How to Make Interior Window Sills

Place a piece of your wooden window casing against the side of your window where it should sit once installed. Mark the location on the wall where the outer edge of the window casing sits. The mark should be in line with the bottom edge of the window. Perform the same task on the other side of the window. These two marks will help determine the length of your window sill. Use your tape measure to find the distance between your two pencil marks. Add 2 inches to the total. Use your table saw to cut your wooden window sill runner to the length required.

Lay the cut window sill runner in place against the front of the window frame between the two pencil marks you made in Step 1. Use your scribes to measure the distance between the inner edge of the window sill and the window itself. Move your scribes over to the ends of the window sill where it extends past the window. Use your scribes to copy the size of the gap you measured onto the ends of the sill. These markings will allow you to cut away the sill at its ends so that its middle can slide in and fill the gap between itself and the window. The cut ends will then become sill extensions, which lend some style and functionality to the window sill. Cut along the lines with your jig saw.

Insert the cut window sill into place on the window frame. It should slide into place reasonably well. If there remains a space between the back edge of the sill and the window, remeasure with your scribes and cut the sill again the same way you did in Step 2. Set the cut sill in place again and check for any remaining gaps. Use you nail gun to set the window sill in place.

Measure and cut your side and top wooden window casing and rest it on top of your window sill for proper installation.