How to Fit a New Door into an Old Opening

Replacing a door isn't difficult—but it will require a little reconfiguring. We'll show you the easiest way to add a new door while using the old door's opening.

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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.

Before you begin, remove the old door and 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.

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What You Need

  • Tape measure
  • Block plane
  • Square
  • Router
  • Chisels
  • Drill/driver
  • Hole saw
  • 1-inch spade bit
  • Utility knife
  • New door
  • New hardware (if needed)
  • Shims
  • Matchbooks

Step 1: Measure and Trim Door

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.

Step 2: Check for Fit

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.

Step 3: Hang Door

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.

Step 4: Drill for Bolt

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.

Step 5: Attach Bolt

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.

How to Fix a Doorway with No Hinge Mortises

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.

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.

Bonus: Consider a Hinge-Mortising Jig

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.

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