How to Frame a Pre-Hung Exterior Door

Lay a level on the floor in the rough opening. If the floor isn't level, insert two wooden shims underneath the rough opening's wooden sill, about 4 inches apart, to raise any low areas. Lift the prehung door into the opening. Set the level on the door's sill to check that the shims are in the correct locations.

Measure the rough opening's sill, and add 6 inches. Cut two pieces of adhesive flashing to this length. Peel off the paper backing on one of the flashings. Center the flashing over the sill. Move the flashing forward until its front edge extends about 2 inches past the rough opening's sill's front edge. Press the adhesive flashing onto the sill with both ends extending about 3 inches up either side.

Make two small cuts on either side of the flashing with a utility knife so you can fold the flashing's front edge down over the rough opening's sill. Peel the paper backing off the second flashing strip. Center the flashing over the sill with the back edge lined up with the sill's back edge. Press the flashing onto the sill.

Apply two squiggly beads of silicone caulk over the flashing. Run a strip of caulk around the rough opening's edges against which the prehung door's exterior molding will press.

Lift the door into the opening from outside. Ask a friend to hold the door in place while you open the door and go inside. Center the door in the opening. Hold a level against both side jambs and the header jamb. Insert wood shims behind each hinge, between the frame and the rough opening. Insert wood shims on the opposite side jamb at approximately the same height as the hinges. The shims hold the door plumb in the opening as well as fill the gap between the door frame and rough opening. If there is no gap between the frame and rough opening, do not use shims.

Back out one middle screw from each hinge with a drill and Phillips driving bit. Replace each screw with a 3-inch wood screw. Drill 9/64-inch pilot holes through the door frame, wooden shims and into the rough opening. Space these pilot holes approximately 16 inches apart and along both side jambs and the header jamb. Some prehung exterior doors come with these holes already drilled.

Hammer a 3-inch finishing nail through each pilot hole. Bend the extending end of each wood shim back and forth to break it off. Spray expanding foam into the gap between the door frame and the rough opening from the interior side.