The Lowdown on Crown Molding

Learn tips and techniques from a professional trim carpenter.

Despite its reputation, installing crown molding doesn’t need to be difficult or time-consuming. The job requires a lot of measuring, cutting and nailing, but if you take precise measurements and follow a few simple guidelines, you can easily add the beauty of crown molding to your favorite room. The key is to plan before you start.

Your first decision when planning a crown molding layout is whether to miter or cope the inside corners. In a coped corner, one piece is butted against the wall, and the other piece is trimmed to fit over the crown profile of the butted piece. If you execute it correctly, a coped inside joint is virtually indistinguishable from a perfect miter joint. I have coped complex profiles, such as dentils and decorative polyurethane molding, but it is extremely time-consuming, and achieving satisfactory results is difficult.

Now I decide whether to cope or miter based mostly on the profile of the molding. I cope traditional ogee profiles because this method allows me to make strong joints that look perfect even in nonsquare corners. If the molding has a complex pattern or profile, I prefer to miter the inside corners. (Outside corners are always mitered.)

No matter what kind of molding you choose, you’ll never get good results without the right tools and equipment, and a pneumatic nailer is perfect for the job. I prefer a cordless nailer, and I typically use 2-1/2-in. finish nails driven near the top and bottom of the molding every 16 in. and at all stud locations.

Setting up the saw
The first step in crown molding installation is to correctly set up your miter saw and any jigs or accessories. You’ll need to tune the saw so that the blade and the rear fence are square to the base of the saw. To check whether the blade is square, place a speed square on the saw table and lower the blade until the vertical leg of the speed square comes into contact with the side of the blade. Make sure the blade makes full contact with the square all the way up to the arbor nut. If you see any gaps between the blade and the edge of the square, adjust the saw according to the manufacturer’s directions.

A simple jig that replicates the room's ceilings and walls allows you to cut crown molding in position without any tricky compound angles. You can make a jig like the one seen here, or you can buy one from woodworking suppliers or building centers.

Next, check whether the rear fence is square to the base. I cut an auxiliary fence from scrap material and attach it to the rear fence to increase the fence height and add support for the molding stock. The auxiliary fence height should be 1/8 in. less than the saw’s cutting capacity. Make sure both the fence and the auxiliary fence are square to the saw table.

Once you’re sure everything is square, you can set up for the cuts. There are two ways to cut crown molding: in position and on the flat. Cutting in position means that the crown is secured on the saw the same way that it will be positioned on the ceiling and the wall; the saw table represents the ceiling and the fence represents the wall. Cutting on the flat involves laying the back of the molding on the table base and pressing the crown up against the fence. Cutting on the flat is a safe and secure method, but it requires tricky compound cutting angles, so most professionals prefer to make cuts in position.

When cutting crown in position, it is essential that the molding be oriented to the saw fence and table exactly the same way each time a cut is made. Because there are so many molding profiles and sizes, you’ll need to tailor your setup to match the exact profile and size of the molding before you start cutting.

To ensure the molding position is the same for each cut, use crown molding stops. Some miter-saw tables, such as the Sawhelper I use, come with built-in stops designed for this purpose. You can also buy jigs or make your own stops.

The first step in making molding stops is to cut a piece of scrap material that extends about 6 in. beyond the saw table in both directions. Orient a 1-ft. piece of molding in precise position on the saw. Trace the edge of the molding where it meets the fence to make a reference line. While holding the molding in this position, place the scrap on the saw table and butt it up against the molding. Then clamp or screw the scrap to the extension wings of the saw.

Marking the height line on the saw fence also provides the information you need to make a spacer gauge for use during installation. Simply cut a scrap to match the height of the fence line. At each inside and outside corner of the room, transfer the height from the ceiling onto the wall. Connect the marks with a chalkline and you’ve established working reference lines.

Years of experience have taught trim carpenter John Langan invaluable tricks of the trade. Here he offers techniques that can help you save time, avoid mistakes and achieve professional results:

  • If the ceiling area above the corners is slightly higher than the rest of the ceiling, you can avoid a gap at the top of the molding by securing the mating mitered strips at the same time. Nail each strip the near the middle of the wall. Then fit and press the ends together at the corner and nail them in place.
  • Glue every corner — inside and outside — with wood glue. (Titebond Wood Molding Glue has a high tack and sets up fast.)
  • On inside corners, the ceiling and wall deviations could force the miters out of alignment. I use a drywall-taping knife to get under the molding and adjust its position. The small blade of the drywall knife is flexible but sturdy and won’t mar the walls or ceiling.
  • Use 2-in.-wide plywood nailer strips to secure crown molding that’s taller than 5 in. Attach the strips at the top of the wall all the way around the room to provide a nailing surface.
  • To determine the angle for inside corners, cut test pieces at different angles, and test to find which pieces fit best. Undercutting miters to 44.5 degrees usually looks best.
  • Write the miter angle (half of the wall angle) on the top of the wall near the ceiling for easy reference.
  • For outside miters, use a miter gauge to determine the exact angle, and write this angle on the wall.
  • When cutting multiple workpieces at one time, write the intended location on the back of each piece.

Mitering inside corners
To size crown molding for mitered inside corners, simply measure the wall lengths at the four inside corners of the room. Because measuring into an inside corner is difficult with a tape measure, I measure out from one corner first and mark a reference line 10 in. from the corner. Then I can extend the end of the tape all the way to the other corner and measure to the reference line. As long as you don’t forget to add 10 in. to the number, this technique yields very accurate measurements.

Measure the wall length at the ceiling. To get an accurate read, mark a reference line 10 in. out from one corner. Measure up to that line from the other corner and add the 10 in. back in.

One of the trickiest aspects of miter-cutting crown molding is learning to consistently distinguish left-hand miters from right-hand miters. To identify which is the left and which is the right, stand facing the corner where the walls intersect. The inside corner to your left will be referred to as the left-hand corner (and will get a left-hand miter), and the corner to your right is the right-hand side.

To cut an inside right-hand corner, swing the miter saw blade to the left. The piece that will be installed will be on the left side of the blade – what I call “saving the left.” To cut an inside left-hand corner, swing the blade to the right and “save the right.”

I start with the right-hand cut every time for consistency. Make the right-hand cut close to the end of the strip, and starting at the top corner of the cut workpiece, measure the full length of the strip. Add 1/16 in. to pieces that are more than 10 ft. long, and add 1/32 in. to shorter pieces. The extra length will force the molding to press into the joints for a tighter fit. Then make the left-hand cut. Once you’ve cut all of the strips, they’ll snap right into place if you’ve measured correctly and cut accurately.

Add 1/32 in. to the length of crown molding pieces shorter that 10 ft. and add 1/16 in. to pieces longer than 10 ft. The slight extra length will cause the strips to snap into place tightly in corners when pressed against the wall. Nail the molding at 16-in. intervals at the ceiling and at the wall.

The miter joints on outside corners tend to open up over time. You can minimize this effect by gluing and nailing the joints. Use a low-drip, high-tack molding glue (inset photo), and lock-nail the corner in several spots.

Coping inside corners
Coping is the process of removing material from the thickness of the molding along the profile of the miter-cut crown. When coping miters, you don’t need to worry about the exact angle of the wall to get a tight fit. But it is important that the coped strip match the orientation of the butted strip.

Tack a long piece of molding into position so it butts up against the adjoining wall in each corner. Do not nail near either end. Miter-cut a test piece at 45 degrees and cope it to match the molding profile.

One way to check this is to cut a 2-ft. test piece from the same stock. Cope each end of the test piece with a coping saw (see “Learning to Cope," below) to represent a right-hand corner and a left-hand corner. I use this test piece to simulate the full length of the crown piece before I nail the butted strip. This allows me to adjust the butted strip on the wall until it mates perfectly with the coped test piece. Once I get a perfect fit, I nail the butt joint tightly to the wall.

Hold the test piece up against the first installed piece and check the fit.

Measuring for molding length is more complicated when you plan to cope the inside corner joints. First I use the same method as for mitered joints to find the full length of two opposite walls. Because both ends of these pieces will butt against adjoining walls, I call these pieces “butt-butts.”

Adjust the long, tacked-on piece until the profiled surface mates perfectly with the test piece.

You’ll get the best results if you install the butt-butt pieces first and then measure for the other two pieces, which will have coped ends that fit over the profiles of the butt-butts. The final length of the coped moldings should be measured between the faces of the bottoms of the crown profiles.

Press the test piece back against the tacked-on piece as a guild while you nail the end of the tacked piece. Cut the end of the actual workpiece to the same angle and profile as the test piece.

Joining outside corners
Outside corners are harder to make, but they are important because they show off the profile of the crown. They comprise three molding strips: Two have one inside corner and one outside corner (we’ll call them “inside-outside”), and one has two outside corners (“outside-outside”). To cut right-hand outside corners, swing the miter saw blade to the right and save the right. To cut left-hand outside corners, swing the blade to the left and save the left.

The best way to measure for these pieces is to start with an inside-outside piece. With the mating molding for the inside corner installed, cut a piece of molding that’s too long and then miter or cope the end that forms the inside corner joint. Position the workpiece so the inside corner joint fits, and hold a piece of scrap with a 45-degree mitered end against the wall on the other side of the outside corner. Press the scrap up to the inside-outside strip, and trace the scrap profile onto the strip with a sharp pencil. Cut up to, but never beyond, the line.

To find the length of the outside-outside strip, tack the first inside-outside strip in position and miter-cut the mating end of the outside-outside strip. Mark the cutoff line at the other outside corner using the method described above. Measure and cut the other inside-outside piece before you install the outside-outside piece with two outside corners.

Wherever possible, preassemble all three strips of the outside corner assembly. It’s much easier to hide adjustments on the inside corners. After gluing, I lock-nail the outside miter joints with 5/8-in. brads at the top and bottom.

Making is a coped joint is one of the few carpentry skill remaining that can’t be done with a power saw. I use a standard coping saw with a fine blade (20 tpi). Before coping, make your 45-degree miter cut. Then, secure the workpiece. I prefer to lay it face-up on my power miter saw extensions and clamp it down. Don’t clamp too aggressively, especially when working with delicate urethane moldings. And make sure the clamp jaws are clean. Before starting the cope cut, make several relief cuts in the molding so the blade doesn’t bind in tight curves. To make the coped cut, take full-length strokes using light pressure, not short, choppy strokes. Let the blade do the work, following the contour of the crown profile. Leave about 1/16 in. of material so you can remove the rest right up to the profile line with wood files. The photos below show the step-by-step process.

1. Start the cope cut.

2. Remove wood with relief cuts as you continue coping.

3. Use a wood file to remove wood up to the surface.