Is your door frame a little off? If you have a crooked door, and it doesn't hit the floor – at least not as it should – it might be the jamb that is causing it to slam. It's not a serious undertaking if you need to correct a crooked door. Get out your tools and carve out a small chunk of time to get your door plumb within its frame.
Take a close look at the door's frame when the door is closed. There should be a consistent gap between the top and down the latch side of the door that is approximately eighth of an inch. The weight of the door tends to push its bottom toward the hinge side. If this happens, the top will be uneven as well. A frame has to be level and plumb in order for the door to operate properly. When it isn't, you may need to pull the frame and get down to the studs to get the door to close correctly.
Removing the Door and Casing
To get to the frame, close the door and tap the hinge pins loose with a hammer and a long, large nail. If the door locks, lock it so it doesn't fall. Once the pins are visible by about an inch, pull them out. Ease the door off the hinges. If this proves difficult, slide a pry bar under the bottom of the door to help take the weight off the hinges, and slide the door away from the hardware. Clean off any paint or caulk where the casing meets the door jamb, then use a flat pry bar to gently remove the casing.
Tweaking the Jambs
Level the hinge side jamb first with a 4-foot level. Have a pack of wood shims to make the jamb level as you work your way up from the floor to the top of the jamb. If you are installing a prehung door, make sure to remove the slab so that the frame is easier to manipulate. With a 2-foot level, level the header of the door frame. Use wood shims for adjustments as you move along the header. Make sure there is an even gap between the existing frame and the door slab. Hand the slab onto the frame and secure the door closed. Adjust the non-hinged jab so that its opening is consistent with the frame. Use wood shims if needed.