How to Fix a Sagging Door

Deck Screws Fix Sagging Doors  
A heavy door can sag after it's been hanging for a while. Its weight can also cause the door jamb to twist. But before you consider trimming the door to make it fit, give this trick a try.
The problem in most cases is that the fasteners holding the door in place can't support the door's weight. The jamb is typically held in place with a few 6d finish nails, and the screws holding the door hinge to the jamb are fairly short. To support a heavy door, you've got to use long screws so you can connect with the wall framing.
The drawings show what I do to fix a sagging door. First remove two of the three short screws in the hinge and replace them with 2" deck screws. These longer screws will reach into the stud and help secure the door. If the door is really heavy, I'll even remove the door stop and drive 2" deck screws through the door jamb above and below the hinge plate. This helps keep the jamb from twisting as the casing and framing swell and shrink. The stop will hide the screw heads after it’s reattached.