Two Ways to Cut Jack Miters
Part miter, part butt joint, this hybrid for beaded casing is elegant and stable.
Synopsis: Finish-carpentry specialist Gary Katz describes the usefulness of jack-miter joints for casing, which allow the head and leg casing to move independently while the joint stays closed in comparison to a regular miter joint, which will often open along the face of the joint with changes in temperature and humidity. In a series of step-by-step photographs, he walks through the process of creating a jack miter, first by measuring the head casing and measuring and cutting the casing legs. He then gives two options for handling the head casing—using a pull saw and a jigsaw if working only on one or two doors, or using a router and a jig for a bigger project. The article includes a drawing of the jig and steps for creating the template.
Jack miters are a hybrid joint, combining a small miter with a much longer butt joint. They’re commonly used on quirk-and-bead casing. As Jim Chestnut, a seasoned New England carpenter, explained to me, before modern miter saws were available, cutting a long miter was a tough task. And miters in wide moldings tend to open, sometimes a lot, when moisture levels change. During the winter, when the weather is cold and dry, wood shrinks and the short point of a miter opens. When the humidity and moisture content rises in the summertime, the long point often pulls apart. Butt joints are a lot less susceptible to problems from wood movement, and if they do move, the joint doesn’t open across the face like a miter, it just cracks the paint a little along a line of no visual importance.
That makes it sound like every carpenter should be an expert at cutting jack miters. But in California, where I spent decades trimming houses production-style, I’d never seen or…