Exterior door thresholds provide a weather-tight seal at the bottom of your outside doors. Many thresholds have a replaceable weatherstrip along the top of the threshold. If this is the case with your door, and the attachment to the floor seems secure, replace the weatherstrip to save money and avoid extra work. When the weatherstrip cannot be replaced, or the threshold comes loose from the floor, replacement may be your only option. Choose a threshold that is the same width and height as the piece you are replacing.
Remove the mounting screws from the top of the threshold using a cordless drill. Cut along the edge of the threshold with a utility knife to release the old caulk. Pry the threshold up from the floor using a pry bar. Start from one end and pry along the length of the threshold to release the seal from the old caulk. Remove the threshold from the bottom of the door frame.
Place the old threshold on top of the new and use it as a template to mark the new piece to length. Trace any notches from the ends of the original threshold onto the new piece with a marker. Cut the new threshold to length and cut the notches with a jigsaw and bimetal blade.
Scrape any old caulk from the sill where the original threshold was positioned in the bottom of the door frame. Sweep any debris from the area to prepare it for the new piece.
Apply a generous bead of 100-percent silicone caulk approximately 1 inch from the outside edge of the bottom of the new threshold. Tap the threshold into position in the bottom of the door frame with a rubber mallet.
Drive one 3-inch treated deck screw through each mounting hole in the threshold. Snug the screws tight to pull the threshold firmly against the sill underneath.
Caulk between the ends of the threshold and the doorjamb to provide a good seal using 100-percent silicone caulk. Install the weatherstrip on top of the threshold, if your threshold has one.