DIY Exterior Windowsill Replacement

Open the window. Remove the screen as well, if your window has one. Most screens pull out of a window opening easily when you pull up the bottom corners and push them out to release them from the track. Slowly lower the screen until you've freed the top corners from the track as well.

Use a utility knife to cut the caulk that covers the gap between the rough opening and the exterior window trim. Place a pry bar behind the window's exterior trim. Pull the trim forward and off the house. Now would be a good time to replace the window trim, if you wish. If you plan to put the trim back on, you can leave the upper trim portion intact and remove just the bottom and two sides.

Hold a reciprocating saw blade perpendicular to the windowsill, near the center. Rest the saw blade's tip directly behind the stool. Make a cut through the windowsill. Move slowly so you do not cut into the window's rough opening. Move the saw blade down about 2 to 3 inches, and make another cut.

Hold a chisel directly behind the stool, on the windowsill. Hit the chisel's handle with a hammer to push it through the windowsill and separate the sill from the interior sill. Pry the cut rectangle out from the center of the windowsill. Work the back of a claw hammer underneath the chunk and pry it up and out, if necessary.

Hold the chisel on the windowsill, directly behind the remaining portion of the sill. Hammer the chisel through the sill to separate it from the stool. Work your way down both sides of the remaining windowsill.

Wiggle one side of the windowsill gently. If it does not move at all, check along the side jamb to make sure it is not nailed to the sill. If you do find a nail, hold the chisel against the nail's head. Hammer the chisel's handle to work the chisel blade under the nail head. Move all around the nail until the head has been raised slightly above the sill's surface. Grab the nail head with nippers and rock the nippers back and forth to pull the nail up and out.

Pull the remaining windowsill portions out from underneath the side jambs. Use the chisel and hammer to clean up the stool's edge and make it as straight as possible.

Apply a bead of wood glue to the new sill's interior edge, where it will touch the stool. Slide the new sill underneath the side jambs and press it tightly against the stool. Hold a scrap piece of wood against the sill's exterior edge, and hammer on the scrap wood to help push the sill in place.

Drive a screw through the sill's underside and into both of the side jambs to pull the sill snug against them. Apply a bead of caulk along the new sill's sides, including the gap where the sill touches the stool. Either put the old trim pieces back around the window, or cut and fit new trim pieces and nail them in place. Apply a bead of caulk over the gap where the sill meets the trim pieces.