How to Make Window Sills & Casings

Hold a piece of window casing against each side of the window opening as though you were installing it. Use your pencil to mark the location on the wall at the bottom of the window where the outside edge of the casings end. These marks will determine the length of the window sill you need. The window sill should run at least to the edge of the side casings, if not a bit further.

Measure the gap between the two marks and add 1/2 inch on each side. Cut a strip of 1-by-4-inch hardwood to fit with your circular saw. Measure the space between the inside edges of the window jamb. Measure the distance between the wall surface and the window itself to determine how big a notch should be cut into your sill. (Reference 1)

Mark the measurements onto the sill material using your pencil and straight edge, then use your jigsaw to cut along the lines. Insert the sill into the slot beneath your window frame and use your level to ensure that it is plumb. If not, use wooden shims to correct it, and use your power drill to make pilot holes through the sill and into the sash below.

Countersink the pilot holes with your countersink bit, then use your screw gun and wood screws to fasten the sill to the sash below.

Measure the interior edge of the window jamb. Transfer these measurements to strips of window casing. Use your circular saw to cut the bottom end of each piece of side casing flat. Use your miter box and saw to cut the upper end of each piece of side casing, and both ends of the top casing at 45 degree angles. The mitered angles should start at the pencil mark and extend outward from there, increasing the longer end of the angle beyond the measured pencil mark.

Install the cut casing strips around the window opening. Use your hammer and finishing nails to fasten each strip in place with the bottom edges tight against the sill, and the top edges in tight corners. Drive one nail every 16 inches or so.