How to Install a New Door into an Old Opening

Measure the doorway opening and purchase a new 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.

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.

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.

For the bolt, drill a 1-inch hole through the edge of the door with a spade bit. Make sure the bolt hole is centered from front to back and aligned with the center of the strike plate.

Insert the bolt into the hole in the door's edge. Align the bolt plate with the edges of the door and trace around it with your utility knife. Remove the bolt and remove the wood inside the outline to create a mortise for the plate. When you're finished, the plate should be slightly below the surface.

Editor's Tip: 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 or a metal collar attached to the router's base that surrounds the bit and runs along the mortise template.

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 might 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.

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.