6 inches deep, cut the wood board to a width of 8 inches. Use the same type of wood as the window frame. Window frames and sills are often made from hard woods like pressure treated oak.
- 3
  Sand the window sill with a sander equipped with fine grit sand paper.
- 4
  Apply wood stain or paint to the sill with a paint brush. Use a color of stain or paint that will pair well with the color of the house. Let the stain dry before installing the sill. You may need to apply more than one coat to match the color of the other wood.
- 5
  Sit the window sill on the bottom of the window frame.
- 6
  Align the edge of the window sill with the inside of the window frame. The window sill should extend outside of the building.
- 7
  Set a leveling tool on top of the sill. If needed, slide wood shims underneath the window sill to make it level.
- 8
  Pound 1 1/2-inch roofing nails through the sill into the frame with a hammer. Don’t drive the nails so hard that you tilt the window sill.

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