When hanging crown moldings, a problem that you may experience is the corners of a room that are out of square. This is actually a common problem because the majority of older homes have settled, and newer homes are being thrown together with no care about how square the corners and seams are. In order to prevent your crown molding from having gaps or cracks due to these out-of-square problems, most professional finish carpenters would avoid using the basic pre-made 45-degree corners. They will rather choose to cope one of the pieces of molding, instead, meaning that one piece of crown molding gets shaped to fit the profile of the opposite piece that is already installed in the corner.
This may seem like it is a complicated job that requires a lot of experience and extensive knowledge in mathematical equations, but the truth is that this task can be accomplished by any homeowner that has a little knowledge and with a few basic carpentry tools.
Step 1: Cutting the First Piece
Begin by measuring the length of the wall to find out how long of a piece you need. Working from left to right, cut the left end of the molding on the square if it is to butt against a wall, and from that end of it, transfer this measurement to the front of the molding making sure that you crosscut at the exact measurements so the piece of molding fits tight from end to end. But if the left end is to be fitted to perfectly match the counter profile of an adjacent crown molding (coped joint), you may choose to miter cut it at 45° instead, to further cope it to match as described in step 2. The crown molding can be crosscut on a miter saw equipped with a fine-toothed crosscutting blade. Before using a power tool, however,
If you have to change the blade, make sure to first unplug the saw.
Wear an eye protection device—goggles or glasses— before operating the saw.
Make sure your hand that's holding the piece is placed well beyond the reach of the blade when you start cutting.
Once it is cut, you can secure the molding in place on the wall with finishing nails, making sure that you nail into the wall studs so that the molding does not move.
Step 2: Cutting the Second Piece
Measure the wall for the 2nd length of molding and once again get a molding long enough so that you don't end up with an unnecessary joint on a straight wall. Place the molding upside down on the miter saw with the bottom of it up against the fence and the top of the molding sitting down on the table as it will sit on the wall except for being upside-down for now and forming a triangular gap on the back of the piece. This will put the molding on the left side of the blade, making it easier to cut the left end of the piece at a 45° angle. With the miter adjusted to the left, hold the molding firmly and make the left end miter cut. Once this is done, you can hook your measuring tape onto the tip of that miter cut and transfer the measurement taken from the wall to the face of the molding along the edge that will rest on the wall, since this is where the molding will be at its longest—an inside corner leaves the top edge much shorter. Crosscut the other end of the molding on the mark.
Step 3: Coping the Molding
Coped corners are made to fit tight. Even if an inside corner is out of square by some degrees, a cope joint will fit tightly with only slight touch-ups done with a wood file. But before proceeding to make a coped joint on the actual piece to be added next, it is strongly suggested that you take scrap pieces lying around to practice with. To get an accurate cut and an accurate fit, make sure that the piece is firmly clamped to a workbench or table. Next, take a pencil and slide the long edge of the lead along the inside or front edge of the miter cut to highlight where the cut needs to be done—it's always easier to follow a pencil line. A regular basic coping saw can be used to complete the cutting. Using the coping saw, cut along the line you just made to get the exact counter-profile of the crown molding. If you were to cut perpendicular to the molding, it would fit perfectly on a perfect 90° corner but would leave a front gap on an obtuse corner. So to always get the perfect fit, tilt the saw blade to remove slightly more material from the back of the molding and try to leave the pencil mark while cutting as close to it as you can. With the coping cut done, use a wood file to remove the excess wood right up to the pencil mark. Try your "practice piece" up against the crown molding up in the corner and look at perfection. With this newly acquired confidence in your skill, repeat the same operation on the actual piece of molding.