How to Replace Odd-Size Interior Doors in an Old House

Side and Top

• Interior door openings rarely -- if ever -- exceed 36 inches in width or 84 inches in height, which is an industry standard. But some older homes have been built with door openings that do not fit contemporary building standards. Even if your doors have hollow cores, they still have solid-wood frames around the perimeters you can trim off. The doorknob side of the door is angled slightly, and doorknob settings shouldn't be tampered with, so always trim off the hinge side. The top and bottom perimeter doesn't matter, but if you trim off the bottom of the door and chip it slightly, it isn't as highly visible as the top -- so if you need to trim the door for length, trim off the bottom instead of the top.

Remove

• Start by removing the old door. Use a drill/driver to remove the screws from the hinges securing the door to the jamb. Place the door across two sawhorses and remove all the hardware from the door. Keep all the parts so you can put the original parts back on. Remove the door from the sawhorses and place the new door on the sawhorses. If the door has hardware on it, remove it. Place the older door on top of the new door. Flush the tops of the two doors together, and the doorknob sides of the door together. Using the old door as a template, trace around the new door at the bottom -- if necessary -- and the hinge side. Use a pencil with a soft lead that easily cleans off. The differences likely are only a fraction of an inch. If it's more than 2 inches, you're better off purchasing a new door because cutting more than 2 inches off of any door can compromise the integrity of the door.

Saw

• Install a cross-cutting blade or a combination blade on a table saw. Don't use rip blades or any type of blade with aggressive teeth that tilt from side to side. This type of blade will chip and splinter the door. Don't use a circular saw. This type of saw is too difficult to use for this purpose. Measure across the door perpendicular to the grain from the doorknob side to the traced-out line on the door on the hinge side. Set the blade on a table saw to the measurement. Using an assistant to help you steady and hold the door, turn on the saw and slowly slide the door, parallel with the grain, across the saw to trim off the hinge side. Trim the bottom of the door -- if necessary -- by placing the bottom of the door against the fence. It likely is only a few inches at most, or only a fraction of an inch between the blade and the fence. This produces a small cut-off piece, so keep a small push-stick handy to clear the cut-off piece as you and your assistant slide the door across the saw perpendicular to the grain on the door.

Hinge Mortises

• Use the old door as a template to cut mortises on the new door. Mortises are shallow recess where the hinges fit on the side of the door. Place the two doors together on the sawhorses. Use a tri-square and pencil to mark the mortise locations, using the old door as a template. Use one of the old hinges as a template and trace the hinge outline at the marks on the side of the door. Use a chisel -- beveled side facing up -- to cut the shallow mortise recess on the new door. Gently tap the end of the chisel with a hammer to cut the mortises flat. Test-fit the hinges, and trim the tracings out with the chisel until the hinges fit flat inside the mortises. Screw the hinges back onto the door, install the doorknob and hang the door. Test the door by opening and closing it. If it rubs or doesn't fit, mark the locations where it's rubbing with a pencil.

Adjust and Finish

• Remove the door from the hinges. Place it across the sawhorses and place a clamp on one end to hold it steady. Use a belt sander to sand 1/8 inch off the sides or bottom where it's rubbing, according to the pencil lines. Test-fit and sand again if needed until it fits. Sand and round all the corners using 100-grit sandpaper. Apply stain and at least two coats of lacquer to the side and bottom where the door was trimmed.