Lumber shrinks across the grain. You learn that as a school kid. Well, you learn it if you hang around construction sites and bug the pros with lots of questions.

Treated lumber shrinks a lot across the grain. It’s treated by forcing water-borne chemicals into the boards, and most treated lumber is sold wet. The wetter lumber is the more it shrinks. Lots of deck builders snug wet deck boards against each other, because they know that the gaps will open as the wood dries.

Lumber shrinks much less with the grain, or end to end.

Except on my deck, where wide gaps have opened between the ends of treated boards in a couple of places. One spot is worse than on any deck we’ve ever owned.

I called Bill Wilhelm, the deck guru at Hoke Lumber in Davidson, to ask his wise counsel.

I learned a lot, and if you’re planning a deck project this spring you can probably learn something, too.

Treated lumber has always shrunk a little end to end, he said. But years ago the shrinkage was predictable and manageable. The newer stuff seems to pose bigger problems. (And bigger gaps.)

He’s not exactly sure why. “It could be the new treatment,” he said. “It could just be the lumber.”

About a decade ago, wood treatment chemicals with arsenic were phased out for residential use. The new deck lumber chemical has copper, like the older treatment, but no arsenic. The amazing shrinking boards on my deck were treated with the new chemical.

But it could just be that today’s treated pine is less stable to start with. “Years ago, you had a lot of old-growth lumber,” Wilhelm said. “Not today.”

So, what should deck owners do?

One solution is to use composite or PVC for deck surfaces. Treated lumber is strong and stable enough for posts and joists, Wilhelm said, and the new products are more stable for decking. “We used to sell lots of composites. … Now, more customers are turning to PVC.”

You could use treated lumber that has been kiln dried after treatment. It will be stamped KDAT. But it’s expensive, and harder to find.

Hoke doesn’t stock it. “All we sell is wet,” he said.

The best bet, he said, is to buy quality treated lumber and let it dry a bit when you get it home. Stack it flat, in a way that air can circulate. Stack it under a roof, out of the weather. You want to keep the rain off, and hot summer sun will make it curl.

After it has dried some, cut it a little bit long and install by jamming the ends of the boards as tightly together as you can.

Be sure to use quality galvanized or coated fasteners suitable for today’s treated lumber. The new chemicals will quickly corrode improper nails and screws. Regular sealing will help reduce wood movement.

That’s not a perfect solution, but about the best you can do, Wilhelm said.

“Everybody wants a no-maintenance deck,” he said. “There’s no such thing. The only solution is not to have a deck.”

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