How to Install Interior Door Casing

Measure from the floor up to the top inside corner of the door frame with a measuring tape and add 1/4 inch to it. Measure from one top corner to the other and add 1/2 inch to it. The additional length allows the trim to be set 1/4 inch out from the casing all the way around.

Measure two pieces of trim to match the measurement of the floor to the top of the door plus 1/4 inch, and make a mark with a pencil. Measure a third piece to match the measurement across the top of the door plus 1/2 inch. For the top door trim piece, start the measurement in from one end about 3 inches.

Mark one of the side trim pieces that is to run from the floor up as the left side of the casing. Place it on the circular saw so that the mark is under the blade and the length of trim is running out to the left side. Adjust the blade of the saw to make a 45-degree angle that will cut up and to the right. Place the blade on the mark and make the cut. When stood up, the trimmed piece should have a cut edge that starts at the mark and angles up and to the left. Repeat with the other piece or trim, but make the angle of the cut the opposite of the first.

Cut the top piece of trim to have a 45-degree angle cut on both ends. Make the cuts starting at the marks so that the angles on either end go in opposite directions. This way, one side of the trim is longer than the other.

Paint all the trim pieces with a paintbrush and leave them to dry for a few hours.

Put one of the side trim pieces up to the door frame so that it covers the frame up to the edge. Check it with a level to ensure that it is perpendicular to the floor. Tack it in place with finishing nails and a hammer.

Place the top piece of trim in place and line the angles up at the corner. Level the piece and tack it in place. Repeat with the other side trim piece. If it is too long, sand the bottom down, and if a side piece turns out to be a little short, slide it up to fit. The shoe molding will cover the error.

Countersink all the nails by taping the tops of them with another nail. Fill over top with wood putty. When the putty dries, sand it flush and touch it up with paint.