Sometimes a door, usually an interior door, will slowly close all by itself. Barring supernatural forces in the home, there is a very logical reason for this phenomenon and a simple cure as well.
The thing you have to remember is that a door is a large slab of wood (usually) that represents a fair amount of mass or weight. The idea is that doors are designed to swing open and then swing back to close. Some doors slide on a track but we’re not talking about that kind here. The swinging door is “hung” on a series of metal hinges. Depending on the height and weight of the door there can be as little as 2 hinges or as many as 4 or more. A hinge is nothing more than 2 metal plates that interlock and are connected together with a metal pin. In most hinges the pin looks like a large nail and is removable. Some are fixed in place. Removing all the pins from a door setup allows the door to be temporarily removed from the opening for whatever reason you might require that.
Basically, one of the plates from the hinge is attached with screws to the door jamb. This is the fixed piece of wood that runs around the perimeter of the door opening. The other plate is attached to the door. Depressions in the jamb and door called mortises allow the plates to recess into the surface so as to be flush. The mortise compensates for the thickness of the plate. The plates are arranged very carefully in order that they will interface with one another and to allow the door to be properly positioned within the door jamb. At a minimum, there is one hinge near the top of the door and another near the bottom. If more are required then they are positioned between these two hinges.
After the hinge plates are attached the door is positioned so that the interfacing hinge plates line up and the pin is slid in from the top so that it won’t fall out due to gravity. That’s why it is shaped like a nail, the head prevents the pin from sliding down and becoming dislodged from the hinge. The pin is essentially an axle for the door to rotate on.
So why is my door closing by itself you ask? The number one reason has to do with its weight and the door jamb we spoke of earlier. If the door jamb that the hinges are connected to is not perfectly vertical in all directions but instead is leaning slightly then this will cause the door to slowly pivot all by itself. The term we use for being perfectly vertical is “plumb”. You may have heard of the term plumb bob which is basically a string with a weight attached to it and allowed to hang freely. The string in that case represents a perfectly vertical or plumb line that doesn’t lean in any direction. Ideally that is what your door jamb should be, perfectly plumb. But we don’t live in a perfect world and sometimes jambs are installed out of plumb or they become out of plumb as the home ages. This is due to structural settlement and sometimes can’t be avoided.
So the ultimate way to fix this would be to rehang the jamb so that it is plumb. In some cases that is warranted. But that is usually a lot of work so in most cases there is a quick fix that works most of the time.
The fix involves the hinge pin that we talked about earlier. When the hinge is brand new the pin is arrow straight, as it should be, in order for the hinge to rotate smoothly. But in our case we want to purposefully put a little bend into it that will create some resistance when the hinge is operated. This is how we do it. If the door has only 2 hinges, remove the pin from the top hinge. Be careful as the weight of the door will no longer be supported by the pin, and you need to either have a helper assist you or you need to place something between the bottom of the door and the floor to support it. If there are 3 hinges or more remove the pin from the hinge that is just below the top one. The door will still be supported by the other hinges in normal circumstances.
Now you need to put a slight bend into this pin. The ideal way is to place it between two rigid objects ( such as bricks) that are spaced apart. The space should be slightly less than the length of the pin. Place the pin on top of the 2 bricks with the head on one brick and the bottom of the pin on the other. Using a hammer, gently tap the pin in the center in order to impart a slight bend into it. Don’t bend it too much or you will never get it to slide back into the hinge knuckle. Replace the pin into the hinge by tapping it in with the hammer. Open the door and see if the door no longer closes by itself. If it doesn’t, then you should be all set. If it still closes or even partially closes, then you need to repeat the procedure with another pin. Or possibly you didn’t put enough of a bend in the first pin. Sometimes it’s a trial and error method that works best but you should be able, with a little work, to prevent your door from closing itself. Good luck!