For a door to function properly, the single most important consideration is that its frame be "square." In carpenters' terms, this means that both upper corners of the door frame meet at an exact 90-degree angle. If the door frame is not square, one edge of the door will bang against the door frame instead of fitting snugly within it. Other door performance issues may also exist, such as squeaks or the door swinging by itself. If the door frame is not square, the door will never function properly. Fortunately, there is a simple method homeowners can use to measure and square the door frame as needed.
Remove the door by driving out the hinge pins, using a hammer and slotted screwdriver. Tap the screwdriver gently against the head of each hinge pin until it is loose, then remove it by hand. Set the door aside.
Measure the door frame for squareness by placing a carpenter's square -- a metal tool shaped like a triangle -- in each upper corner of the door opening. If the door frame is perfectly square, each corner will measure exactly 90 degrees on the carpenter's square. If it is not square, one corner will measure more than 90 degrees and the other will measure less than 90 degrees.
Measure the precise amount of adjustment needed to bring the door frame back into square. Measure from the top corner on one side of the door frame to the bottom corner of the other side of the door frame, using a tape measure. Do this twice, once from each top corner to the bottom corner. Picture a big "X" in the door frame; that is how the tape measure should be placed.
Calculate the difference between your measurements by subtracting the smaller number from the larger number. If the numbers are not equal, that means the top corner of the door frame with the larger measurement is higher than the other side of the door frame. For example, if one measurement is 265 cm (106 inches), and the other is 262.5 cm (105 inches), the door is out of square by 2.5 cm (1 inch). The 265 cm (106 inches) side is the "high" side.
Adjust the door frame by removing the attachment screws on the "low" side of the door frame. Gently tap a few wood shims beneath the bottom of the door frame on that side. Keep tapping and adding wood shims until the door frame moves upward by half of the difference between your two side-to-side "X" measurements.
Measure the door frame again using the "X" pattern as before. Continue to adjust the door using shims until both "X" measurements are equal.
Install the door frame attachment screws tightly with a screw gun. Add additional wood screws if necessary to ensure that the door frame does not drop to its original position. Remove the shims. Hang the door back on its hinges, and insert the hinge pins. Test the door for correct operation.
If the door frame seems stuck when you try to adjust it, carefully cut along the outer perimeter of the frame with a utility knife to break the caulk bead or dried paint that is holding it. When only a small adjustment is required, it is a good idea to add a few more frame attachment screws so that the original attachment screws don't simply go back into their original screw holes.
Be sure the door frame is secure before putting the door back on its hinges.