Rebuilding a Window Sill

Measure the length, width and depth of your existing window sill. Trim a solid block of matching lumber or cellular PVC material according to the existing sill measurements. Set it aside for later installation.

Use the putty knife to score the calking line between the siding and casing. Pr off the existing caulking along the edge of the sill to separate the casing from the wall. Tap the pry bar or knife handle with the hammer to loosen and remove each of the pieces. Ensure that the flashing behind the casing is undamaged.

Slice through the old paint seam between the trim and window sill. Put on your protective goggles and cut the sill with a circular or pull saw, making a vertical incision down the length of the damaged sill. Position your pry bar underneath the sill and lightly tap it with the hammer to pry up the pieces. Remove any remaining nails from the wall.

Apply epoxy filler to repair areas along the old wood frame that show moisture damage or rotting.

Squeeze a line of waterproof, marine-grade adhesive along the old wood sill. Trim the back edges of the new board to ensure a secure fit. Pre-drill holes approximately 16 inches apart from each other across the front and back edges of the new sill.

Push the new sill into place, applying pressure along the areas with adhesive. Drive in 3½-inch deck screws until the heads are approximately 1/4 inch above the sill surface. Cover the screw heads with wood filler and allow to harden. Wait 30 minutes before sanding the adhesive smooth.

Apply a line of caulking next to the ends of the siding and reinstall the casing into the window frame, using 8d stainless steel nails and trim boards as needed. Cover the nail heads with adhesive and sand once dry.