How to install door casing: mitered and craftsman, done right

Door casing is the trim that frames a door opening and hides the gap where the jamb meets the drywall. Installing it well comes down to two things: an even reveal — the small, consistent setback of the casing from the edge of the jamb — and corner joints that stay tight. You set the reveal, mark and cut your pieces, join the corners (45-degree miters on mitered casing, square butt cuts on craftsman casing), then fasten the casing into both the jamb and the framing behind the drywall.

How do you install mitered door casing, step by step?

Most door casing is three pieces: two legs up the sides and a head across the top. On mitered casing the two top corners are cut at 45 degrees so the profile wraps around the corner. The order below is how the crew runs it, and the reveal is the first thing that gets set because everything else references it.

• Mark the reveal. Run a light pencil line about 3/16 to 1/4 inch in from the inside edge of the jamb, on both legs and across the head. A combination square set to that depth keeps it identical everywhere. The inside edge of the casing lands on this line.
• Cut and fit the head. Measure between the reveal lines at the top, then cut a 45 on each end of the head casing. Tack it to the reveal line before you commit nails so you can still adjust it.
• Cut the legs. Stand each side piece on the finish floor, mark where it crosses the head reveal, and cut a 45 at the top to meet the head. The bottom is a straight square cut sitting on the floor. Cut a leg short and it sits up off the floor, which is one of the details that gives a bad install away.
• Dry-fit the miters. Hold each top corner together and check it closes tight, on the face and across the top, before any glue goes in. If a joint gaps, shave the cut now, not after it is nailed.
• Nail it off. Fasten the inside edge into the door jamb and the outside edge into the framing — the trimmer stud behind the drywall — so you have two solid lines of hold. Brads catch the jamb; heavier finish nails go into the framing. Casing fastened only to drywall works loose.
• Glue and pin the miters. Put glue in each top joint, close it, and drive a pin through the edge of the head into the leg and another back the other way. That locks the two pieces together so the corner can't slide open later.

How is craftsman or butted casing different?

Craftsman and other flat-stock casings skip the 45s. The corners are square butt joints: the legs run straight up, and a flat head casing sits across the top of them, often wider or thicker than the legs, sometimes with a backband or a small cap on top. You still set a reveal the same way. A butt joint won't creep open the way a miter can, but it shows any gap in the cut, so the square cuts have to be dead-on and the head has to sit flat. For the different profiles and how they read in a room, see our door and window casing styles guide. Window casing follows the same reveal-and-corner logic, laid out in how to trim a window.

Why do door casing miters open up in East Tennessee?

Wood moves with the seasons, and East Tennessee humidity swings enough that casing expands and contracts through the year. A miter that looked tight the day it went in can open a hair by the next season if it was only nailed. That is why the top corners get glued and pinned instead of just tacked: the glue holds the joint face to face, and the pin keeps the two pieces from sliding apart as the wood works. It is the same reason baseboard gets coped at the inside corners rather than mitered, so the joint survives movement. In older mid-century homes around Oak Ridge, and on out-of-square openings in custom builds, that seasonal movement is where a careful install shows.

A reveal is the consistent setback — usually about 3/16 to 1/4 inch — between the edge of the casing and the face of the jamb. Keeping it even on all three sides of every door, and matching it from door to door, is the difference between clean casing and builder-grade. When the reveal wanders, the eye catches it.

Running a trim package on a build, or a cabinet or door shop that needs pre-hung openings cased out? We install to the finish schedule and hold the same reveals and tight miters from the first opening to the last. Our scope stops at the install — filling nail holes, sanding, and paint are the finisher's trade. What a door package costs comes down to the number of openings, the casing profile and any build-up, paint-grade versus stain-grade, and how far out of square the walls run, so send the plans and door schedule and we will bid it. See how we run interior doors, or send the plans and we will price the casing, doors, and base as one consistent package.