DIY Window Sill Replacement

Cut through the caulk around the window's exterior trim, with a utility knife. Wedge a crowbar behind the trim and pull it forward, off the house. If you want to remove the entire window, go inside and remove the interior trim with the crowbar. To avoid damaging the drywall, put a putty knife between the crowbar and the wall.

Insert a reciprocating saw blade between the window frame and rough opening. Run the blade around the window's edges to cut through the nails or screws securing the window in place. Remove the window from the opening.

Cut a 2-inch-long chunk out of the sill's center, using the reciprocating saw. Do not cut through the rough opening's wood underneath the sill. Insert a putty knife underneath the chunk and hit the knife's handle with a hammer to loosen and remove the chunk.

Work the sill's two side pieces out from underneath the side jambs. Use the putty knife and a hammer to loosen the pieces. Look under the sides for any nails or screws holding the sill pieces in place. Remove them if necessary.

Insert the new window sill into the gap left by the old window sill. If you decide to make your own window sill, use your old one as a template to determine the appropriate length, width and slope. Making your own window sill requires a bit of carpentry knowledge.

Drive screws up through the new sills underside and into the side jambs to hold the sill in place. Put a tube of silicone caulk in a caulk gun. Cut off the caulk tube's tip with a utility knife. Apply a bead of caulk around the window sill's exterior edges.

Mount the window back into the opening and replace the window trim, if you decided to remove it.