How to Install Post and Lintel Trim

Measure the inside edges of the doorframe, across the top and down both sides. Assuming that both sides are exactly the same could lead to an uneven appearance, especially if there's a difference in floor covering, or carpet that has worn unevenly.

Mark each piece of trim for cutting. To make the trim fit neatly together at the corners, cut each end at a 45-degree angle, leaving the ends that will go against the floor straight. For the posts, apply your measurement of the height of the door to the narrower edge, if there is one, of the trim. Add the width of the trim to that measurement, and mark the outside of the trim at that length. The lintel piece will have 45-degree cuts on both ends, so mark the inside edge with the measurement from the top of the doorframe, leaving space equal to the width of the trim at either end.

Cut the trim using a miter saw. Either powered or manual, a miter saw allows you to cut the trim at a perfect angle. Ensure that the trim is supported along its length and is level with the base of the miter box so that the cut is as clean and accurate as possible. Cutting the post trim should connect both marks made from the initial measurements.

Place the trim on the door without attaching it to make sure the measurements are accurate and the cuts good. You may need to sand the cut ends a little to get rid of splinters, but be careful sanding the facing surface, as that can ruin the neat appearance of the finished trim.

Paint or stain the trim if you're working with real wood. Some trim is made of pre-colored composites and should be installed without paint or stain. Allow the finish plenty of time to dry before installing.

Place and nail in the posts first, making sure the bottom edge is flush against the floor and the lower end of the 45-degree cut at the top neatly meets the corner of the doorframe. If you're using a hammer and finishing nails, as opposed to a nail gun and brads, hammer in the nails about 90 percent of the way, then use a counter-sink tool to drive the nail in the rest of the way. This will help hide the nail head in the wood and avoid damaging the trim with the face of the hammer.

If everything has gone right up until now, the lintel piece of trim should rest neatly on the posts and look finished without doing anything else. Use the same technique as attaching the posts to fix the lintel piece of trim into place.