You might think the easiest and best way to install crown molding at the ceiling is to miter-joint where it meets in inside corners.
But a lot of carpenters swear by the traditional method. Coped joints can give you a better fit, even if the wood shrinks, and while there’s extra work in some areas, there’s less in others. For example, for three of the four sections in a rectangular room, you don’t have to worry about your length being spot-on.
Now bear with me here, because this is tricky to describe. Get some scrap crown molding and practice, and you’ll quickly catch on.
First the good news: you don’t need to do any fancy cuts on the first piece of crown molding you install. Just make 90 degree cuts and use a couple nails to hold it in position. Don’t nail in either end yet. Now, you’re ready for your first coped joint.
First, you have to miter it as if you were doing an ordinary miter joint. Put a length of crown molding in your miter box upside down and backwards, and make a 45 degree cut.
If you hold the piece in place, and look at the surface straight on, you’ll see the exposed end-grain of the wood at the miter. This is what you’ll remove with your coping saw. Carefully follow the profile of the design on the outside surface of the trim, cutting back 45 degrees or more. When you’re done, this piece will fit snugly over the piece you’ve nailed up. Now, cut the other end of the molding 90 degrees and install. Move the end of your first piece up or down till it fits nicely. Follow the same steps for the next piece. For the final section (in a rectangular room), you’ll need to cope both ends.