The sill is the foundation of the door frame. Sills rest underneath the threshold and are typically only found on exterior doors. Replacing the sill is a fairly simple process. Begin by examining the sill. If it is intact and not suffering from rot. Repair it by patching the damaged area with epoxy body putty. Sills that are damaged with missing pieces or soft from rot will need to be replaced.
Prop the door open securely before beginning your repair. Locate the threshold. The threshold is the plate directly underneath the closed door. Remove the screws from the threshold and lift it out of position. Set it aside for reinstallation.
Tap a pry bar under one end of the sill. The sill is the flat, wooden plate located directly underneath the threshold. Pry the sill up to loosen the nails. Move the bar along the width of the sill, prying the sill up as you go. Tap the sill out from under the jamb with a hammer on the inside edge of the sill. Drive the sill plate toward the outside of the door. Cut stubborn sills down the middle with a circular saw with the depth set to the thickness of the sill. Remove the two pieces. Set them aside to use as a pattern.
Measure the sill. Cut the new sill blank out of lumber to the same length as the original. Mark the ends of the new sill to match the original. The ends of the sill are notched to fit around the door jamb. Cut the notches with a jigsaw for best results. Dry-fit the new threshold and make any adjustments needed to get a proper fit. Sand the sill, bevelling the faces slightly that will come in contact with the door jamb for a better fit.
Apply a coat of primer to the surface of the sill. Make sure to coat both top and bottom to seal out moisture. Allow the sill to dry thoroughly before installing.
Install the sill in position. Tap it into place with a rubber mallet or hammer and wood block. Nail the sill to the wall-sill plate with a hammer and 16-penny nails. Use at least four nails spaced across the width of the sill.
Caulk around the edges of the sill. Apply two coats of exterior latex semigloss paint. Follow the label instructions for best results. Be sure to allow adequate drying time between coats.
Reinstall the threshold. Use new screws for the best attachment and replace the weatherstrip along the top of the threshold for a good seal.