Installing baseboards, crown molding, and other types of millwork means that you eventually have to turn a corner. The long, straight runs are easy to install, but when it comes to an inside corner, what to do? With an inside corner, your two options are either to cope the joint or miter the joint.
Mitering is generally the easier and faster technique for beginners since all cuts can be made on the electric miter saw. Yet many carpenters prefer to cope the joint because the result is cleaner and there is less of a chance of gapping.
No single answer applies to all projects—or all people. Learn about mitered joints vs. coped joints for inside corners and which one is best for you.
Inside Corner and Outside Corner
An inside corner is a turn where the two walls face inward, or toward each other, at a 90-degree angle. Contrast an inside corner with an outside corner: a 90-degree angle where the walls face outward. Coped joints only work with inside corners, not outside corners. Mitered joints work with both inside or outside corners.
What a Coped Joint Is
A coped joint starts with one molding piece that is cut square and simply butted into the wall corner. The mating molding piece is then cut to conform to the profile of the first piece. This second piece butts into the face of the first piece. The profile of the second piece of trim is cut with a coping saw or with a jigsaw.
Basics of Coped Corners
The biggest drawback of mitered joints is often realized when you nail the molding to the wall. If either of the two pieces doesn't remain vertical after it's nailed, the miter joint will open up. For example, if there's some thick drywall compound (also known as mud) at the bottom of the wall and this forces the molding to lie back, even a little bit, the mitered joint will be open at the top. By contrast, coped joints tend to be less affected by imperfect walls.
Trim carpenters and other professionals often prefer coped joints because they tend to open up less than miters when the wood shrinks during dry weather. Coped joints also accommodate out-of-square wall corners better than miters, which require a 90-degree corner for a perfect fit.
Coping is the traditional method of baseboard joinery and is considered a mark of craftsmanship. For this reason, it is often preferred for work with historic or period moldings.
On the downside, coped joints take more practice and skill than mitered joints. With miters, the saw does all of the cutting, and the trickiest part is taking accurate measurements and cutting the molding to the proper length. Coping requires a special technique and precise cuts made with a handsaw. Once you've mastered coping it can be just as fast as mitering, if not faster, but that kind of speed takes practice.
4 Reasons to Have Coped Joints
Area Is Humid or Moisture-Prone
Use coped joints with wood moldings in climates with radical seasonal shifts in humidity levels. Large seasonal swings make it more likely that corner joints will visibly open up, and coped joints are less likely to show these gaps.
You may also want to favor coped baseboard joints in moisture-prone areas like bathrooms.
Room's Walls Are Not Square
Many finish carpenters rightly argue that coped joints are better able to accommodate walls that are out of square—where the planes do not meet at exactly 90 degrees. With a coped joint, it's always best if the two sections meet at 90 degrees. But, if necessary, the coped board can be angled a few degrees with little visual discrepancy.
To Create a Polished Look
Period homes seeking to duplicate the look of original craftsmanship will benefit from using historically accurate joinery methods—using coped joints rather than mitered joints.
For Chair Rails and Crown Moldings
Chair rails, crown molding, and other trim that is high on the wall: all are much more visible than ground-level baseboards. When the trim will receive more than a cursory visual inspection, it's often best to cope the joints.
Pros and Cons of Coped Corners
Less prone to opening up
Best for out-of-square walls
Good for more noticeable trim like chair railing
Difficult to cut
Precise marking required
Hard to fix if cut too short
What a Mitered Corner Is
A mitered joint is made with both ends of the molding cut at 45-degree angles and fitted together to form a 90-degree inside corner.
This is the type of joinery used in most picture frames and many other woodworking projects where perfect 90-degree angles are required. Making a mitered joint requires either a power miter saw or a hand miter box and saw.
Basics of Mitered Corners
There is a reason why do-it-yourselfers and production carpenters building stock houses use mitered corners on moldings: They are easy and quick to make. Modern power miter saws fitted with fine-tooth blades make quick work of cutting moldings to perfect angles for mitered joints.
Even an inexpensive miter box and hand miter saw can make accurate miter cuts, but most of these can cut only 45- and 90-degrees; they can't make custom angles.
The biggest drawback of mitered joints is often realized when you nail the molding to the wall. If either of the two pieces doesn't remain vertical after it's nailed, the miter joint will open up. For example, if there's some thick drywall mud at the bottom of the wall and this forces the molding to lie back, even a little bit, the mitered joint will be open at the top. By contrast, coped joints tend to be less affected by imperfect walls.
4 Reasons to Have Mitered Joints
When Using PVC or MDF Trim
Shrinkage is a moot point with polyurethane and MDF moldings, so this is not a reason to cope the joints. Poly doesn't expand or contract, and neither does MDF, at least under normal conditions (direct water contact will cause MDF to swell or contract).
When Walls Are Square
Inside corner walls are rarely perfectly 90 degrees. But some are closer to 90 degrees than others. When the walls are in that 88 to 92 degrees range, it's simple to set the miter saw anywhere between 44 and 46 degrees (or sometimes even right at 45 degrees), to make the perfect cut.
Where Skill Is an Issue
By contrast, copes are tricky and take practice. The board must remain steady. The coping saw's blade often becomes twisted within the saw handle. Even the slightest wrong move might send the saw blade on the other side of the pencil mark.
Where Speed is Important
Most homeowners making mitered joints will be finished with an entire room at the same time it takes to learn the basic technique of coping. Mitered joints are the quickest method for installing baseboards and other moldings requiring inside corners.
Pros and Cons of Mitered Joints
Simple to cut
Easier to mark
No manual cutting required
Can be difficult to square on some walls
Joint can open up
More seam visible from above than coped joints