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 heard of the joint. I’ve nailed up miles of quirk-and-bead casing, but I always mitered the corners. After all, casing is casing, and as a production finish carpenter, if I couldn’t do the job with a tablesaw, miter saw, and nailer, I was lost. Then Jim taught me the advantages of cutting jack miters, which go beyond creating a joint that will stay closed.
How many times have you installed casing around a door near a corner, and there isn’t enough room for the full width of casing between the jamb and the adjacent wall? Scribing a bit off the back of a piece of casing doesn’t look so bad, but scribe more than
1⁄2 in. and you begin to see that the miter’s been chopped, and it looks silly—like someone made a big mistake. If you use a quirk-and-bead pattern and jack-miter the casing, you can scribe any amount you want off the corner legs, and the joinery still looks good.
And how many times have you had two doors or windows so close to each other that you can’t install two pieces of casing, one on each jamb? If you use a quirk-and-bead casing, you can run the legs and head casing around the group and rip mull casing to any width you want to bring it all together.
Making a jack miter is somewhat more involved than making a miter or butt joint, but once you get a system down, it can still be done with a fair level of efficiency. The casing legs take two cuts on the miter saw and the head casing can be cut with a handsaw—if you only have a few joints to make—or with a router and jig if you’re trimming a whole house.
For both methods, measure first, then cut the casing legs
Whether you cut the jack miters by hand or with a router and template, start by measuring the head casing. Using a piece of casing or a tape measure, make a light pencil mark where the outside edge of the casing legs will land on the wall on each side of a door jamb.
Measure the distance between the marks to get the length of the head casing. You can also do the math: measure the casing width, add 1⁄4 in. for the jamb reveal, double that number, and add it to the inside dimension of the door jamb. To find the length of the casing legs, measure the jamb leg and add 1⁄4 in. for the jamb reveal. Cut the 45° miter on the casing legs with the short point at this marked length.
Swing your saw to 90° and make a second cut, daylighting at the location where the miter and the shoulder of the quirk intersect. From here, you can handle the head casing in one of two ways.
The two methods detailed in this article are (1) use a pull saw and a jigsaw, and (2) use a router and a jig. To see details on these methods, please click the View PDF button below.
Also check out this associated video: A Better Joint for Beaded Casings