How to Install Door Trim in 8 Steps

A clean workspace is a clean mind, so be sure to clear the space before you start measuring your trim to prevent injuries or confusion in your calculations. Remove objects you could trip over and clean the wall around your doorway to make marking the places for the trim easier.

Before touching the trim, you’ll need to determine where to place it along your doorframe. A reveal line will help you install the door trim ever so slightly away from the frame, giving the hinges room to operate. The easiest way to draw reveal lines is with a combination square and a pencil. 

Set the combination square anywhere from 1/8 to 1/4 inch away from the jamb in the top two corners of your doorway. Draw one horizontal and one vertical line in each corner to mark your reveal lines.

To calculate the trim pieces, grab the measuring tape to measure from the ground up to where it’s parallel to the horizontal line. Repeat on the other side to ensure you have accurate side trim measurements. 

Then, measure the distance between the two vertical marks to get the measurement for the inside of the top piece—this is called the header trim. Now you have the lengths for the thin sides of your trim pieces. Mark these measurements onto the inner edges of your trim.

“If you’re not painting the trim, it’s essential that the mitered pieces match perfectly,” says Bob Tschudi, Angi Expert Review Board member and general contractor in Raleigh, NC. “For painted trim, you can fill small imperfections with caulk before painting.”

With your safety glasses in place, use a miter saw to cut your trim at a 45-degree angle. The header trim will need two 45-degree angles on each end to fit against the side pieces. The side pieces only need one 45-degree angle on the end that fits into the header trim; the other end of the side pieces should be a 90-degree angle, as they’ll extend down to the floor.

Cut from the thickest side of the trim across to the thinnest side, as you will install the thickest side furthest away from the doorway. Your two side pieces should be mirror images of each other to ensure the joints fit properly together.

For the next step in how to install door trim, find your hammer or nail gun. Use 1-inch, 15- or 18-gauge nails to attach the thin side of the trim to the wall and 2-inch, 15- or 18-gauge nails for the thicker side of the trim. Space the nails approximately 1 inch away from the edges of the trim and 1 foot apart.

Measure one inch in and one inch above the thin, inner edge of the trim. You’ll want to match the spacing and distance for the outer edge of the trim, as well. This will leave two rows of nails in each piece of trim.

It might be wise to enlist a friend to hold the trim in place while you nail it to the wall. Consider using a piece of cardboard underneath the board while hammering to avoid damaging the wall.

Now it’s time to fit the two side pieces. If your cuts are off and the two pieces aren’t flush against the header trim—this might be your most challenging part of installing the door trim. Place the side pieces one at a time against the top trim until they’re flush with each other. The edges should go flat against one another. 

If there is a gap or uneven section, you’ll need to sand down the trim until it evenly connects to the header. Generally, if there’s a gap, you’ll need to sand the opposite side to get it to lay flat.

Once both sides fit flush against the head trim, you can attach them to the doorframe. Before you nail them into the wall, place wood glue on the end of each side piece. Spread it evenly across the entire edge of the piece before placing it firmly against the header piece. Then, using 1-inch nails on the inside of the trim, nail the piece into the wall. When nailing in the outer portion of the trim, use 2-inch nails as you did with the header trim. 

“In some cases, especially in older homes, you’ll have perfectly mitered wood, but the wall depth is slightly different,” says Tschudi. “Carpenters simply ‘shim’ the lower piece using small pieces of cardboard behind the trim before nailing.”

For extra security, you can predrill 1/16-inch holes in the top and side corners to better hold the two pieces together. Alternatively, you can simply drive four 3d (1 1/4-inch) nails into the corners.

Once you’re finished installing the trim, it’s time to touch up any marks or smudges made by the nail gun or miter saw. A damp microfiber cloth can remove any scuff marks, but you’ll need to touch up the paint if there are any significant nicks.