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Quality table tops are built from many smaller pieces of wood, which are then glued together. (Here we focus on table tops, not on butcher blocks, where multiple long beams are glued, then cut into successive slices.)

The pattern on the table top easily identifies how it was assembled.

building a table top from many pieces of wood

The advantage of assembly in this way is to reduce the effect of shrinkage or expansion of any one piece of wood. Even if one warps, its effect will be small, and there is a good chance it will be held in place by the adjacent pieces. If the stress does cause trouble, the difficulty will be limited to the development of a crack on one side of the table, and an additional bit of glue may well solve the problem by maintaining the integrity of the table top.

Despite all this, a table top made as shown in the pattern above can eventually warp.

cupped table top

What causes this cupping? The many pieces of wood have now cupped as a single element, and so the stress from any one of them is not responsible.

One thought is the following: If a table top has cupped and now looks like the sketch above, then that means that the bottom side has absorbed more humidity than the top side, because wood expands when the space between the cells absorbs humidity.

Is this correct? Sag is of course potentially another cause, but we assume here that the table top was resting on a rectangular frame underneath that reached to nearly cover all four sides.

We're not even talking yet about how to fix the problem, merely about the effect of humidity on such a table top.

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What causes this cupping?

This is a little more complex than just about moisture loss and gain, it's to do with long-term changes in wood as these effects take place.

What in essence makes a tabletop (as well as deck boards incidentally) cup in this way is compression set. The wood on top gets wet/wetter and tries to expand, as wood naturally does when it takes on moisture, but it is unable to do this as much as it needs to. So the wood fibres become compressed.

Over time this effect builds up, and eventually you reach the point where the top of the deck board/tabletop is substantially compressed, and weakened — which is why it may be impossible to rectify this problem on a tabletop by e.g. moistening the top to get it to expand, holding the top in its flatter state with clamps and letting it dry out. What very often happens is that once the clamps are removed the top bends again, at least partially1, back to its former shape. This is why I said in your other current Question that some fairly major intervention may be necessary to resolve this issue.

Read more about compression set at the following links:
Finish Both Sides? Not Necessary. by Bob Flexner on Popular Woodworking.
Decking boards: which side up? on Fine Homebuilding (answer by R. Bruce Hoadley, author of the very important Understanding Wood).
Straightening a Warp [ Compression Set ] on the Millcreek Woodworking blog.


1 But sometimes almost completely, which is very disheartening!

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  • Thanks for the reference to popularwoodworking.com/article/finish_both_sides_not_necessary . Indeed what was puzzling me most is exactly the feeling that exactly the following should have happened: "if the unfinished side made a difference, it should have caused the top to warp in the other direction – convex".
    – Sam
    Dec 8 '20 at 17:31
  • At the same URL: "You can encourage the flattening even more by introducing steam using wet cloths and a hot iron." My intuition was again entirely wrong. Had I not asked here, I intended to do exactly this (wet towel + hot iron) on the top side, and then quickly afterwards refinish to seal at least some of the moisture.
    – Sam
    Dec 8 '20 at 17:32
  • From your third reference millcrek.wordpress.com/2012/08/16/… : "As the bound water evaporates the cell walls begin to shrink and continue until the wood stops losing water." Good grief! How do boats that stay in the water for six months, and then hang outside to keep them from cracking from ice ever survive? Or do they really have a lifetime of (maybe) ten years and then they're gone? But that's clearly a completely separate question, and perhaps a boat made of wood is a bit of a stretch for the scope of this group.
    – Sam
    Dec 8 '20 at 17:33
  • This sort of thing regularly causes mental stumbling as people (very much me too!) struggle to visualise with what has happened on one side or the other and therefore envisage what fix to apply to that side, or the opposite, to rectify the issue. One of the standard solutions for just a basic cup on a board or panel is to place it on grass in the sun, but it's not at all unusual to get confused about which side goes down and which goes up! Aaand the confusion is amplified by the difference between what to do just after this occurs v. your current situation, where the fixes seem to contradict!
    – Graphus
    Dec 9 '20 at 8:48
  • Re. boats, there's a lot of flexibility in standard boat construction, that allows for quite a bit of shifting about of various elements as conditions change. But also bear in mind that boats are typically painted (and heavily) or otherwise coated (chiefly with epoxies these days it seems) which greatly limits the amount of water transfer that's possible. The marine environment is one of the few areas of woodworking where the word 'seal' may mean exactly what it says, rather than in the world of interior objects (boxes, turned items) and furniture where it's used very loosely or informally.
    – Graphus
    Dec 9 '20 at 8:54
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"Is this correct?"

No.

As a rough start, you should assume that moisture is evenly distributed throughout the wood. What happens is that the curved grain structure of the wood (a consequence of circular growth rings) will change in dimension as the moisture content changes. Since the wood was milled at one moisture level, at any other level the board will show curvature. This accounts for the fact that the board only curves along one dimension. In the other, the structure of the wood is straight, so there is no change in shape.

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  • Moisture is never evenly distributed throughout milled wood. It's always drier on the exterior and wetter towards the interior, which is why resurfacing old and very stable boards can lead to subsequent warpage (particularly if the boards are not used immediately or stored poorly). "at any other level the board will show curvature" What? When you buy wood that is flat and straight from a lumber yard where it was stored correctly, take it to a home or workshop environment for acclimation you don't expect it all to warp do you? "Acclimation" is just a shorthand way to say a change in MC.
    – Graphus
    Dec 7 '20 at 8:34
  • 1
    "The board only curves along one dimension" - so bow, crook, and twist movement don't exist? Dec 7 '20 at 16:39

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