How To Install Door Trim

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.

The 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 Trim

For the 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 with a sharp knife even with the casing, then caulk the joint with the 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.

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.

Cutting Trim for Square Casings

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 as square casings.

What You Need

Tools and Materials