Got a call today from someone wanting wooden countertops in an L shape. I called John Boos (company that does butcher blocks and wooden kitchen counters). I was told they have been making these tops prepped for titejoint fasteners and precut to a 45 for a very long time without any complaints. I asked... Really? No complaints? What about wood movement? I was told as long as I leave a 1/8" gap at back (caulk joint or splash covers gap and is only fastened to wall) I would be fine. I said, "I have been in woodworking for nearly 20 years and my better judgment tells me this can't be."
I was reassured it is fine and that if the miter does open up, "that's the great thing about titejoint fasteners, you just have to re-tighten the fastener." Beauty? Huh? Re-tighten it? Like, in every season? Well, after one minute of second guessing myself and thinking this would work, I thought, no, it won't. At least nothing I would want my name on or a callback for. So, what do you think? I'd like to be able to offer this. In another post, someone mentioned sliding dovetails in miter. Sorry if I am being redundant. Just confused.
From contributor M:
You are best off not using a miter on any joint in solid wood longer than a couple of inches. As the two pieces which meet at the miter shrink or expand widthwise, they change the angle of the miter cut on their ends, and the joint has got to open up somewhere. Your instincts are correct according to my experience.
For a solid wood countertop corner joint, you are better off joining one endgrain piece up to one edgegrain piece with some kind of alignment scheme, like a tongue and groove, to hold the top surface flush as the end grain piece shrinks and swells.
Check out clam clamps. I used to trim houses and used these on a couple of jobs. Kind of flies in the face of common wood principles (wood moves and miters will open). Trim is, obviously, smaller than a 25" deep countertop, but there are plenty of trim guys that use these on 6" wide casing without issue. All despite an unheated/cooled environment and battling all of the moisture introduced by sheet rock mud, trim that has been stored in unconditioned spaces, etc.
You deliver, install and whistle as you leave. After a year, the client repaints the entire house and adds several gallons of water into what was once a stable environment. Or the basement floods. Or the in-laws visit for a month. At any rate, the RH increases, so the EMC increases, and the wood swells. Goes to 24-3/8" on both parts.
Now the miter changes - becomes less than 45 degrees on each part, and if the joint is forced to stay tight (tite-joint top fasteners, for instance), then the two legs of the top will want to open, making for a greater than 90 degree joint/inside corner.
Well, let's say you are aware of this likelihood and use a plywood spline under the top and glue and screw and fasten the bejeebers out of the two halves. All to prevent any change in width. If the tops don't bow up as they take on moisture, they will still take on moisture - can't prevent it. Since the wood cannot expand, it will crush all the fibers along the points where it is restricted - at the joint. Then as the house dries back out, the crushed fibers will separate and cracks will develop.
Long story short, the top is doomed. From here it depends on your view of your work over time. Should it only last a couple of years? 10 years? Longer? Is it correct to depend on things outside of your control for the length of the life of the top, or do you have a responsibility to know all this and develop a strategy that will avoid the inevitable?
Boos makes a fine product, but they are not immune from the physics or the reality. Look at their docs and I'm sure there is no guarantee/warranty that the joint won't open. If people have not had a problem, all that means is the customer(s) never called - not that the joint did not open. When I ask owners of tops with open miters, people say everything from "he is probably out of business" to "I have no idea who made the tops." There will be exceptions, but the reality is that wood moves, period.
The trick is to float the countertop. The anticipated movement that contributor D provided will happen. If you screw the counter down hard, the joint will move. Therefore I use oversized holes with fender washers to allow the counter to move at will. You can choose one location that stays put. Do not restrict movement anywhere else.
This is why Boos specs the expansion area at the wall. They expect the countertop to move under the back splash. Do not be afraid of this project. If you understand the basic principle, you can get a very nice product.
By the way, I am a fan of clam clamps but I don't see the relevance for such a long miter. They are great for trim work, but for counters?
The long miters in question will last just fine - if the RH does not change from a narrow region. That RH could be stable for many years, and the miter will look fine. No magic or special glues or fasteners - only RH.
You see chart recorders in museums around the world. Their insurance carrier insists on tracking RH and temp to provide insurance for priceless artwork that depends on stable environments to last. They will not pay a claim if the RH is not stable, and the chart recorder is there to document everything. Not something for the average homeowner.
That is why I asked what a maker's responsibility is? As you stated - "Pay me now and we'll see if it works..."