Wide Miters in Wood Countertops

This long discussion about a proposed custom project includes good explanations for why miters in wide solid wood don't work, and some suggested methods to use instead. November 14, 2014

I have been asked to modify an existing built-in that resides in a Frank Lloyd Wright house and is of the era (maybe 50's). The owners want to keep with the look but want to replace the countertop which is about 16' long, 18" wide and 3/4 thick (edged PB with Sapele veneer and 1 1/2x 3/4 solid Sapele edge). This existing top has flip-up doors for access to the inside of this catch-all cabinet. They would like to house a TV and have a TV lift incorporated into the design but want to have a new top so as to have a one piece (no seam) look other than the pop up TV's wood cap (which would obviously have some reveals). The only thing I know of that comes 16' long is actual wood. The trick is the counter is an "L" and I am very fearful of movement (even if the board is left in their house for a couple weeks). Dominoes/biscuits, epoxy? Thereís breadboard T and G between the two mitered boards. Veneer in 16 footers would be ideal if I had a 16' vac bag.

Forum Responses
(Furniture Making Forum)
From Contributor C:
Make a 16' vacuum bag. Ever see pictures of vacuum bags for spiral staircase banisters or other very long projects? They make their own bags. Contact one of the veneer bag suppliers and ask about doing this. They should be able to supply you with the bag material, the connectors, and some adhesive for sealing the bag. If it is a one-time use, you should not need to buy the expensive bag material. If you already have all of the vacuum equipment and knowledge, this part should be easy for you.

From the original questioner:
Finding 16' sapele veneer is key. They don't want a seam. They might get one though.

From contributor R:
What if you put the seam at the opening for the TV? There will already be seams around the cover. Another option would be to inlay a contrasting veneer into the top. Maybe a line around the top and then a couple pieces cross grain? One of those would be at the seam in the veneer. Run those inlays over the edge, and you could do the top in a couple pieces and butt them together on site. Hauling a 16' L shape to the site won't be any fun either.

From the original questioner:
Thought of that too but they want no inlay or the like, just the lid to the TV lift box visible (designers). I still am wondering about solid.

From Contributor R:
Solid will fail in an 18" wide miter. It makes no difference how many splines, biscuits, screws or prayers are thrown at it. This is why veneer exists - to the things that solid can't do. Look around, I'll bet there are several shops nearby that can provide it for you already laid up.

Here is a lengthy reasoning of why the solid miters won't work, over time: If the two tops are 24" wide, solid wood, say quartered maple (thickness is irrelevant here), and at 6% MC coming from the lumber supplier. Your shop is about 30% RH, maybe little less. You do an excellent job of cutting the miters and they match perfectly, and each defines a 45 degree angle. Finish goes on and all is well. 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 increase, and the wood swells - goes to 24-3/8" on both parts.

Now the miter changes - becomes less than 45 degrees on each part (you may want to read that again - it is key), 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, letís say that you are aware of this likelihood and use a plywood spline under the top and glue and screw and fasten the heck 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 then separate and cracks will develop. Long story short, the top is doomed. From here it depends upon your view of your work over time. Should it only last a couple of years? Ten years? Longer? Is it correct to depend upon 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. Who knew wood movement could cause a philosophical self-examination.

From Contributor H:
I've built L-shaped solid wood counters that are 3/4" thick x 24" deep. The trick is to do a slot dovetail butt joint, not a miter joint at 45 degrees. If you can convince them to make this change you can use solid wood.

From the original questioner:
Contributor O, if you don't teach, it seems you should - great explanation. As for the sliding dovetail, it still seems iffy after reading Contributor Oís post. How long has your project remained the same way it looked when you delivered Contributor H? I am assuming a traditional sliding dovetail and not a "loose" sliding dovetail.

From Contributor H:
Yes, traditional sliding dovetail. Running pin on the butt end of one side and tail slot in the long grain of the side section. It's been through two years of seasonal changes in CT with insignificant change. The front inside corner is fixed and both counters are allowed to move in and out underneath the back splash.

From Contributor R:
Contributor Hís solution is as good as they get. You have to let the design allow it. I have done countertops where the front inside corner is fixed for four-six inches and the rest floats. Any movement is hidden under the backsplash. I do several hybrid tops a year, where 1/4" solids go on either a man-made core or a torsion box - both sides for balance - and you are then free to miter at will since the assembly is a stable construction. These tops can be sanded, scarred, refinished, abused and come up looking fine due to the thick veneer that allows the stable construction.

From Contributor H:
I'll add that the sliding dovetail corners were on three 90 degree bends in a fairly large kitchen. All three have remained tight over time. This was only 3/4" thick material which made for a fairly small sliding dovetail. I think that it would be better in thicker material and would probably want to use at least 5/4 stock if I did it again.

From Gene Wengert, forum technical advisor:
Sapele is not exceptionally stable with MC changes. Wood will be wood, and your customer needs to know that. I agree with Contributor H.

From contributor A:
With the sliding dovetail seam, you still need to provide a way for the wood to expand and contract when you fasten it back down to the cabinetry.

From Contributor H:
That is correct. You need to guide the expansion/contraction of the material. As such you pin these to the cabinets at the front edges of the counters and let the wood expand and contract at the back. In the case of the kitchen counters I've referred to here that expansion/contraction takes place under the back splash.

From contributor W:

If you want to do exactly what the customer wants, just veneer the countertop. You could even make your own veneer. You don't need to buy it as a sheet. You could grab some long planks of ribbon sawn sapele, re-saw them, send then through a drum sander and then start laying them out. I would recommend using a slow epoxy rather than wood glue. If you use wood glue, you will have to veneer the bottom of you substrate so that the top does not bow or cup, and believe me it will. Once you have the veneer laid out, vacuum bag it. you can buy vacuum bag material in tubes - basically 4' wide, by whatever length you want. I see this as being the most bomb proof system. Then put a UV cure finish on it. You will end up with the warm wood look, but with the strength of composites.

From contributor B:
I am very surprised nobody has questioned the wood. Are you sure that the piece you are replicating is sapele? This would have been a very uncommon wood for a 50's era house. Even more uncommon considering it was a Frank Lloyd Wright home. Which home is it? Almost all of his homes are cataloged and have information available on them. Especially his later homes because of his fame and the advent of new photography methods, etc. Do you have any pictures?

From the original questioner:
Contributor B you are right. The house is a Frank Lloyd Wright inspired house. A student of his built it as I am told - mahogany everywhere. The house is in San Rafael Ca. A quick Google search for type of wood mahogany Frank Lloyd Wright used spat out Philippine Mahogany. The paneling in this house does look like P. Mahogany but the tables/built-ins/floating shelves have a grain that looked closer to Sapele to me. I never told myself that it was an unusual wood for the time. I am familiar with P. Mahogany for sheet goods and it looks awful in my recollection (Luan).