How to Install an Exterior Door to a Rough Opening

Measure from the bottom of the new door's sill to the door's bottom. Then measure from the rough opening up to the flooring. Add a treated 3/4-inch plywood board to the rough opening if opening-to-flooring measurement is less than the door measurement. Measure the opening's width at the bottom and cut the treated plywood to this length. Secure the board in place with construction adhesive and 2-inch screws.

Measure the rough opening's width at the top. Cut a metal Z-flashing strip to this length using a tin snips. Push the strip onto the opening's top board and secure it in place with screws.

Insert a silicone caulk tube into a caulk gun. Cut the tube's tip off with a utility knife. Run a caulk bead along the rough opening's exterior edges.

Lift the new exterior door into the rough opening from outside. Insert a wood shim between the jamb and the rough opening on either side of the door near the top to hold the door in place.

Set a level against one of the door's side jambs. Adjust the door until it is sitting straight in the opening. Lay the level on the door's sill. Insert wood shims between the sill and the rough opening until the sill is level.

Go inside and check for gaps between the hinge jamb and rough opening. Insert a shim between the jamb and rough opening to close the gap and keep the door plumb. Secure the door by driving 2-inch screws through the jambs and into the rough opening. Use three to four screws on each side.

Score any exposed wood shim with a utility knife. Bend the shim back and forth on the scored line until it breaks.

Insert spray foam insulation into the gap between the door jambs and the rough opening. Allow the foam to harden according to the manufacturer's directions. Use a putty knife to scrape off the excess foam.