How much will my tabletop move?

Why this question has no good answer online

Search for it and you will be shown product listings for furniture dollies and moving straps, because the search engine reads "tabletop" and "move" and reaches the wrong conclusion. Behind that, the woodworking answers are forum threads, and they mostly say "leave a bit extra". This page exists because the question is a good one and nothing was answering it.

Fix the middle, free the edges

A solid top has to be allowed to change width, and the way you get that is to pin it in exactly one place across its width and let everything else slide. Fasten it at the centre, and each edge then travels half the total swing. Fasten it at two points far apart and you have built a clamp: when the wood shrinks it cannot, so it splits instead. More well-made tops die this way than any other, and it is always the outer screws.

The breadboard end is the classic trap

A breadboard end runs across the grain of the top, which means the top changes length along the breadboard while the breadboard itself does not. Glue the whole joint and one of them loses. The convention is to glue only the centre tenon and to leave the outer tenons in slots with pegs through elongated holes, so the top can breathe past its ends.

The slot needs the full seasonal swing, not half of it. Half is the mistake, and the reasoning behind the mistake is sound: movement is symmetrical about the fixed centre, so each side only travels half. But the pin in the outer tenon is not at the centre — it is out near the end, where the accumulated movement of everything inboard of it has piled up. Cut the outer slots for the whole figure.

Build it in the middle of its range

Wood assembled bone dry can only swell, and wood assembled damp can only shrink. Neither is ideal, because a joint designed around one direction of travel has to absorb twice as much of it. If you build at the midpoint of the range the top will actually see, it moves half as far in each direction and every allowance you cut is doing half the work. The tool prints that midpoint.

Asked at the bench

I glued up narrow boards. Does that reduce the movement?

Not at all. A panel glued from six 100 mm boards moves exactly as much as one 600 mm board of the same species and cut — the glue lines do not resist anything. What ripping narrow and alternating the ring direction does buy you is flatness: the cups of adjacent boards oppose each other, so the panel stays flatter even though it changes width just the same.

What about plywood or a veneered top?

Different problem entirely. Plywood is cross-banded, so each layer restrains the next and the panel is dimensionally stable across its face. That is why a veneered top can be screwed down solid. It also means you must never mix the two: a solid edging glued across the end of a plywood top will be fine, while the reverse arrangement on a solid top will not.

Does a thicker top move less?

No. The percentage is the same at any thickness. A thick top does respond more slowly, so it lags the seasons and tends not to reach the extremes a thin one does, but that is a damping effect rather than a reduction — over a few years it gets where it was going.

My top already cupped. Can I flatten it?

If it cupped because one face was sealed and the other was not, or because it was stored flat on a damp floor, then evening out the exposure will often bring most of it back. If it cupped because it is a wide flatsawn board doing what wide flatsawn boards do, planing it flat at one time of year just means it is thinner and cupped the other way six months later.