If you already installed the door and baseboard moldings, you probably found it easy to fit properly because they are basically rectangular in shape. They also do not draw the close attention of attractive crown moldings. Crown moldings can make the difference between a nice room and a beautiful room.
Crown moldings, by design, are basically triangular in shape. Today, in order to reduce the material requirements, they are usually undercut in the back (partially hollow). This makes it more difficult to get a good corner fit even using an accurate miter saw.
Just take a moment and think about the geometry of a triangular-shaped crown molding. Since the top of it extends out farther from the wall than the bottom, the length from end to end of the top must be shorter than the bottom. You must remember this when making your saw cuts.
To make the cut on the proper angle, you should cut the molding upside-down and backward with the miter saw. This will give you the proper 45-degree angles on the mating edges of the two pieces in the corners.
Before you ruin much more of your expensive crown molding, and your mental health, cut two 1-foot-long practice pieces of molding. Place one piece of molding in the miter box so that the lower edge of the molding is facing upward and sloping downward from the saw fence to the base.
Adjust the blade accurately at 45 degrees and make your cut. This will produce one-half of the joint. Now, repeat the identical cutting process on the other 1-foot piece with the blade positioned in the opposite direction. The two pieces should now fit together, forming a perfect 90-degree corner.
Hold them up in each one of the corners in your dining room. Don’t be discouraged if there is still a gap between them in some of the corners. You did nothing wrong, the original builder did. Very few rooms are perfectly rectangular, so all the corners were not exactly 90 degrees to start with.
You have two options at this point. You can use some filler if the gaps are very small, or you can use a coped joint. In the past, nearly all experienced carpenters used coped joints for a truly professional-looking job. Even with seasonal weather changes, the joints will still look good.
A coped joint means that one piece actually fits over the other piece at the corner; therefore, the angle of the walls at the corner is not critical. This method allows you to wiggle and twist the molding just enough for a gap-free fit.
With one molding piece cut on a 45-degree angle as discussed above, use a coping saw to carefully follow the cut edge profile. Hold the coping saw on a slight angle to make sure that the edge will rest tightly against the piece of molding under it.
Cut off the end of the mating molding piece squarely (no miter). Slide this piece up into the corner and nail it in place. Place the piece with the coped end over it and nail it in place, too.
