Why do deck boards crack and warp over a Long Island winter?
Water gets into the wood, freezes, and expands. It does that dozens of times over a Long Island winter, and each cycle pushes the fibres apart a little further until the board checks, cups or splits at the ends.
What is actually happening to the board?
Water in the timber turns to ice, and ice takes up more room than the water did.
Every freeze while a board is wet is a small wedge driven into it from the inside. Thaw, re-wet, freeze again and the wedge goes in a little deeper. The board ends suffer first because end grain drinks fastest.
That is why a deck can look fine in October and rough in April without anything having happened to it except weather.
Why do boards cup rather than just split?
Because the two faces dry at different rates, and the wetter face swells while the drier one does not.
The underside of a deck board stays damp longer than the top, which sees sun and wind. That difference is what pulls the board into a curve.
Cupped boards then hold water on their upper face, which makes the next cycle worse. It compounds.
What does salt air do?
It accelerates corrosion of the metal in the deck more than it damages the timber itself.
Near the water, fasteners, joist hangers and post bases work harder. Corroded connectors are a structural problem long before they are a visible one, which is why the connections deserve a look on any coastal deck.
Road salt tracked onto a deck at a side door does the same thing in a smaller area.
Does snow damage a deck?
Snow itself rarely does. Clearing it badly does.
A metal shovel dragged across boards gouges wood and scratches the shell on a composite. Working across the boards rather than along them, with a plastic blade, avoids most of it.
Work across the boards rather than along them and most of the risk goes away.
Why do railings go loose over winter?
Because the timber they are bolted through shrinks and swells with every wet and dry cycle.
A guard post carries a load and is fixed through timber that moves. Over enough cycles the fixing loses its grip, and a guard that moves when leaned on is the most dangerous thing on a deck.
It is worth pushing firmly on every guard post at the start of the season, every season.
What actually helps?
Keeping water off the wood and out of the joints, and looking at the connections once a year.
Clear the leaves out of the board gaps in autumn, because trapped debris holds water against the timber all winter. Keep the deck clear enough that it can dry.
On a wood deck, a sealer in good condition is the difference between a board that sheds water and one that drinks it. On any deck, the annual check of the ledger, the posts and the guards is the one that matters.
Does any of this happen to composite?
Not the cracking and cupping, because there is nothing to soak up water in the first place.
Composite and PVC boards move with temperature rather than moisture, which is why they are installed with specific gaps. Done correctly, a winter does very little to them.
The framing underneath a composite deck is still timber, though, and it is exactly as vulnerable as it would be under a wood deck. The surface changing does not change the structure.
More questions on this
Should I cover my deck for the winter?
Generally no. A cover that traps moisture against the boards does more harm than the weather it keeps off.
Can a cupped board be fixed?
Sometimes a board can be turned or replaced individually. If most of the deck has cupped, it is telling you about the deck rather than the board.
When is the best time to look at winter damage?
Early spring, before the deck is in use, while there is time to do something about what you find.