A woodworker on the Internet announces he is "planning" a wall shelving unit made of old oak. It is five-eighths inch thick on the side supports. The shelves will be a half-inch thick.
"Dadoing into the five-eights-inch lumber to help support the shelves is the area of question," he continues. "One-quarter inch deep sounds too shallow. Three-eighths of an inch is slightly over half the board. The shelves will be only about 16 to 18 inches wide. Any recommendations?"There was considerable confusion over his statement that the shelves would be 16- to 18-inches wide. Some responders took that to mean that the shelves would be up to 18 inches deep and of an unknown width. Since the fellow used the word "only," I'm assuming he means the shelves will occupy 16 to 18 inches of wall space. Otherwise, he is a very weird woodworker.
Several questions arise other than the matter of dadoes. Shelves half an inch thick, even in oak (and how old is old oak, anyway?) are pretty minimal for an unsupported 18-inch run. If this is to hold knickknacks or things that aren't too heavy, it will be all right. I wouldn't stuff it full of hard-bound books, however.
Interestingly, one responder said, "My tastes run toward lighter-looking construction, so I wouldn't automatically discount his intentions. Using overly thick stock is one of the most common design errors."
I'm not privy to statistics on design errors, but I would tend to doubt this, at least among amateurs. Most of the design flaws in amateur work that I've seen involved wood too thin, often the result of trying to save money. Also, it depends on what you're building. In a piece that does not need to withstand outside pressures or any kind of abuse, the smaller it is the more it demands thin wood.
But in load-bearing work, whether the load to be borne is books or people, it's better to err on the heavier side.
How deeply the shelves should be set into the sides has a lot to do with how the sides are secured. The correspondent doesn't mention what he's going to do at the top and bottom of the unit. If, for instance, the top and bottom pieces (or shelves, if you like) are going to be dovetailed into the sides, then a quarter of an inch, or even 3/16ths, is plenty of inset for the interior shelves.
Incidentally, the shelves should be set into the sides with shouldered dadoes, rather than full dadoes. If the shelves are half an inch, then cut the dadoes to 3/8 of an inch and trim the ends of the shelves to fit, leaving 1/8 inch of shoulder below the dado. This shoulder will fight against the wood's tendency to droop right out of the dado.
On the other hand, if this gentleman thinks these dadoes are actually going to hold the entire piece together, I fear he is in for a rude shock. The shouldered or rabbeted dado is sufficient to hold shelves into an already secure case, but by themselves they will not keep the case together.
The problem, you see, is that while there are four separate mating surfaces involved - top, end, bottom and shoulder - not one of them involves mating two glue surfaces. End grain is involved in each section of the joint, and end grain, of course, is not a viable glue surface.