Tips for Installing Crown Molding

Installing crown molding can be a challenge the first time you try it.. The following tips were compiled from discussions and information on another site.

Most Finish Carpenters consider it poor form to miter inside corners, they should be coped. It allows for slight wood movement, and it allows you to perfectly match the outside corners by allowing some slight leeway in the length of the piece that will be covered by the cope.

You will see that if you take a coping saw and keep the blade parallel to the miter saw table, and perpendicular to the molding, you can cut along the molding edge intersection with the miter cut, and make a perfect cope. To help visualize, take a pencil and blacken all of the mitered surface. Cut all of this away by cutting along the molding profile.

In practice, make your miter cut a little long, then adjust to fit with whatever it takes, usually a sharp chisel, sometimes (judiciously) a gouge or rolled coarse sandpaper. It is also usual to slightly undercut the profile so just the visible edge contacts (see figure 4).

I usually cut any cope cut first, and the other end last, That way if the cope goes really wrong, it can be re-cut before the stick is too short. It is seldom necessary to make a cope cut on both ends of a molding if you have planned the room right. When it does happen, it is usually the last piece, and there will have been ample time to figure how the ceiling rolls, so to speak, so you can guess pretty well where to compensate. This brings to mind, of course, Always do your longest runs first; hopefully your closing pieces will be short ones and you will have three or even four pieces of short end to get it right without actually wasting anything.

ut my walls aren't really Square?

Most wall/ceiling junctures run somewhere around 92° to 95° on the really bad ones today. Stick a framing square up and and check the corner at a number of places around the perimeter of the room. What happens is that even if the framing of the house is square, (pretty likely) the drywall finishers tape the corner, then bury the tape, then feather this out into the rest of the surface. So in the corner, the mud may be 3/16" (or even 1/2") thick, feathered both ways to nothing in 8" to 12", leaving greater than a true right angle.

Knowing this, A custom shop making millwork often makes the back angle more than 90* (95* is not untypical-see Figure 1) - you want the top and bottom edges to touch before the inside edges bottoms out, leaving a gap. 

Generally, the ceiling is the reference for the miter angle, so you would want that surface flat to the table of your Miter Saw. Practically speaking, it may be necessary to split the difference. Position it by hand, or stick a splinter or paper match somewhere to keep it from rocking.   There a lot of little adjustments you can make if a joint is almost, but not quite, there. For instance: outside corner to outside corner a piece is a smidgeon too short (we're talking 1/32's). Plane the inside bottom of the molding (wall juncture) of the three pieces slightly and the joints will close up because the corners are brought in closer to the wall. You can't fix a botched job this way, but you can improve dramatically an "adequate" installation. If the job is botched, re-cut the pieces while they are still useful for somewhere else, and start over.

Good Luck!  If you are looking for a great book on Crown Molding and Trim Carpentry, check out Trim Carpentry Techniques by Craig Savage, which covers many of the "secrets" professionals use to get those nice tight joints.    Other highly rated, finish carpentry references that provide detailed information on crown molding installation are the Finish Carpenter's Manual, by Jim Tolpin  and  Finish Carpentry, by Gary Katz.

If you are looking for a source for some truly stunning crown molding, check out the Rope Accent Crown Molding at Rockler.com.  It is available in paint grade, cherry, oak or maple.