Water causes wood to swell, so most people think that wetting one side and not the other will cause the wetted side to bow – that is, increase in width so the center is higher than the edges.
If the wood is thin enough, this will be the case initially. But the overall swelling or shrinking after many wettings and dryings out, no matter the thickness of the wood, will be the opposite. The wetted side will shrink and the wood, or boards, will cup. The four accompanying pictures show examples of this. With a little thought, you will most likely come up with examples from your own experience.
The explanation is a phenomenon called “compression set” or “compression shrinkage.” When one side of wood is wetted the wood cells want to swell. If the thickness of the wood prevents them from doing this, they compress from cylinder shapes to oval shapes, and they don’t return fully to their cylinder shapes when the wood dries out.
The compressions are cumulative. So after many wettings and drying outs, the result is shrinkage. Eventually, the wood can’t shrink enough on the wetted side, so it splits. On a deck, for example, this shows up first as small “checks,” or splits all across each board.
None of this has anything to do with heartwood or sapwood up, or planesawn vs. quartersawn. The continuously wetted side will be the side that shrinks.
This is the reason wood exposed to water on one side should be protected with a finish in good shape and thick enough so water can’t get through. It’s the reason deck stains and water repellants are not very effective, and it’s the reason the message of the Antiques Roadshow, “Don’t Refinish,” has been so destructive to our furniture heritage.