How to Replace a Broken Door Jamb

Break the paint seal from the old jamb and the door casing, using a putty knife. Using the sharp end of the putty knife, cut the seal and scrape off debris. Break the seal for the entire length of the jamb.

Break the paint seal from the base molding and the wall attached to the jamb.

Pry the broken jamb away from the molding, using a pry bar until the nails used to secure the molding become visible. Pry enough room to see the nails, but not so much that the molding becomes damaged, requiring replacement. Leave 1/4 inch exposed.

Saw the exposed nails in half, using a blade-mounted hacksaw. This will detach the molding from the door casing. Once you remove the nails, you are able to see how the side jamb interlocks with the top jamb.

Cut the side jamb horizontally, using the hacksaw or a power saw. Cut the jamb a few inches below the top jamb. Using the pry bar, detach the side jamb completely. Use a wood chisel to break off any remaining pieces of the side jamb still attached to the top jamb or any area that impedes installation of the new door jamb.

Measure for the new jamb lengthwise with a tape measure, then cut the piece to fit. Cut the new jamb with a hacksaw or power saw. Check the measurements by placing the new jamb in place to see if it is plumb. Make any adjustment cuts based on how the side jamb fits with the top jamb. You may need to use wood spacers to make the side jamb even with the top jamb.

Nail the jamb in place with a hammer and replace the molding at the base. Paint the new jamb to match your remaining trim. Use wood filler on any damaged area of the molding, nail back in place and paint with a paintbrush.