How to Install Basement Window Sills

Scrape the window ledge to loosen dirt, debris and old caulking, using a putty knife. Remove loosened debris from the window ledge using a vacuum cleaner or brush.

Draw a line across the approximate center of the unfinished window ledge and new windowsill.

Measure the distance from the center line closest to the window frame on the unfinished ledge to the left corner closest to the window frame and transfer this measurement to the same location on the windowsill. Measure the distance from the center line closest to the front edge of the window ledge to the front edge of the left corner and transfer this measurement to the same location on the windowsill. Repeat this step for the right side. This step provides a tight fit for windows that are out of square.

Use a speed square and pencil to connect the two measurements on each side of the sill. Cut the windowsill on each end with an angle grinder and attached diamond-cutting blade. Check the fit of the windowsill and make any required adjustments. It should fit in the opening without any binding, which could break the sill during installation.

Cut the tip of a tube of silicone with a razor knife to allow for a 1/4-inch opening and load it into a caulk gun. Apply three of four dollops of silicone to the unfinished window ledge.

Set the windowsill in place and align properly. Use a small level to check and adjust the sill. Remove excess silicone with your finger, a rag and acetone.

Apply a bead of latex caulk around all joints of the windowsill. Smooth the joints with your finger and remove excess latex caulk with a damp rag.