Solid Wood Table-Top Thickness and Warping

      Ideas about how to make a solid wood table-top that will stay stable. March 26, 2007

Question
We are planning to produce maple table tops that will rest unattached on metal bases. We are debating what thickness the table tops should be, in order to resist warp. The dimensions will be 47" long by 21" wide. Stave width will vary from 3/4" to 3 1/2", with most at 2 1/2". The tops will be finished all over with a water-based varnish. We would rather not attach battens to them. Is there an industry standard, or even a rule of thumb regarding minimum thickness for a maple panel, at a given width?

Forum Responses
(Furniture Making Forum)
From contributor K:
We used to build solid oak tops in a drastic environment. After a few years of trying to make it work with little success, we switched to plywood with solid oak banding. Now that we have gone CNC, I'm considering designing solid oak tops that are machined on the bottom side to remove as much material as possible after the tops are glued up and surfaced. The tops would appear to be 1.5" thick, but pocketed down to .25" in the middle should reduce the amount of movement through the seasons. If this works as well as I predict, this would allow you to successfully produce any design of top you wish to manufacture. I hate when materials limit a great design.



From contributor P:
Why would making the top thinner reduce movement? And what are you going to do with the sawdust?


From contributor K:
Less wood equals less volume for the wood to absorb moisture in the air from a change in humidity, causing less movement. A working example would be veneer - veneer works good in all climates since the thickness is minimal. We are experimenting on the right thickness for our climate. Hopefully .25" will be sufficient, but these tests will take lots of time, at least 12 months. As far as the sawdust goes, we have this giant square machine in the back of our shop that actually sucks the dust and small wood chips up and puts them into a barrel - it's amazing. As a byproduct, we sell our sawdust to locals for horse bedding. The slab itself starts out at .875" with the perimeter built up to the desired thickness, so you're only taking .625" of material away. To do this successfully you really need to understand the way the wood you're working with moves and changes with humidity. Another idea would be to make relief cuts in certain areas on the bottom side after pocketing to give the wood a place to expand. This would be all one process, done on a CNC after glueup and surfacing. The cut of wood you choose (quatersawn, flatsawn) and how you choose to glue up (edge to edge, face to face, face to edge) will make a huge difference in the direction of movement.


From Professor Gene Wengert, Sawing and Drying forum technical advisor:
This is an interesting theory, but my experience in drying shows less warp with thicker stock. Likewise, thicker table tops (2") warp less than thin (1"). Have you ever seen veneer buckle? I have, but I have not seen lumber buckle. Paper loves to curl and warp.

In any case, moisture is what makes wood shrink or swell, so anything we do to reduce the percentage of moisture gain or loss is positive. With a thicker piece, although the surface may gain or lose MC, the core is rather stable and that is what holds the wood stable.

In answer to the original question, I know of no standard, but I would certainly use 2" for this size.



From contributor K:
In over 20 years we have never had a problem with veneer, and have never seen veneer buckle. I'd think the veneer might buckle if it was attached to an unstable substrate. Our only problems with wood movement have been with our .875" solid oak tops. From your experience, Doc, doubling this thickness will create a stable core? Won't the surface still absorb moisture and expand and contract, causing splitting on the ends? This has been our main problem.


From Professor Gene Wengert, Sawing and Drying forum technical advisor:
Splitting results because the edge is shrinking while the core is not. The cure is to make sure that both core and surface are at the same low MC; the correct MC is determined by the in-use MC, but is typically 7% MC or 35% RH. Then, make sure that the edge is coated with a moisture vapor resistant coating that is thick enough to control MC.

In my earlier posting, I was thinking about a thin piece of wood (veneer) that was not fastened to anything else. It is likely to warp and buckle.

Regarding a .875" top, it will likely warp much more than a top that is twice as thick. Oak is a wood that moves quite a bit with RH or MC changes. Are the splits found at the glue joints or between the joints? If at the joints, then we need to work on making a better (stronger) joint, as the joint should be stronger than the wood itself. If it is in the wood, we need to make sure that what we are seeing is not an end split from drying that was not trimmed off in manufacturing. These drying splits are often impossible to see, but will reopen when in a panel.



From contributor D:
Thinner wood equals less movement/warping? That's all wrong. I'd be concerned about the different thickness too. A thick and thin wooden top sounds like an invitation to inconsistent movement and moisture content. The effects that moisture have on different thicknesses in terms of amount of movement are the same. The difference lies within the speed that the movement occurs. Faster with thin, slower with thick, which is where you'll run into big problems. That thin wood will take off and leave the slower thick wood behind, but the thick wood will have the strength to make the thin wood go crack, like tying a bicycle to a fire hydrant with a ten foot rope. Everything's fine for the first 9 feet...


From Professor Gene Wengert, Sawing and Drying forum technical advisor:
I did not mean to imply that a top is made of two different thicknesses (unless veneer is added to a solid thick core). Two different thicknesses does invite problems.


From the original questioner:
Thanks for the input. 2 inch thick panels may be cost prohibitive from a raw material standpoint. How would your recommendation change if we were to use battens?


From Professor Gene Wengert, Sawing and Drying forum technical advisor:
2-inch material is not cost prohibitive, if the 1-inch material does not perform well in a changing RH environment.


From contributor S:
As the ancients found out, for stability, the top was thin and the under frame solid. Now we try all kinds of other schemes. Going back to the English bogors and Windsor chair making, they used a solid seat with the pins of the legs going right through and wedged in the seat. But they didn't waste good wood or break their clients' backs with anything over inch and a half!

An idea for thinner tops and frameless leg structure could be to let in two rails in the underside crossing the grain. Allow for movement with the rails made shorter than the groove and dry screwed.



From the original questioner:
Our cost for sticks to make a table top 2 inches thick would be so high, it would prohibit anyone from buying it. Because of our product mix, we would be better off to buy 6/4" sticks and make the tops 1 1/4" thick, rather than using 5/4" sticks to make 1" tops. We have had some success in the past with smaller pieces, by cross bracing the undersides with battens. Using the dimensions of 47" by 21", approximately, how much cross bracing would be called for, if the table top was 1.25 inches thick?

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