If you want to keep an old door but the old jambs are damaged or you want to install an old door in new construction, purchase a door frame, sometimes called a jamb kit, at a lumberyard or home center. The kit consists of three pieces of lumber 9/16 inch thick and 4-3/8 inches wide. The two side jambs are milled across the top to receive the third piece, the head jamb.
The first step is to make the door fit the opening or vice versa. The door should be about 2 1/4 inches narrower than the rough opening and 1 inch shorter than the distance from the header to the finished floor. If you have to cut a door a significant amount in width, trim from both edges to keep the stiles symmetrical. With hollow-core doors, try not to cut beyond the solid edges. If you have to cut into the hollow area, save the solid edges and glue them back into the door to reinforce it.
Expect to spend between 1 and 3 hours on this project. You need to be able to accurately measure, use power tools, and drive screws. Before you begin, make sure that the rough door opening is square and the used door is square and flat.
What You Need
- Table saw
- Circular saw
- Tape measure
- Layout square
- Utility knife
- Jamb kit
- 2 1/2-inch screws
- Doorstop molding
Step 1: Measure and Cut
Hook the tape measure into the side jamb rabbets or dadoes and measure down a distance equal to the length of the door. Add 3/4 inch clearance for an interior door or 1/4 inch for an exterior door, plus the thickness of any flooring that will be added later. Cut the jambs to this length.
Step 2: Mark Mortise Locations
Hold the hinge jamb along the edge of the door and lay out the hinge locations on the jamb. The top of the door should be just slightly more than 1/16 inch below the edge of the rabbet or dado. Mark the mortise locations with a sharp pencil.
Step 3: Scribe Hinge Outline
Scribe the hinge leaf outline onto the door edge with a sharp utility knife. Set a 1/2-inch straight router bit slightly deeper than the hinge leaf thickness. Rout close to the scribe lines. Finetune the mortise with a chisel.
Step 4: Attach Hinges
Cut the head jamb to a length equal to the door width plus the combined rabbet or dado depth plus slightly more than 1/8 inch. Screw the door frame together with 2 1/2-inch screws. Attach the hinges to the jamb and the door.
Step 5: Hang Door
Hang the door as you would a prehung unit. Check its fit in the opening before securing the jamb. Plane the strike-side edge of the door if necessary. Cut the stop moldings and install them.
What If the Door Needs New Hinge Mortises?
If a used door must be trimmed to size, cut or plane the edge to remove the old mortises. This prevents having to find hinges to match the old mortises.
Most doors require three hinges (hollow-core doors need only two). If neither the door nor the jamb has mortises, center the middle hinge and place the other hinges about 6 inches from the top and bottom of the door.
Cut the mortises in the door first. You can cut them with a router or a chisel. Clamp some 2x4 scraps on both sides of the door to keep the router from tipping. To make the job easier, buy a hinge mortise template for routing. Routed mortises have rounded corners; square them with a chisel if your hinges have square corners. Transfer the locations to the jamb.
What If You Don't Have a Router?
You can cut hinge mortises by hand with a chisel. Hold the hinge in place and scribe around it with a utility knife. Mark the depth of the mortise on the edge of the jamb with your knife, using a combination square as a guide. The depth of the mortise must be slightly more than the thickness of the hinge leaf.
Sharpen your widest chisel (1 to 1 1/2 inches is best). Cut down close to the mortise depth all around the perimeter of the mortise.
Once you have defined the outline of the mortise, pare away the wood until the mortise is cut to its full depth. Keep the bottom of the mortise as flat as possible so the hinge will seat evenly.