How to Change Door Trims in House

Remove the old door trim by setting the front edge of your prybar against the outer edge of the trim, where it meets the wall. Tap the prybar so it slides behind the trim, then gently pry the trim off.

Take the measurement of the top horizontal span of the door opening, measuring from the insides of the two upper corners.

Transfer the measurement to a piece of molding, with two marks on the bottom edge of the piece and the measured length between them.

Set the trim face-up on a miter saw, with the bottom edge against the fence of the saw. Slide the piece so one of the marks is in front of the blade. Turn the blade 45 degrees outward, to point to the closest end of the trim. Make the cut.

Move the trim to put the second mark under the blade. Turn the blade to 45 degrees in the opposite direction as before, which will again point outward. Cut it.

Set the cut trim along the top of the door opening, so it’s lined up along the edge of the casing. The mitered ends angle out and up from the two upper corners. Secure it there with your trim nailer, shooting nails every foot or so along the top and bottom.

Measure the first vertical side of the opening. Cut the upward-facing edge of the trim in the same manner as before, at 45 degrees. Cut the downward-facing edge at 90 degrees, straight across.

Install the side piece with the upward-facing miktered end butted to the mitered end of the horizontal piece, so they form the corner. The flat downward-facing end should sit on the floor. Repeat the process for the other vertical side.