How to Build an Interior Windowsill Out of Wood

Measure the window for the interior sill. The sill is 1 inch wider on each side than the window casing -- the molding that surrounds the window -- or 2 inches wider on each side if there is no casing.

Transfer the sill width to the 1-by-4 board and cut the board to length on the miter saw. Hold the board horizontal against the lower part of the window opening and mark the edges of the opening on the board.

Measure the distance from the sash frame to the wall. (The sash frame is the part that encloses the pane of glass and moves up and down.) Take the measurement at the bottom of the window opening and on each side of the window. The sill will fit against the sash frame.

Align the square with the mark you made on the board. Draw a line across the board along the square. Measure along the line the same distance as from the sash to the wall, and make a mark. Place the square on the end of the board and align it with the new mark. Draw a line from the end of the board to the mark. Repeat on the other end of the board.

Cut along the lines with the jigsaw to remove a rectangular section at each end of the board. The cut board should now fit into the window opening against the sill and the surrounding walls.

Place the level on the sill parallel to the window. Insert shims beneath the sill to adjust the height at either end until the sill is level. Nail the shim to the wall framing under the window opening with finish nails until the nail heads are 1/16 inch above the sill. Place two nails at each end and every 16 inches between the ends.

Place the point of the nail set against the nail heads and tap the nail set with the hammer to set the heads below the surface. Fill the nail holes with wood putty.