Read on to discover how to trim a door yourself, whether you're installing a new door or replacing the trim for cosmetic reasons.

You've either installed a new door or opted to replace the trim on your existing door for cosmetic reasons. Trimming a door requires bridging the gap between the door jamb (the frame from which the door is hinged and where the latch contacts) and the wall. This is accomplished through casing molding.

• A miter joint is formed by cutting the ends of two pieces of the molding at identical angles—45 degrees for a square corner.

• A butt joint is formed at 90 degrees by the square-cut ends of the legs meeting the edge of the head.

A butt joint is formed at 90 degrees by the square-cut ends of the legs meeting the edge of the head.

Prepping the Door for Trim

The jamb and the face of the wall must be in plane with each other for the casing to sit flat and the head joint to connect properly. This is especially true towards the top of the door, where the joints reside.

Hold a straightedge against the face of the wall and stretch it to the jamb—it should just kiss the edge. Plane the jamb flush if it protrudes significantly from the wall. If the trim is to have a natural finish and will not be caulked to the wall, this is the ideal option. If the trim is to be painted, the jamb can be left alone. Instead, nail the trim to the jamb first, then put tapered shims into the gap between the wall and the back of the casing at the spots where you'll nail it. These shims will prevent the nails from pulling back at an angle on the case. After nailing, carefully trim the shims even with the casing with a sharp knife, then caulk the junction with the painter's caulk.

If the wall extends beyond the jamb

In this scenario, the strategy will be different. If the difference is 1/8 inch or less and the wall surface is drywall rather than plaster or skim-coated drywall, you can simply knock the drywall down with a hammer—rough but effective.If this is not the case, rabbet the back of the casing where it will contact the wall with a power planer or table saw. When the jamb is recessed by more than 14 inches, tear extensions to nail to the edge of the jamb to bring it flush with the wall.

Steps for Installing Door Trim

The casing should be 1/8 to 14 inches away from the face of the jamb. This is known as a reveal, and carpenters use a sharp pencil and combination square to draw it all around the jamb.

After you've marked the reveal, position a casing leg with its edge on your pencil marks. Examine how the leg touches the floor—if it is not level, you may need to trim the bottom of the leg to meet it fairly.

When you're satisfied, indicate where the reveal on the jamb head meets the leg. This is where the leg will be severed. Rep the process on the opposite side

If the casing is to meet miters, cut the legs at a 45-degree angle with a miter saw. Cut one end of the casing head as well. Cut the left side if you're right-handed. If you are left-handed, do the opposite. This simplifies the next step.

Align the cut you just made with the reveal on the jamb leg while holding the head in place on the jamb. Mark where the reveal on the uncut side meets the casing without moving the head. Cut this miter on the head with a miter saw. Lay the two casing legs and the head on a worktable, completely coat the mitered ends with carpenter's glue, and clamp the miters together if you have miter clamps.

After around 30 minutes of clamping, you can install the casing as a whole. Install the casing one piece at a time if you don't have miter clamps.

Use 4d finish nails or 1 12-inch, 15-gauge gun nails spaced about 16 inches apart to fasten one leg to the jamb first.

Align the casing's edge with the reveal marks.

Fit the head to this leg for a test fit. You may need to tweak the cut somewhat. To close a slightly open miter, it's often enough to use a block plane, knife, or rasp to trim the rear of the miter cut.

Spread glue in the joint, then nail the head to the jamb along the reveal line.

Squeeze the joint tightly and insert a nail from the side. Rep the process where the second leg meets the casing head, correcting the leg cut if necessary.

After you've completely fastened the casing to the jamb, switch to 6d or 2-inch nails and nail the casing legs to the wall. Pay close care around the joints—if the casing isn't flush against the wall, shim behind it before nailing, or the nail will pull the joint open. When hand nailing, you may need to predrill for the top nail to avoid this problem. Do not nail the casing head's top to the wall. The framing above doors moves up and down due to seasonal fluctuations in humidity. If the casing is nailed to the framing here, it will also move and crack.

Square Casing

Butt joints are used instead of miters in some trim styles. One significant difference is how the length of the skull is calculated. Square casing heads typically overhang either side of the legs by 12 inch or more unless there is a back band (a second layer of molding that wraps the outer border of the casing).