If an existing door is damaged or worn, you can put a new door into the doorway. Often the easiest way is to remove the existing jambs and casing and install a prehung door. However, if the doorway is square or if the existing trim is difficult to replace, you can keep the doorway and replace the door.
Unless your house is very old, you’ll have little problem finding doors to fit. The most common door height is 80 inches, although most are available 78 inches tall as well. Common interior door widths are 24, 28, 30, 32, and 36 inches. Height and width are often stated in feet and inches: 2-6 for 30 inches or 3-0 for 36 inches, for instance.
Check that the doorway is square. If not, scribe and plane the door. If the doorway is well out of square, remove the casing, cut through the nails holding the jambs, and reattach the jambs so they are square.
About 3 to 4 hours
Tape measure, block plane, square, router, chisels, drill/driver, 21?8-inch hole saw, 1-inch spade bit, utility knife
Measuring and laying out, planing, cutting mortises, drilling
Remove the old door, check the opening for square, and acquire new hardware if necessary.
New door, new hardware (if needed), shims, matchbooks
If you have more than one or two doors to hang, consider investing in a hinge-mortising jig, which is a template for guiding a router to cut perfect mortises for hinges. Several models are available. The simplest (and least expensive) ones cut one mortise at a time, leaving the placement of the matching mortise up to you. More complex jigs come with multiple templates that will position matching mortises on the jamb and door edge.
Most hinge-mortising jigs work with a router equipped with a template guide. The guide could be a roller bearing on the bit, as shown here, or a metal collar attached to the router’s base that surrounds the bit and runs along the mortise template.
Measure the opening and purchase a door that fits. If you have to trim the door, take an equal amount off each side. Plane a slight bevel (about 5 degrees) in the direction of swing on the strike side of the door to ease opening and closing.
What If. There are no hinge mortises in the opening? Step 1
Some houses have openings that are trimmed out like a doorway with jambs and casings but have no door. These might be found, for example, between a kitchen and dining room or a den and a hallway. If you decide to add a door, you’ll have to cut mortises in the jambs to hang the door and install stop molding.
If the opening is finished only with drywall, you may be able to treat it as you would a rough opening and install a prehung door. If the rough opening was framed to a standard size and 1/2-inch drywall was used, check to see if the opening is square. If it is you can nail the door frame directly against the drywall opening. Otherwise you’ll need to remove the corner bead and the drywall from the jamb and header faces to make room for shims.
Check carefully for nails, then lay out the hinge mortises on the jamb. Rout close to the layout lines. Finish the mortises with a sharp chisel. Hold the door in the opening with shims and transfer the marks.
Check the door fit. Ideally there should be a gap of about 1/8 inch at the top and along each side, and about 3/8 inch at the bottom. Use cardboard spacers or folded matchbooks (four thicknesses equals about 1/16 inch) along with shims underneath to maintain the spacing. Mark the mortise locations and cut the mortises.
What If. There are no hinge mortises in the opening? Step 2
After the door is hung and the lockset installed, rub a little lipstick on the bolt to mark the jamb for the strike plate. Using a spade bit, drill a 1-inch hole for the bolt; mortise the strike plate into the jamb with a chisel.
Hang the door. New locksets come with a template to help you locate where to drill holes. If you are reusing a lockset, extend a line across the face of the door with your square, making sure it is centered on the strike plate. Measure the lockset to determine the distance the hole should be from the edge of the door. Drill a 2-1/8-inch hole through the door with a hole saw.