David Lamb replies: A breadboard end should be attached by way of tenons from the top to mortises in the breadboards. Also, there should be a continuous haunch or ‘tongue and groove’ that goes from tenon to tenon and out to the end so when all trimmed out it looks like a tongue and groove fit.
The tenons should be at least 3/4˝ long (at min.) and preferably 1 to 1-1/4˝ long. Depending on the width of the table you would use 3 – 5 tenons, equally spaced. The center tenon may be fully glued to the mortise, may be spot glued on the next set out, and no glue out at the ends.
The tenons should also be pegged – firmly in the center and through slotted holes (in the tenons) on the outside tenons. This slotting allows for wood movement yet keeps the ends tight to the top.
Always expect movement in this joint and avoid surface finishes that would whiten if the surface breaks from this movement. A client should never expect the edges and surface of this joint to remain as new. If they do – use a plywood panel, veneer it and glue it tight.
Expansion depends on the materials, but I would expect 1/8˝ to 1/4˝ of movement on a 36˝ top over the span of the four seasons. It all depends on the environment it is in and how dry the stock was when built. A climate controlled house with no direct sunlight on the piece should expect little movement.
Al Breed replies: When attaching breadboards, I generally use three blind tenons into the breadboard. Draw pin and glue the center one fast.
The outer most tenons I do the same but without glue, and I elongate the hole in the tenons a little parallel to the shoulder so that the table top can expand and contract. Make these mortises a little wider than the tenon so that it can seasonally slide back and forth. Remember that the table top is going to expand and contract while the breadboard and the pins in it will not.
A shallow haunch can be left between the tenons to keep the top sliding level with the breadboard.
I have an 18˝ inch pine top that moves about 3/16˝ seasonally, but different species will act differently. Bruce Hoadley has a formula for this in his book on wood technology if you want to do the math, but I never do, I just guess. Good luck.