If your home is very old, first check if the top corners are square. One corner could be up to 1/4" (6 mm) higher than the other. Use a large framing square if you have one or use a square piece of cardboard. Measure the width of the opening in three places, because if a doorjamb is bowed, the width will be less in the center. [1] X Research source Measure the height of the opening at both sides of the doorway. If they are not the same, check if the floor is level. If the floor is higher at one side, the door should be cut to have this angle at the bottom. [2] X Research source
Allow for a gap of 1⁄8 inch (3. 2 mm) along the doorknob side and above the door. If a gap is too narrow the door may not close in hot, humid weather because it will expand. Allow for a gap of 3⁄8 to 1⁄2 inch (9. 5 to 12. 7 mm) under the door. If the floor is not level, cut the door at an angle to allow for gap that is the same on both sides. Allow for a larger gap at the bottom if the door swings out over a carpet.
If you’re replacing an old door, you can lay the old door on top of the new door and trace around it to easily mark where your cuts will be. If you must take off more than about 1⁄8 inch (0. 32 cm) of the door, use a circular saw. For smaller amounts, use a plane or a belt sander.
Before marking the locations, determine the direction that the door will open. The recesses will be narrower than the door’s width, but how much narrower depends on the width of the hinges. The door should open into the room, and you must be able to reach the light switch when entering, that is, the door knob side should be the side with the light switch. Hold one hinge 7 inches (18 cm) from the top of the door and trace around it with a sharp pencil. Hold the other hinge 11 inches (28 cm) from the bottom of the door and trace around it. [4] X Research source
Be careful. If you chisel out too much, it will be hard to fix.
The hinges must be perfectly flush with the door edge when screwed on, and with no angle. .
While holding the door in place with the hinges in the recesses, drill pilot holes through the centers of all the holes in both hinges. Use a Vix-Bit if you have one. [8] X Research source
If there is no gap at the top or the side of the door, use a plane or belt sander to create a gap. If the door does not completely close, the hinges may be mounted at a very slight angle. To correct this, loosen the screws that mount the door to the frame. Slip in a thin strip of cardboard to change the angle, then tighten the screws. You may have to make it thicker by using 2 or 3 thin strips.
Many doorknob kits come with a template to help you drill and chisel out the door to install the latch and doorknob. Mount the strike plate. It will be in the box with the doorknob and latch.
