How to install door casing yourself: step by step
45° miters and a steady 1/4" reveal. Two sides per opening. The reveal line is what makes it look professional, not the miter.
Difficulty 2 of 5 · about half a day · you can do this
Photo of your room, the look you pick, then this guide with the materials counted for your room.
What you need
- Door casing (both sides of the opening)
- 1-1/4" 18 gauge brad nails (into the jamb)
- 2" finish or brad nails (into the framing)
- Wood glue (for the miter joints)
- Lightweight wood filler / spackle for nail holes
Tools
- Miter saw with a fine-tooth finish blade — $35–60/day in LA. Rent it for the whole trim weekend: casing, baseboards, and closet shelving all need it.
- Finish/brad nailer (18 ga is plenty for casing) — $30–50/day, or hand-nail with 4d and 6d finish nails and a nail set.
- Combination square (set it to 1/4" and use it as your reveal gauge)
- Tape measure and sharp pencil
- Nail set and hammer
- Block plane or sanding block (shaving a miter to fit)
Step by step
Step 1 of 12
Check the jamb sits flush with the wall
Run your hand across every jamb-to-wall joint. A jamb proud of the drywall tips the casing; a jamb behind the drywall leaves it hanging. Plane a proud jamb down or add a thin extension before you cut trim.
Step 2 of 12
Mark the 1/4" reveal all the way around
Set a combination square to 1/4" (6 mm), run it along the inside face of each jamb leg and the head, and tick a pencil line every foot or so. That line is where the inside edge of the casing goes — top and both sides, both faces of the opening.
Step 3 of 12
Decide the order: legs first, head last
Cut and tack both legs, then measure the head between them. Doing it the other way means fitting two miters at once and both will fight you.
Step 4 of 12
Cut the leg bottoms square and stand them on the floor
Square cut at the bottom, sitting on the finished floor. If the floor is not level, the leg still sits on the floor — the difference gets absorbed at the top miter and hidden by the baseboard.
Step 5 of 12
Mark and cut the top miter on each leg
Stand the leg in place on the reveal line, mark where the head reveal line crosses it, and cut a 45° miter with the long point on the outside edge. Tack each leg with two brads only — you may need to shift it.
Step 6 of 12
Measure the head long point to long point
With both legs tacked, measure between the two outside (long) points of the leg miters. That is the length of the head piece, long point to long point, with a 45° cut on each end.
Step 7 of 12
Dry-fit the head and read the joints
Hold the head in place. If a miter gaps at the outside, the angle is a touch too steep; if it gaps at the inside, too shallow. Adjust in half-degree steps on the saw or shave the face with a block plane.
Step 8 of 12
Glue the miters and close them
Put a dot of wood glue on both faces of each miter, press the joint tight, and drive a pin or brad through the edge of the head into the end of the leg to lock it. Wipe squeeze-out with a damp rag right away.
Step 9 of 12
Nail off both edges
Inner edge: 1-1/4" brads into the jamb, every 12–16 inches. Outer edge: 2" nails through the drywall into the framing, roughly opposite each inner nail. Keep nails back from the thin outer edge so nothing splits.
Step 10 of 12
Do the other side of the opening the same way
Every opening gets casing on both faces. The reveal on the second side is measured from that side's jamb edge, not copied from the first.
Step 11 of 12
Set every nail below the surface
Sink each nail head just under the wood with a nail set so filler can cover it. Filling and caulking happen in the next task, once all the trim in the room is up.
Step 12 of 12
Quality check
Stand back six feet on both sides of the opening and look down the reveal line before you call it done.
Quality check
- Reveal is 1/4" and consistent on both legs and the head
- Both top miters are closed, glued, and pinned
- Casing lies flat on the wall with no rocking
- Nails in both the jamb edge and the outer edge, all set below the surface
- No splits at the outer edge
- Both sides of every opening are cased
Common mistakes
- Casing set flush with the jamb, or a reveal that drifts. Flush casing shows every flaw in the jamb and looks homemade. Set your combination square to 1/4" and mark the line all the way around the opening before you cut anything — then install to the line, not by eye. Mistake #1 in this task.
- Measuring the head piece to the wrong point of the miter. The head piece is measured long point to long point — the outside of the miter. Measure to the short point and the head lands short with a gap at both top corners.
- Miters that gap because the wall is not flat. The corner is rarely exactly 90°. Cut a test pair from scrap, hold them up, and adjust the saw a half-degree at a time until the joint closes. Then glue the joint and pin through it from the edge so it stays closed.
- Nailing too close to the edge. Brads near the thin outer edge split the casing and the split shows through paint. Stay back from the edge, and on hardwood trim pre-drill at the miters.
- Cutting the legs before the floor is in. Casing legs sit on the finished floor. If flooring is going in later, the legs end up short and there is a gap under them. Install casing after the floor.
- Nailing only into the jamb. Casing needs two nail lines: short brads along the inner edge into the jamb, and longer nails along the outer edge into the framing behind the drywall. One line alone lets the casing lift off the wall.
Draft guide written from standard practice, not yet reviewed by a licensed pro. Follow product labels where they differ.
Get this job planned for your room