How to Trim an Interior Door


There are two distinct methods of adding trim to an interior door. The most common method--mitering the joints at the top--is not that hard once you understand how it works. The second method uses decorative blocks at the top and sometimes the bottom for a distinctive look. Either method requires careful attention to measuring and cutting.

Things You'll Need

  • Combination square Pencil 10-foot tape measure Step stool Molding for door casing Plinth blocks - method 2 only Miter saw or Miter block and saw Hammer Finish nails Countersink Paintable caulk or wood putty

Mitered Corners

  • Begin by setting the combination square to make a 3/16 inch mark. Use the setting to mark a 3/16-inch reveal along the top and sides of the door jamb on both sides.

  • Measure from the floor to the left mark on the top jamb. Be exact. Stand on the stool so you can look straight at the measurement. Write down the measurement. Repeat this measurement for the right side.

  • Measure along the thin side of the casing and mark it for the left cut. Position the miter saw table to cut at a 45-degree angle. With the wide edge of the casing against the table, the cutoff piece you will use is on the left.

  • As a check, the thick side of the molding will be longer than the thin side after cutting. Also, be sure to cut along the right side of the mark. Cut the molding. Reposition the saw table, measure and cut the casing for the right side.

  • Position both pieces of molding on the door jamb. Insure that the marks align with the molding and nail in place with two nails

  • Measure across the top mitered cuts of the molding. Mark the final piece and cut it. Put in place and nail it, making the joints as tight as possible.

  • Repeat Steps 1 through 6 for the opposite side of the doorway if necessary. Finally, countersink all nails and fill the holes with paintable caulk or putty.

Decorative Blocks

  • Begin by setting the combination square to make a 3/16- or 1/4-inch mark. Use the setting to mark a reveal along the top and sides of the door jamb on both sides.

  • Nail the top and bottom plinth blocks in place. Insure they are perfectly aligned with edge of the door jamb by using the combination square reveal setting as a gauge.

  • Measure for all three pieces of casing. On the left and right, measure between top and bottom plinth blocks. Along the top, measure between left and right plinth blocks. Be exact; stand on a stool so you can look right at the measurement.

  • Measure, mark and cut the three pieces according to the measurements. Be precise.

  • Align the casing with the reveal marks and the inside edges of the plinth blocks. Nail in place.

  • Repeat Steps 1 through 6 for the other side of the doorway. Set all nails below the surface of the wood. Fill the holes with putty or painters caulk.

Tips & Warnings

  • Be precise in your measurements and cuts. Always view measurements up close and straight on. Use a step stool or three-step ladder to get yourself eye to eye with the measurements. Keep the tape straight and make every measurement the same. When marking for cutting, always mark the same way and cut accordingly. Be precise in your cutting. The finished job will reflect the care you take when cutting. Always check to make sure you are cutting to leave your work piece the correct length. The blade of the saw removes 1/8 to 3/16 of an inch of material, more than enough to leave a gap in your work if you cut on the wrong side or too close. A miter saw takes less practice to make perfect than a miter block. You can use a miter saw to "sneak up" on a measurement. Make the cut slightly long, then shave a little at a time to make the cut perfect. The ends of moldings are rarely cut square when you purchase them. Always cut a quarter to half inch off the ends before measuring them for cutting. A finish nailer will save you time by setting the nails below the surface for you. Nail the casing to both the rough door frame and the door jamb. A typical doorway needs four to six nails in both the jamb and the rough frame on each side. A total of six along the top piece is more than enough. The time to make sure a door frame is square is before you begin trimming. Hold a 2-foot level against the top jamb and a 4- or 6-foot level against the side jambs. If the top is not level or the sides plumb, make adjustments. You'll find it nearly impossible to make mitered cuts line up on an out-of-plumb doorway, and the joints between blocks and casing will show gaps as well. If you will be painting the trim and doorway, use painter's caulk to fill any gaps left when you are finished. Always paint with primer first, even if the casing came primed. The primer they use is cheap and you'll end up putting two coats on anyway.
  • Always wear safety glasses when cutting wood, pounding nails or using a nail gun. Always wear a dust mask when cutting wood. Hearing protection is a must when using a miter saw.

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