Q. Does biscuit joinery improve the strength of edge-to-edge joints when gluing up panels? If not, why bother spreading glue in the slots or on the biscuits? Just how strong are biscuit joints, anyway? Are they stronger than joining with dowels?
A. The fact that a well-prepared and glued edge-to-edge joint will break not at the glue line, but adjacent to it confirms that such a joint needs no reinforcement. The only reason to use biscuits in an edge joint is for alignment purposes; it’s a good approach when dealing with bowed boards. Gluing biscuits in their slots isn’t necessary.
Biscuits can be effectively used in butt joints. But because of a biscuit’s relatively shallow penetration into mating workpieces, it won’t be as strong as a proper mortise-and-tenon joint. When it comes to reinforcing miter joints, biscuits are very easy to install, and typically plenty strong for the job.That’s the application I mostly use them for.
It’s hard to draw strength comparisons between biscuits and dowels because strength depends on the type of joint and the size and number of biscuits or dowels used. Because dowels can more deeply penetrate the joining parts, they typically offer better mechanical strength. On the downside, dowel joints are fussier to make and don’t offer nearly as much long-grain to long-grain glue surface as biscuits. Thus, they tend to pull apart over time in heavy use. I’ve repaired lots of ‘em over the years.
Senior Editor, Woodcraft Magazine