Sprung joint: Your last layer of moisture defense
One of the glued-up shelves that I built split along the glue-line at the shelf’s ends. The shelf is floating, so there’s nothing restraining seasonal movement. What should I do to prevent this?
—Lanny Cunningham, Dallas
Sounds like the boards of the shelf are pulling against themselves, Lanny. This sometimes happens because the ends of the shelf absorb and shed moisture more easily than the interior, thus swelling or—as in your case—shrinking at a disproportionate rate.
One preventative solution that the old-timers used was a “sprung” joint. Both boards of a sprung joint have slightly concave mating edges.
To create a sprung joint, first joint the mating edges as you normally would. Pair the boards in your bench vise with their ends and top edges flush. With a hand plane, make a light pass over the center two-thirds of the boards’ edges. Then make two full passes to smooth the edge into an arc about 1⁄32 " deep.
Dry assemble the boards. Only the ends should be touching with a gap in the center where you formed the arcs. When you glue the boards together, clamp from the center outward, ensuring that the joint pulls tight and leaves no trace of the gap. Now the ends can shrink without forcing the glue line to separate.