How to Fix a Door That Closes by Itself. Most of us have had them at one time or another, a door that just won't stay open. You open it up and walk away and when you come back the door has swung partially or fully closed. Sometimes it's the result of moving air (like when your furnace comes on), but in many cases it doesn't have anything to do with your furnace. Here are some ways you can fix your interior doors and stop them from closing by themselves.
Understand that a door closing by itself is usually caused by the door's hinges being out of plumb. What that means in plain English is the top hinge isn't directly over the bottom hinge; the hinges are out of alignment.
Try checking to see if any of the screws holding the hinges in place are loose. Simply tightening the screws up may be all that's required to stop your door closing by itself. Slightly longer screws may be able to get a better bite and be less prone to working loose.
If tightening the screws doesn't work, you can try to get the door hinges back into alignment. Start by removing the screws from the bottom hinge at the doorframe. (You don't need to take the hinge off the door itself.)
Using the hinge as a template, mark and make the holes on a thin piece of cardboard, then put the cardboard into the hinge mortise and reinstall the screws. The piece of cardboard will change the alignment of the bottom hinge and hopefully bring the top and bottom hinges back to plumb.
Alternatively, you can remove one of the hinge pins (the middle one if there are three hinges on the door).
Place the hinge pin on a concrete block or brick (something your don't mind marking up a little), then hit it with the hammer to bend the pin, just a little.
Put the hinge pin back into the hinge. Since you've bent the hinge pin it may not slide in easily, but try tapping it gently to force it into the hinge.
Once the bent pin is in place, the increased friction in the hinge should be enough to stop the door from closing by itself.