When joining sections of baseboard molding at inside wall corners, you have a choice between creating a mitered joint, or a coped joint. When cut properly, the two methods produce joints that look largely the same; which method you choose will depend largely on your skill level and what tools you own.
Both joinery styles can be used to join segments of molding at interior (concave) corners that are set at roughly 90 degrees. The differences between mitered joints and coped joints are fairly simple to understand:
Each baseboard piece is cut at a 45-degree angle (as shown here). When joined, the two pieces form a 90-degree angle that fits perfectly against the inside corner where two walls meet. Mitered joints work quite well when the walls meet at exactly 90 degrees, but where walls are not perfectly square, achieving a perfect mitered joint may require some adjustment to the cutting angle on the baseboard pieces. For example, if a wall meets at an 86-degree angle, the baseboard pieces need to be cut at 43 degrees in order for them to make a perfect joint. Mitering is also the only method that is used on outside corners—where molding pieces meet on convex outside corners, such as in L-shaped rooms or where baseboards follow the profile of partial walls on archways.
Each baseboard is cut square, at a 90-degree angle. One piece of baseboard is positioned so the square end rests in the wall corner against one wall, then the adjoining piece is manually cut with a coping saw so the end conforms to the profile (contour) of the first piece. Coping can be a good choice when walls are not perfectly square; coped joints are also less susceptible to gapping when changes in humidity cause wood moldings to expand or contract.
Which Method Should You Use?
Most homeowners who own a power miter saw or a miter box opt for mitered baseboard joints since they are very easy to cut. But professionals swear by coped joints since this method is well suited for all types of wall conditions. Above all, professional carpenters want to avoid call-backs to solve problems, so they generally opt for the method that gives the best results.
However, for amateurs, coped joints can be tricky to cut perfectly. If you attempt coping, make sure to practice your technique on scrap wood; if you get good results, coping is the way to go. If you find coping difficult, however, you will be best served by using mitered joints for your baseboards.
Mitered-Cut Baseboard: Top View
This top view shows how the adjoining pieces of baseboard molding are cut and fit in a mitered joint. When done correctly, the corner looks perfect, with no gaps—provided the corner is a perfect 90 degrees, to begin with. Viewed from the front, there is little visual difference between this mitered joint and a coped joint.
How It Is Made
With either a manual miter saw and miter box, or with a power miter saw, cut each baseboard pieces at a 45-degree angle. Fit the two pieces together to form a seamless 90-degree-angle corner. Some slight adjustment of the cutting angles may be necessary if the planes of the wall are not at a perfect 90-degree angle.
Miter joints are very easy to make, provided walls meet at a perfect 90-degree angle and your miter saw is calibrated correctly so that it cuts at angles that exactly match the miter scale. However, if the walls do not meet at 90 degrees, or if the saw does not cut at precise angles, you may be left with a gap in the corner. Seasonal expansion and contraction of wood moldings can also cause visible gaps to open up. With painted baseboards, you can disguise such gaps with a small bead of caulk.
Coped Baseboard Joint
This oblique front view shows how a coped joint fits into a corner. Note how one piece of molding fit is square-cut to fit flush into the corner, while the adjoining piece is contoured to fit the profile of the molding. When cut correctly, a coped joint looks very similar to a mitered joint, though a trained eye may be able to distinguish the craftsmanship involved.
How It Is Made
Cut one baseboard piece at 90 degrees so that it will butt into the wall corner. Use this baseboard piece (or a short scrap piece of identical molding) as a template to draw a profile of the molding shape on the face of the second piece of baseboard.
Next, use a manual coping saw to cut the second piece along the marked line. Make sure to keep the saw blade angled slightly inward (back-beveled) from the front face of the molding, ensuring a sharp edge that will fit tightly around the adjoining molding.
Fit the second piece of baseboard molding into the corner, so that the shaped end fits around the contours of the first piece. If cut correctly, the coped joint should fit tightly, with no visible gaps.
Pro tip: Some professional carpenters make coped cuts by first cutting the molding piece at a 45-degree miter, then using the coping saw to cut precisely along the line where the saw blade has sliced into the molding. The edge of this miter cut will conform exactly to the molding contour once you cut along it with the coping saw. When using this technique, a carpenter normally cuts the baseboard piece an inch or two longer than the required length. Once the piece is first mitered then coped along the edge of the miter cut, the other end is square-cut to fit the precise length needed.
Cutting a good coped joint is admittedly a bit tricky if you don't have experience. The flexible blade of a coping saw can be difficult to control, and for best results, you need to back-bevel the cut to ensure a sharp edge that fits tightly around the adjoining piece of baseboard molding. It is best to practice on scrap pieces of baseboard before coping the long piece of molding you intend to install.
If done perfectly, coped joints are preferable to mitered joints, since they are less likely to reveal gaps due to imperfect wall angles or seasonal expansion and contraction of wood.