There comes a time during crown moulding installation when you are forced to cut join trim sections to a 22.5-degree angle. This happens when the walls are angled inward at a degree which is not square, specifically at a 45-degree angle in a corner area. Cutting crown moulding to a 22.5-degree angle is a quite simple task that can allow you to complete your moulding installation process in a timely manner to create a seamless appearance in the corners of the room.
Measure the length of the walls where the moulding will meet. Cut one section of ½-inch-thick moulding to ¾ inch longer than the measured length of each wall, using a mitre saw. For example, if you measure a wall that is 56 inches long and the joining wall is 48 inches long, cut one section to 56 ¾ inches and the second piece to 48 ¾ inch.
Place the crown moulding on the wall to determine which corners will be angled. Measure back ¾ inches from the end along the front side of both moulding sections where they will join in the corner and mark this location on each section of moulding with a pencil.
Set the mitre saw to cut a 22.5-degree angle. Cut the angle from the ¾ inch line to the back corner on each end of the moulding. This will create a 22.5-degree angle in the moulding in order to install the two sections in a 45-degree corner.
For moulding of different widths, add one and a half times the width during the measure back, and mark from the end. For example, if your moulding is 1 inch thick, you will measure and mark 1 1/2 inch back from each end along the front side of the moulding.