This is an excerpt from “With All the Precision Possible” by André-Jacob Roubo, translation by Donald C. Williams, Michele Pietryka-Pagán & Philippe Lafargue. The following text is part of an essay about Roubo’s workbenches written by Christopher Schwarz.
One of the biggest obstacles, downsides and joys to a French bench is the massive slabs used to construct it. Finding wood that is big enough to use without laminating thinner pieces together can be difficult. Laminating thin pieces together to make the thick pieces required for the top and legs is a lot of work without the help of machines.
If you do find stock that is 6″ thick and 22″ wide for your benchtop, it almost certainly will be wet in the middle and prone to distortion. The first French bench that I built used a 4-1/2″-thick cherry slab that had been seasoning in a lot for about five years. The first couple years with that bench were rough. The top shrank at least 1/16″, leaving the through-tenons and sliding dovetails proud of the benchtop.
After planing those flush, the top didn’t shrink much more, but it sagged a bit in the middle during the third year. And now the benchtop is quite stable – yearly humidity fluctuations have little effect on it. The tops of the legs and the benchtop are always in the same plane and the overall shape of the top is consistent.
The French oak that I used in 2013 was likely even wetter than the cherry. For starters, the oak was thicker. And thick material takes a lot longer to dry than thin material. When we first cut into the oak, we used a moisture meter on the wood and found its moisture content in a few places was off the charts. Most places on the bench were about 30 percent moisture content, which is quite wet by furniture standards.
Two months after completing the bench, the top was so wet that it would rust the surface of a holdfast left in a hole overnight.
Like the slab cherry workbench I’d built years before, the oak benchtop shrank around the tenons by more than 1/16″ during the first six months. And the middle of the benchtop began to sag. I flattened the oak top twice during the first nine months in order to be able to plane thin stock on my benchtop.
This begs the question: How flat does a bench need to be? The answer is: It depends on your work. If you plane woods that are less than 3/4″ thick, benchtop flatness is important. I shoot for getting the front 12″ of the benchtop so flat that I cannot get a .006″ feeler gauge under a straightedge anywhere in that area.
If you work with thick stuff or do mostly carpentry, you can be more cavalier.
So if thick slab workbenches are so difficult to find and fussy at first, why bother?
After they settle down, slab workbenches move very little. The same forces that make the top dry slowly also retard its ability to take on much moisture during the seasons (thanks to Steve Schafer for explaining this via Fick’s Second Law, a diffusion equation). After about five years in your shop, your benchtop should be well acclimated and monolithic.