Things You'll Need
Phillips driving bit
1 1/2-inch screws
When you replace your exterior door, you may notice a slight gap between the new door's sill and the threshold. Sill extenders close this gap and prevent water and cold air drafts from entering underneath the door, making the doorway more energy efficient. Mount the sill extender to the door before you secure the door in the opening. Purchase a sill extension kit that is compatible to your exterior door from your local home builder's supply store.
Lay the exterior door face up on two sawhorses. Insert a tube of silicone sealant into a caulk gun. Apply a bead of sealant to the exterior door sill's two channels. One channel is near the sill's top edge and one channel is in the center.
Line up the sill extension's ridge with the exterior door sill's top channel. Push the sill extension along the exterior door sill until the ends of the two sills line up. Wipe the extra silicone sealant off the sill with a damp paper towel.
Test fit the sill spacer under the exterior door's sill. If the sill spacer seems to bulge out at the ends, trim off about 1 1/4 inch from both ends of the spacer's ridge with a pair of tin snips.
Apply a bead of silicone sealant along the sill spacer's ridge. Push the sill spacer back under the exterior door's sill. Secure the spacer in place with 1-inch screws.
Squeeze sealant between the sill extension and exterior door sill on both the left and right sides. Secure the exterior door in place. Check whether the sill extender extends past the door threshold. If it does, position a sill support underneath the sill extender and secure it to the door frame with 1 1/2-inch screws.