Deck maintenance is mostly cosmetic until it is structural, and the structural part is invisible from the surface.
The ledger board
Where an attached deck fastens to the house. Deck collapses overwhelmingly involve this connection, and it is the least inspected part of the structure.
What to look for: the fasteners should be structural bolts or approved connectors, not nails. There should be flashing above the ledger directing water away from the wall rather than behind it. The board itself and the house framing behind it should be sound, not soft or stained.
In Western Washington, missing or poorly detailed ledger flashing is a common and serious fault. Water runs behind the board and rots both the ledger and the house framing it is bolted to, invisibly, for years.
If you can see the ledger from below, look at it. If you cannot, and the deck is older or of unknown construction, having someone assess it is worth doing.
Western Washington: rot and moisture
Persistent damp means decks here stay wet far longer than in drier climates, and that drives the failure mode.
Check where water sits rather than drains. Places where two pieces of timber meet and hold moisture between them. The tops of posts, particularly where they meet beams. Anywhere debris has accumulated between boards.
Probe suspect areas with a screwdriver. Sound timber resists; rotten timber gives. Pay particular attention to posts at ground level and to joist ends where they meet the ledger.
Moss and algae on the surface make decks genuinely slippery and are a real fall risk. Clean them, but be careful with pressure washing — high pressure damages softwood decking, raising the grain and accelerating deterioration.
Eastern Washington: UV and freeze-thaw
Different problems entirely. Intense summer sun degrades timber and finishes far faster than in the marine west, so the finish is doing more work and needs renewing more often.
Freeze-thaw cycling works at every crack. Water enters, freezes, expands, and the crack widens — each winter compounding the last. Splitting and checking in decking boards is the visible result.
Concrete footings and pads are affected the same way, and inadequate footing depth means frost heave lifts posts over successive winters. A deck that has become uneven is worth investigating rather than shimming.
What to check annually
Railings — push firmly and see if they move. Loose railings cause falls. Stair treads and stringers. Fasteners that have backed out or corroded. Boards that flex underfoot. Anything soft, split or visibly deteriorated.
Clear debris from between boards, since accumulated organic matter holds moisture against the timber continuously.
Finishes
West of the Cascades the finish is working against moisture. East of the Cascades it is working against UV. Both need renewing, and the eastern one generally more often.
Apply during a genuine dry window and on timber that is actually dry, not merely surface-dry after rain. Sealing moisture into the wood achieves the opposite of the intention.
When it is beyond maintenance
Widespread rot, a compromised ledger, structural members that have failed, or footings that have moved. These are not maintenance items.
Deck construction is permit territory above certain heights or where attached, and rebuilding to current requirements — particularly snow load east of the Cascades — matters structurally rather than bureaucratically.