Fight Physics with Lapped Dovetails
Lapped dovetails are the right joint for a top rail
For a lot of woodworkers just getting acquainted with the intricacies of dovetail joinery, the idea of using a lapped dovetail to join the top rail of a carcase or table to the adjoining leg seems like overkill. Why not just use the tried-and-true mortise-and-tenon? A recent Q&A in Fine Woodworking magazine highlighted this very topic, and it’s worth some thought. A mortise-and-tenon might last you a decade, but a dovetail will likely last you a lifetime.
Q: In caracase and table construction, I often see a lapped dovetail joint connecting a top rail to the legs. Why is this joint preferred over mortise-and-tenon joinery?
– John Dennis, Temple, New Hampshire
A: Lapped dovetails are used on a narrow rail above a door or drawer, where there isn’t enough thickness for a strong mortise-and-tenon joint. A kick to the bottom of the leg, or the act of repeatedly sliding the table across a floor, creates racking forces at the top that want to pull the leg away from the rail. The lapped dovetail really shines here, because it has a mechanical advantage over mortise-and-tenon joinery. The angled sides of the tail pull the joint tight and lock it together, which means it can’t pull apart, even if the glue fails. A single lap is strong, but I think a double lapped dovetail, in which the upper rail is joined to the leg and side apron, is stronger. Because a double lapped dovetail requires a wider rail and has two locking points, it resists racking forces even better.
– Steve Latta is a contributing editor
A double lapped dovetail is even stronger. Having two tails on a wide rail makes the base even stronger. Use this version of the joint when possible. It will require dovetail sockets in both the top of the leg and the side of the apron. You'll also need to incorporate a wider top rail.
A single lapped dovetail locks the parts together. Angled sides prevent the tail from pulling out of the socket and provide resistance to racking forces in every direction.
Small tenons are a bad idea. Because the tenon joining the top rail to the leg is in line with the forces working to pull the joint apart and offers no mechanical resistance to them, you're relying on your ability to fit the joint and the strength of the glue to keep the joint together. An inadvertent kick to the bottom of a leg creates a lot of force up top.