A door that won't stay closed can be more than just a nuisance. Doors that close properly and completely are not only safer, but they also are better for controlling the temperatures in your home. In the winter time, doors that close tightly keep more cold air outside where it belongs. When it's hot in the summer, that cool air produced by your air conditioners will stay inside where it will keep you comfortable. There are a few things that you can do to fix a door that won't stay closed.
Even if the strike plate isn't causing your door problems now, it could in the future. Check to see where the door hits the strike plate. Is the latch securing into the hole effortlessly? If not, you'll want to address this first. All you have to do is use a metal file or another similar tool to widen the hole where the latch goes into the strike plate. Once you have made the hole wide enough, the door should fit into the strike plate hole more easily, and the door should shut tightly. If the door continues to come back open, you will need to consider one of the other causes and fixes.
Fix the Stop Molding
The stop molding is the strip of material that prevents the door from swinging open and shut freely. What often happens is that the stop molding hits the door cleanly at the bottom, but as you look up toward the door knob, there is a gap between the door and the stop molding. This is another easy problem to fix, but it requires two steps. The first step requires a wood block and a hammer. Place the wood block against the stop molding, and strike it with the hammer until the stop molding moves far enough to be properly aligned. The only problem is that this will leave a small strip of unfinished wood where the stop molding once was installed. A staining pen will solve this problem easily. All you do is trace the small strip with the staining pen.
Fix the Door Jamb
There should be a nice, wide gap between the door and the door jamb so that the door can fit into the frame and close properly. To create a bit more space, you might want to put a small piece of material behind the door hinge. This material should be sturdy, yet soft enough so that you can drill the screws back through it. Unscrew the flap of the door hinge. You then must place the material behind where that piece of the hinge is situated. Screw the screws back into place, and there should now be enough space. You now have a door that closes properly.