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Tom Silva works on trimming a door.

Either you’ve installed a new door or decided to change the door’s trim for aesthetic reasons. Either way, to trim a door, you need to bridge the gap between the door jamb (the frame from on which the door is hinged and where the latch engages) and the wall. This is done with molding called casing.

  • A miter joint is where the ends of two pieces of molding are cut at equal angles—45 degrees for a square corner.
  • A butt joint is where the square-cut ends of the legs meet the edge of the head at 90 degrees.

Casing can be molded or flat, and whether the casing legs and head meet at the top of the door in miter joints or butt joints depends partly on which you choose.

Prepping the Door

For casing to sit flat and for the head joint to meet well, the jamb and the face of the wall need to be in plane with each other. This is most important at the top of the door where the joints are.

Check this by holding a straightedge on the face of the wall and extending it to the jamb—it should just kiss the edge. If the jamb projects slightly from the wall you can plane it flush. This is the best option if the trim is to receive a natural finish, and won’t be caulked to the wall.

If the trim is to be painted you can leave the jamb alone. Instead, when you nail the trim in place, nail it first to the jamb, then insert tapered shims into the gap between the wall and the back of the casing at the points where you’ll nail.

These shims will keep the nails from pulling the casing back at an angle. After nailing, carefully cut the shims even with the casing using a sharp knife, then caulk the joint with painter’s caulk.

If the wall extends beyond the jamb

If this is the case, the approach will differ. If the wall surface is drywall and not plaster or skim-coated drywall and the difference is 1/8 inch or less, you can just beat the drywall down with a hammer—crude, but effective.

If that’s not the case, use a power planer or table saw to rabbet the back of the casing where it will hit the wall. When the jamb is recessed by more than ¼ inch, the solution is to rip extensions to nail to the edge of the jamb to come flush with the wall.

Steps for Installing Door Trim

Casing should sit back from the face of the jamb by 1/8 inch to ¼ inch. This is called a reveal, and carpenters mark it all around the jamb with a sharp pencil and combination square.

  1. Once you’ve marked the reveal, hold a casing leg in place, with its edge on your pencil marks. Check how the leg meets the floor—if the floor is out of level, you may need to trim the bottom of the leg to meet it fairly.
  2. Once you’re happy with that, mark where the reveal on the jamb head meets the leg. This is where you’ll cut the leg. Repeat the process on the other side.
  3. If the casing is to meet with miters, cut the legs using a miter saw set to 45 degrees. Also cut one end of the casing head. If you’re right-handed, cut the left side. Do the opposite if you’re left-handed. This makes the next step easier.
  4. Holding the head in place on the jamb, align the cut you just made with the reveal on the jamb leg. Without moving the head, mark where the reveal on the uncut side meets the casing. Take the head to the miter saw and cut this miter.
  5. If you have miter clamps, lay the two casing legs and the head on a worktable, thoroughly coat the mitered ends with carpenter’s glue, and clamp the miters together.
  6. After half an hour of clamping, you can then install the casing as a unit. Lacking miter clamps, install the casing one piece at a time.
  7. Nail one leg to the jamb first, using 4d finish nails or 1 ½-inch, 15-gauge gun nails spaced about 16 inches apart.
  8. Align the edge of the casing with the reveal marks.
  9. Test fit the head to this leg. You may need to adjust the cut a little. Often, trimming the back of the miter cut with a block plane, knife, or rasp is all that’s needed to close a slightly open miter.
  10. Spread glue in the joint, hold the head in place, and nail it to the jamb along the reveal line.
  11. Squeeze the joint tight, and nail into it from the side. Repeat the process where the second leg meets the casing head, this time adjusting the cut on the leg if needed.
  12. Once you’ve nailed the casing fully to the jamb, change to 6d or 2-inch nails, and nail the casing legs to the wall. Pay attention near the joints—if the casing doesn’t sit tightly to the wall, shim behind it before nailing, or the nail will pull the joint open. To avoid this problem when hand nailing, you may need to predrill for the top nail. Don’t nail the top of the casing head to the wall. Seasonal changes in humidity make the framing above doors move up and down. If the casing is nailed to the framing here, it too will move, and the joints will crack.

Square Casing

Some styles of trim call for butt joints instead of miters. One big difference is in how the length of the head is determined. Unless there’s a back band (a second layer of molding that wraps the outer edge of the casing), square casing heads usually overhang each side of the legs by ½ inch or more.

This is more than a stylistic detail. The legs will expand and contract in width with seasonal humidity changes, and this will be noticeable if you install the head with its ends flush with the outside of the legs. So, the length of the head will be the jamb width, plus the reveals, the combined width of the legs, and whatever overhang you want on each side.

If you have a biscuit joiner or a pocket hole jig, the joints between the legs and the head on square casings are an excellent use for these tools. Otherwise, the nailing sequence is identical with square casings.