The joint that is most often associated with interior trim work is the miter joint. The miter joint is a straight forward 45-degree joint that is designed to bring together two boards that meet at a right angle (90-degree). The joint that is the most uncommon joint is the scarf joint. The scarf joint is used whenever you want to join two boards in a straight line. Making scarf joints is quite easy after a little practice.

For example if the baseboard trim is not long enough to reach from corner to corner an extension is required. Joining the two pieces together with a scarf joint is the only way to join the two pieces and have a finished professional finish.

When a scarf joint is done properly, it can be almost invisible, especially if the boards are primed and painted. Because this joint is used so infrequently, it’s a good idea to master it. Practice by following the steps shown here before you tackle any actual trim jobs.

Step 1: Start a scarf joint by cutting a 45-degree miter at one end of a board. This is referred to as the scarf cut. You can use a power miter saw or a hand-powered miter box. Keep in mind that the joint will be only as good as the cuts you make on the two mating boards.

Step 2: Attach the cut board to the wall by nailing with 6d-finishing nails along the bottom edge into the wall plate. Nail the top of the board into the wall studs that are usually spaced 16 inches apart. Spread carpenters glue onto the miter cut with a small disposable brush.

Step 3: Cut a matching miter joint on the end of the second board and then cut the board to the proper length. Spread glue on this miter cut too. Then push the board in place so the joint is flush. Drill small pilot holes through the boards for installing the finishing nails.

Step 4: When the glue is dry sand the joint surface until smooth, then apply primer and finish paint.

That is all there is to making scarf joints.

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