How to Trim a Square Archway

Trimming out a square archway or other opening in your home is essentially the same process as trimming out a doorway, except the door isn't there. The trim doesn't serve a structural purpose, but rather dresses up the edge of the opening so it looks clean and finished. By using trim that matches with trim around the windows and along the floor and ceiling, you can visually tie the archway into the rest of the room.

Tape measure
Pencil
Flat trim
Miter saw
Trim nail gun
Caulk

Instructions

- 1
  Measure the width of the archway along the top horizontal span, from corner to corner. Transfer the measurement to a piece of door trim, with two marks that indicate the beginning and end of the measurement.
- 2
  Lay the trim on a miter saw, so the bottom edge runs along the back rail of the saw, with the face of the board looking up at you. Turn the blade to point 45 degrees out, so it points toward the nearest end of the trim. Make the cut.
- 3
  Slide the trim so the second mark is under the blade. Swivel the blade 45 degrees in the other direction, so it's again pointing to the near end. Cut it.
- 4
  Hold the trim along the upper span of the opening, so the measured bottom edge is lined up with the edge of the opening and the two mitered cuts angle up and out from the upper corners.
- 5
  Affix the piece in place with your trim nail gun, setting the nails every 10 to 12 inches along the top and bottom edges of the piece of trim.
- 6
  Measure one vertical edge the opening, from the upper corner to the floor. Use the miter saw to cut the top end at 45 degrees. Cut the bottom end straight across (at 90 degrees).
- 7
  Secure the piece with your trim nail gun, so the mitered ends butt together and form the corner. The bottom edge should sit flat on the floor.
- 8
  Repeat for the other vertical edge. Run beads of caulk along the edges of the trim where it meets the walls.

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