Replacing a window sill is a fairly simple repair. You will need to have a piece of lumber the same width and thickness as the sill. Removing the sill to take for comparison when shopping can save time and confusion. Examine the other parts of the window while you have the sill removed. Patch or replace any other parts that show damage or rot. Keeping up with window repairs can extend the useful life of your windows.
Remove the horizontal moulding at the bottom of the window directly underneath the sill. Use a flat pry bar and a hammer to remove the moulding. Pry the moulding away from the wall, using the pry bar. Work your way across the width of the window, moving the pry bar as you go.
Open the sash of the window to the top. Cut around the edges of the window sill with a utility knife to release any paint or caulk that may hold the sill in place. Tap against the bottom of the sill with a hammer. Use the pry bar, if necessary, to pry the sill out of its place. Cut stubborn sills down the middle with a circular saw with the depth set to the thickness of the sill. Remove the two pieces with the hammer.
Cut the new sill board to the same length as the original. Sills are notched at the ends to fit into the window frame. Use the original sill as a template to mark the ends of the sill. Cut the notches with a jigsaw for best results.
Sand the sill thoroughly. Slightly bevel the edges that will make contact with the window frame for a better fit.
Prime all surfaces of the new sill with oil-based primer to seal the wood. Allow the primer to dry thoroughly before proceeding with installation.
Tap the new sill into place using a rubber mallet or hammer and wood block. Nail the sill to the subsill with a hammer and finishing nails. Fill the nail hole with solvent-based wood filler. Allow the filler to dry and sand it down smooth to blend with the surrounding sill surface.
Caulk the edges of the sill with latex painter's caulk. Allow the caulk to dry before applying high-gloss latex paint to complete the repair.