Coping Trim Molding
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When installing trim molding, two methods can be used to join inside corners: mitering and coping. Mitering a molding joint requires cutting each piece to be joined at complementary angles. For example, two 45° miters can be joined to make a 90° turn. Coping a molding joint requires cutting out along the profile of one of the pieces so it butts against the other piece to form a seamless 90° union.
Coping an inside corner is sometimes the best method for joining trim molding because a coped joint will usually not develop a crack along the seam as miter joints can. Gluing a miter joint will greatly diminish the possibility of cracks developing but if you want to be sure you should cope inside molding joints where possible.
Install Piece One
Plan your molding layout and install the first piece so it butts against the wall on both ends. If necessary use two pieces and splice them together in the middle of the wall.
Miter for First Cope
Cut the first piece of molding for an adjacent wall at 45° as if making a mitered joint. Use a compound miter cut if coping crown molding. Run a pencil lead along the profile to highlight the edge to be cut.
Cope Piece Two
Use a coping saw to cut along the profile and remove the wood from the back. Hold the blade at an angle sufficient to cut the back off the molding face.
Make Multiple Entry Cuts
Make multiple cuts from all directions to allow the blade to cut smoothly along each line of the profile. Make as many entry cuts as needed to make cutting easier and chip away at the back of the trim.
File to Finish the Joint
Check the fit of the coped piece against the installed molding and use a file to remove any wood that prevents a neat fit. Work carefully to avoid rounding the point on the front of the profile.
When the two mate smoothly, cut a miter on the opposite end of the coped piece to form a splice and run the remaining molding to finish the wall.
