Door thresholds keep water out. They also serve to seal the bottom of the frame to keep moisture from wicking up into the frame, causing rot. Installing a threshold is usually a simple matter of driving screws down through the provided holes into the threshold into the sill plate or wooden sill. When installing thresholds on concrete an extra step is added to provide pilot holes for the necessary concrete screws.
Remove the original threshold. If there are concrete screws in the original back them out with a drill to remove the threshold. Most thresholds on concrete are nailed. Use a large pry bar and hammer to pry a nailed threshold away from the floor. Start at one end and pry carefully to avoid chipping the concrete. Work across the width of the door, tapping the pry bar under the threshold and levering up to loosen the nails.
Remove the loosened threshold and use it as a template to cut the new threshold to length. A jigsaw or reciprocating saw is the best way to cut a metal threshold. A fine-toothed blade on a circular saw works well for wooden thresholds.
Mark the ends of the new threshold using the original as a pattern. The ends will be notched to fit around the door frames. The pattern varies from door to door. Cut the notches using a jigsaw for best results.
Tap the threshold into place using a rubber mallet or hammer and wood block to avoid damage to the threshold. Do not force it. If the fit is too tight, trim it slightly and try again. Make needed adjustments until you have a snug fit.
Mark pilot holes for the concrete screws. With a 3/16-inch rotary mason's bit, drill into the concrete underneath each screw hole in the threshold. Drill to a depth of 1/8 inch to mark the location. Remove the threshold.
Drill a pilot hole in each location you marked in the last step using the same mason's bit. Make your holes 2-inches deep. Wrap a piece of masking tape around the bit, 2 inches from the tip to mark the depth. Blow the dust from the holes for a proper fit.
Set the threshold back in place. Drive a 2 ½ -by-1/4 inch concrete screw into each hole. Drive the screws until they are tight against the threshold. Do not over tighten. Caulk around the edges of the new threshold with latex caulk. Smooth caulking with a dampened finger for a smooth finish.