How to Replace a Sagging Garage Door Header

Open the garage door. Locate the door header -- the long piece of wood spanning the top of the door opening-- from inside the garage. Remove the screws from any garage door hardware or opener that may be attached to the header.

Remove the trim from the exterior wall around the door opening by carefully prying it off with a small pry bar or flat-blade screwdriver. Remove the trim from the vertical and overhead horizontal jamb areas in the same fashion.

Remove the exterior siding from above the garage door opening in the area of the header by carefully prying out the nails with a small pry bar and removing the pieces of siding. Obtain a siding removal tool -- available from home centers or the siding manufacturer -- to remove vinyl siding without damaging it.

Cut through the nails around the perimeter of the header with a reciprocating saw. Remove the header using a hammer and large pry bar.

Obtain dimensional lumber that is the same width and thickness as the lumber used in the old header. Measure the length of the old header, and transfer this measurement to the new lumber. Cut two pieces to length with a circular saw.

Sight down the length of each piece of lumber to find the crown or high point. Position both pieces so that the crowns will be facing the top of the header when assembled. Sandwich pieces of scrap plywood between the two cut pieces to bring the total thickness of the new header to that of the old one. Make sure the edges of all the pieces line up. Secure the pieces together with several 16 penny nails.

Insert the new header -- with the crown facing upward -- into the opening in the wall. Use a hammer to bring the piece into alignment with the other framing members of the wall, if necessary. Secure the header into place by nailing it to all other adjacent framing members with 16 penny nails.

Replace the jamb trim, exterior trim, and reattach any garage door or opener hardware as required. Nail or snap the exterior siding into place. Run a bead of caulking around the exterior trim of the garage door to aid in water intrusion.