Understanding how to fit and install prehung interior doors is a useful skill for the home remodeller. In most cases, it will not take longer than an hour.
If you are installing an interior door in a new home, the door can only be successfully put into place after the door framing is complete. The majority of prehung interior doors have 4 9/16 inch-wide jambs and are designed to fit with walls that have 2x4 construction with 1/2 inch wallboard. If your walls are thicker than this, you must take this into consideration before beginning. Standard prehung doors will still work, but you will need to use jamb extensions attached to the edge of the door frame to make it work.
If you are replacing a door, be certain to take exact measurements to ensure that the replacement door is exactly the same dimensions of the old door.
Place the prehung door unit into the framed opening of the doorway. Position the door so the edges of the jambs are flush with the surface of the wall and the hinge side jamb is plumb. Determine the straightness of the hinge side jam with the carpenter's level.
Using the hammer, drive wood shims into the space between the framing and the hinge side jamb. Space the shims approximately 12 inches apart and insert them alternately from opposite directions. After the shims have been positioned, use the carpenter's level to make certain that the hinge side jamb is still completely level and has not begun to bend.
Using the 8d casing nails, hammer the hinge side jamb completely through the jamb, the shims, and into the stud on the other side. Do this at every point you have inserted a shim. The hinge side jamb should now be firmly anchored to the wall.
Take more shims and insert them between the frame and the latch side jamb as well as between the frame and the top jamb. Again, space the shims every 12 inches.
Shut the door and move the shims until the space between the door's edge and the jamb is approximately 1/8 inch.
Hammer casing nails through the jambs into each of the shims and into the underlying frame.
Cut off the shims using a handsaw to cut them flush to the wall surface, being careful not to damage either the wall or the door jamb.