The secret to coping crown molding

Coping crown molding is not thought of as a science, but more as an art. It involves a set of skills and techniques passed from master to apprentice, or less formally, from one guy on the job to another. After working with trim for a long time and talking to a great many carpenters, I have developed a good handful of crown-coping techniques, and I’d like to pass them along. Created out of need, the techniques are based partly on math, partly on common sense, and partly on learning from mistakes. The most-important concept to realize is that every crown profile is designed for a fixed wall and ceiling projection.

Once you have determined the projection, you need to reproduce the measurement accurately and consistently in your miter cuts and in the installation of the coped joint. This will always remove the complications brought on by irregular corners, bad framing, and lumpy tape jobs. Cut precisely, a coped joint is forgiving and fits every time.

Transfer the ceiling projection to the saw

Every crown profile has a specific wall and ceiling projection. If the crown is cut at the same angle as the projection, the cope will fit every time. To find the ceiling projection of the crown, I fit a square onto the bedding angles of a flat sample piece; the ceiling projection is indicated on the top scale.

Rather than use a compound-miter saw to cut crown on the flat, I’ve found that it’s faster to create a saw-table stop that reproduces the ceiling projection, then cut upside down and backward. After measuring the projection, I rip a piece of plywood to the width of that measurement (in this case,  3 1/8 in.). I use the plywood to set the position of a stop screwed to the auxiliary table. With the fence in place, all cuts will be equal (photo right).

To check the cut, I measure the length of the miter; it should equal that profile’s ceiling projection. Installed at that projection, the cope will fit.

Use gauges and take the guesswork out of the installation

Sprung crown doesn’t make contact with the intersection of ceiling and wall, so measuring from the corner to mark guidelines only introduces errors that result from framing irregularities or joint-compound buildup. Instead, I use the piece I ripped to set the miter stop, glue a slice of the crown into position, and lop off the corner. With the gauge held up to the ceiling, I can mark the crown’s top and bottom positions.

An alternative technique comes from my colleague Dave Collins (www.collinstool.com), who cuts the wall and ceiling projections of a crown profile into a piece of plywood that holds the crown in the proper position. It’s especially handy for laying out and installing outside corners. I measure the angle of the corner, subtract that angle from 180°, and divide by two to get the miter angle. Once the miters are cut, I install the piece, using the gauge to hold it at the proper projections.

My favorite way to establish a guideline is to use a 2-ft.-long piece that’s coped accurately and a 2-ft.-long square-cut piece. Fitted in the corner, the coped piece locates the square piece at the correct projections, and you can mark the top and/or bottom. The longer pieces take into account more of the wall and ceiling conditions than small gauge blocks.

A coped joint that looks mitered

Especially when you’re working with wide stain-grade stock, there are occasions when you’d like the crisp look of a mitered joint but want to keep the integrity and ease of the cope. (Gary M. Katz demonstrated a similar technique in “Baseboard Done Better,” FHB #174) Both sides of the joint need a little attention. After cutting the miter, I cope away the waste but leave the point intact (photo left). I test-fit the mate of the piece and scribe the point onto the opposite piece, then make a shallow cut with a fine-toothed handsaw (photo right).