A door is only as effective as the weatherstripping and other weatherproofing elements that seal it. A damaged sill can allow moisture to penetrate the door frame, causing rot and other damage. Replacing the sill becomes necessary when it has suffered too much damage for it to be effectively sealed or patched. While replacing the sill, it is a good idea to also replace the threshold of the door frame to ensure proper fit and a weatherproof seal.
Prop the door open. If there is a storm door, remove it from its hinges, or use the closer lock to hold it open. Locate the threshold. The threshold is the metal or wooden piece that is directly under the door when it is closed. Typically, rubber weatherstripping is on the top.
Cut any caulk or other adhesive around the edges of the threshold. Remove the screws from the threshold and lift it out. If it is stuck, use a flat pry bar to lever it up and out of its placement. Set it aside to reinstall or to use it as a template for the new threshold.
Pry the sill up and out of its position and set it aside for measuring. The sill is the wooden plate that sits under the door frame directly under threshold. It is typically nailed down and will require vigorous leverage to remove.
Cut the new sill to the same length as the original. Use the original sill as a template to mark out the side notches on the new sill. Cut with a jigsaw for best results.
Brush on a coat of oil-based primer to the new sill. Cover the entire surface top and bottom. Drill pilot holes for at least four mounting screws. Use a 1/8-inch bit. Use a countersink bit to countersink the pilot holes slightly so that the screw heads will run flush.
Set the new sill in place after the primer has dried. Use a rubber mallet or hammer and wood blocks to tap the sill into position. Attach the sill to the sill plate of the house foundation with 3-inch treated deck screws.
Seal the edges of the new sill with latex painter's caulk. Paint the sill with at least two coats of semigloss exterior latex paint. Allow it to dry.
Cut the new threshold to length and notch the ends to match the old threshold if you are replacing it. Use a jigsaw with a metal cutting blade.
Set the threshold into position. Tap it into place with a rubber mallet or hammer and wood block. Drive 3-inch treated screws through the threshold screw holes into the wood sill. Seal the edges of the sill with latex painter's caulk.