I'm meeting a client this morning that would like a wood countertop in her kitchen. The layout is U-shaped, so for the continuous top she desires there will be two joints. I was thinking of using miter joints and MAS epoxy, but in scanning WOODWEB, I've seen a couple of warnings about miters in this situation. Why? I would have thought this would be the best way to go.
From contributor D:
If your tops are 24" wide and have a nice 45 degree cut on one end, and they mate together, all is well, for starters. As we know, wood expands and contracts across the grain in response to changes in humidity. Let's assume you used 6% sugar maple in a mix of flat sawn and quartered pieces. Then the humidity picks up in the house as all the extended family come to visit and check out the new kitchen. Cooking, cleaning, showering, etc. all increase. With all the excitement, maintenance on the tops is not done, and the MC increases to 11%.
This increase in MC will change the width of the tops by almost 3/8" - to 24-3/8". This will effectively change the angle of the miter from 45 degrees to something other than 45 degrees, opening the points or toe of the miter. I'll let someone else do the math. The family all leave, and you get a phone call about the "tops falling apart."
Let's assume you anticipated all the above - astute woodworker that you are - and you added a 3/4" thick, 10" wide piece of plywood under the miter and glued and screwed it in place to reinforce the perfect miter. Then, as the humidity increases, each board - each fiber - will attempt to swell but be restricted. Each fiber - board - will then crush just a little bit to accommodate the attempted expansion. Now, when the tops dry back out in time, little hairline cracks are likely to develop all around that area as the now crushed wood shrinks back in place.
If the humidity stays stable at all times, the miters will be okay, but you do not have control over the humidity. Even if you float the tops and use a spline that is short grain - oriented about the same as the grain in the tops - the angle of the miters will change in time.
This is why mitered solid wood moldings are limited in practical use to about 4-1/2" wide and two or more pieces are used to build to wider widths.
My thinking is that the end section of the U, which will be secured at the front edge, can expand and contract at the edge along the wall, with the movement hidden below the maple backsplash. The movement will be accommodated at either end in the unglued sliding dovetail.
The side wall (sides of the U) counters will also be affixed at the front edge, allowing for movement under the backsplash. With the front fixed in place, and locked to the butt ends of the perpendicular counter surface with the tight fitting sliding dovetail, no gap can develop (at least theoretically) between the counters at the joint.
There is a way to get mitered corners if that is a driving criteria. Like anything, all it takes is time and money. We call it "faux solid" and strip 1/8" thick solids onto both sides of a stable corestock, adding end grain and thick edge bands as needed. Then the balanced panels can be joined as miters, dog legs, etc. But the tops are not solid.
I see two issues with this plan, though. First is that it is going to look substantially different from a plain sawn layout, and that may or may not satisfy your customer. Second is that while movement will be significantly reduced, even if it expands or contracts as little as 1/16" over a 2' deep counter, that is going to happen on both sides of the miter. That means that any gap that develops in the front of the miter due to shrinkage, or the back of the miter due to swelling, will be about 1 1/2 times greater than the amount of movement of each 24" deep surface. Plus once opened, that gap will likely remain.
However, the science is there, and both flat sawn and quartered will move, albeit quartersawn at about half the rate of flatsawn. Some species are much more stable than others. Thickness of the top/boards makes no difference, unless it is decreased to where it can be treated like veneer - 1/8" or less. Gluing or not gluing splines or dog bones or whatever will make no difference. The wood just doesn't care.