Installing Molding Using Mitered and Coped Joints

Decorative moldings like baseboards and chair rails can add a real touch of elegance to a room. While installing these moldings is getting into the realm of finishing carpentry, a handy DIYers can do a pretty good job, using just a few "special" tools, understanding the process and taking the time to do it right.

To install moldings, you need to know three basic joints.

Mitered joints are used for outside wall corners. The ends of two pieces of molding are cut on 45 degree angles and when the moldings are installed around an outside corner, the angles 'marry up" and form a 90 degree corner.

Scarfe joints are used to join two pieces of molding end to end. The meeting ends of both pieces of molding are cut on 45-degree angles so they'll overlap each other. By nailing through the overlapping pieces, the two pieces of molding are solidly joined. Scarfe joints work best when the joint is located over a wall stud so the nails go through the moldings and into the stud itself.

Coped joints are used on inside corners and effectively hide the joint between two pieces of molding. In a coped joint the first piece of molding is cut straight (90 degrees) and then butted directly into the corner. The second piece of molding is first cut on a forty five degree angle and then the material on the back of the molding is cut away, allowing the front edge (or profile) to slide tightly up against the profile of the first piece.

Why not just use miters on both outside and inside corners?

The challenge with mitered joints is that most walls aren't plumb and when you put the two pieces of wood together, they don't line up. The seam is often visible.

As a result, many years ago finishing carpenters evolved the idea of coping joints to address the problems with mitered joints. Coped joints (since they overlap) aren't as likely to show a gap as mitered joints. Plus, no matter how well a mitered joint was installed initially, overtime wood moves and different pieces of wood move at a different pace so even the best mitered joints will tend to open up and a gap will be noticeable.

Another advantage of coped joints is their adaptability. You don't measure an angle and then cut it, you actually fit one piece onto another. Even if the wall isn't perpendicular (and they rarely are) you can adjust the cope so the pieces align and make the joint fit perfectly.

The obvious downside to coped joints is that since they need to be cut and fitted, they are time consuming, but with a little practice, you can figure them out.

So how do you actually cut a 'coped joint'?

On the piece of molding you are going to cope, cut a 45-degree angle with the point facing the corner. (called an inside angle). Take your pencil and highlight the molding profile. Now using your coping saw, cut away the molding material behind the profile. You will end up with the point of the molding being the end profile and no material directly behind it. This point will fit directly against the first piece of molding that was butted into the corner with both profiles aligning (theoretically). Test fit the 'coped' piece, and use your rattail file to remove a little material at a time until the two surfaces do fit perfectly.

Installing moldings

Measure around the perimeter of your room and sum the measurements. Then, add 10 percent for wastage to determine the total number of running feet you will need. Most molding comes in 8-foot sections, but some comes in 10 or even 12-foot lengths, so pick out your molding before you calculate how many pieces you'll actually need.

Clear out the room to give yourself room to work, then use a stud finder to locate the studs in your walls and lightly mark their location on the walls. If you are going to be painting, staining or finishing your molding it's best to do that before you install the moldings. It's easier to paint or stain lying on a sawhorse in your garage or basement than on the walls. Don't forget to save a little of your paint or stain for touch-ups after the molding is installed.

A good way to install your molding is to start working on the wall on the right-hand side of the door and work your way around the room. The first piece of molding will be straight cuts on both ends. Butt one end into the corner, then mark and cut the molding to length to fit in at the door end. (Marking the piece itself is always more accurate than measuring and then cutting).

On your second piece of molding, cut your coping joint to fit into the first corner and then run the molding straight down to the far end. (Remember if the wall is longer than a single piece of molding, join two pieces using scarf joints positioned over a wall stud). At the far end, but the second piece of molding into the corner. Cut the cope on the next piece and install it with the cope over the butted in the end of the second piece.

Continue around the room, following the same process (one end of each piece of molding will be coped and the opposite end butted into the next corner).

If you're working with hardwood moldings, it's a good idea to predrill pilot holes where the moldings pass over the wall studs (so the wood won't crack when you nail through it). Plus, use a nail set to drive the nail heads below the surface.

If you have any outside corners, you'll need to cut miters. (As we've already seen, cutting miters that fit precisely can be a major pain, - an alternative is to cut your miters so they're "pretty close" and use some acrylic caulk to fill the gap. The caulk can be painted and the gap will disappear).

You can add the touch of elegance that moldings provide to your home. Using the proper tools, understanding how to make and install basic joints and probably most important, taking your time - will ensure your finishing carpentry job is a success.