Step-by-Step

You'll probably have to remove the apron, stool, and stop molding to remove a sill. Pry out these pieces carefully to avoid damage.

If the sill is embedded on either side, you may have to saw out a section from the center of the sill. Drive the end pieces inward. Use these as patterns for the new piece.

Gently tap the new sill or saddle into place. Don't force it. If it resists, remove and sand the ends, beveling them slightly.

Using rust-resistant nails, secure the sill from underneath. Countersink the nails, then caulk the nailheads and the ends of the sill.

Nail a saddle at both ends and in the middle. Countersink and hide nailheads with putty. Caulk both ends of the saddle.