A mitered joint is one in which the ends of the molding are butted together with two pieces with ends angled at 45 degrees 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. A mitered joint is fairly easy to make if you have a well-tuned miter saw to make the cuts.
A coped joint is one in which one molding piece is fit square into the corner, with the end of adjoining molding shaped to conform to the profile of the first piece. The profiles cuts are usually made with a coping saw, or sometimes a jigsaw. Many professional carpenters prefer to cope the corners on baseboards and other moldings for a variety of reasons. Coped corners are often regarded as the sign of fine craftsmanship.
To the casual viewer, both joints have the same appearance, so why do pros prefer coped joinery? And should DIYers attempt to copy the pros?
Pros and Cons of Coped Corners
Many home improvement and woodworking experts recommend coping as the preferred method for making inside corners for moldings. According to Gary Katz at This Is Carpentry:
Wood swells and shrinks throughout the year, depending upon seasonal humidity. Coped joints don’t open nearly as much as miters. Cope joints are also faster to install than miters—the pieces of molding don’t have to be cut exactly the right length; in fact, coped material can be cut a little bit long.
This professional opinion is sometimes debated by amateurs attempting to make coped joints since the speed Katz mentions comes only with experience. Coping is an acquired skill that requires practice to make the profiled cuts fit precisely over the adjoining molding piece. Especially for hardwood moldings with fancy shapes, this can be a frustrating exercise for DIYers. But a do-it-yourself who masters the skill is rewarded with a tight-fitting joint that won't open up and will look like the work of a pro.
Places, where coped joints are the best choice, include:
- 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.
- Where walls are out of square. Many finish carpenters 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. However, mitered joints can also accommodate out-of-square walls, simply by adjusting the miter cuts a degree or two off of 45 degrees.
- Where the look of fine craftsmanship is desired. 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. Higher wall molding is much more visible than baseboards, and the perfectly shaped joints made by coping will be evident and appreciated by viewers.
Pros and Cons of Mitered Corners
There is a reason why DIYers 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.
Places where mitered joints make the most sense include:
- With non-wood trim. Shrinkage is a moot point with polyurethane and MDF moldings, so there is no advantage to coping the joints. Poly doesn't expand or contract, and neither does MDF under normal conditions (though it may swell with direct water contact).
- Where skill is an issue. If you have a power miter saw, nothing can be easier than locking it to the 45-degree angle, pressing the baseboard against the fence, and sawing down to reveal a nice open angle. By contrast, copes are tricky and take practice.
- 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 skill of coping. Mitered joints are the quickest method for installing baseboards and other moldings requiring inside corners.
Most DIY homeowners and production carpenters will find it possible to make accurate and acceptable inside corner joints with baseboards using mitering. Especially with baseboards, where the lower wall corners are generally somewhat shadowed, it is rare for gaps caused by shrinkage to be noticeable. Modern power miter saws make it easy to cut precise miters quickly.
Coped joints may still be the method of choice for elaborate moldings where you seek the look of historically accurate joinery. It may also be the best choice for higher wall moldings, such as crown moldings, which are much more visible than baseboards.