A deck is a structural addition attached to the house, which means it is held to the same physics as the roof and walls it connects to, not to the lighter standard homeowners sometimes assume applies to outdoor structures. The two points where most decks eventually fail are the footings and the ledger board connection, and both are essentially invisible once the deck is finished, which is exactly why they matter so much during construction. Footings have to be engineered to handle the specific frost depth of the region, since a footing that does not reach below the frost line will move every time the ground freezes and thaws, slowly working the entire structure out of level over a period of years rather than failing all at once. The ledger board, where the deck attaches to the house, faces a version of the same water intrusion risk as a poorly flashed roof valley or siding seam, except a failure there rots both the deck framing and the wall structure behind it simultaneously. Framing decisions that seem minor, like joist spacing, also compound over time depending on whether the decking material is wood or composite, since composite boards flex differently than lumber under the same span. None of these problems are visible on the day a deck is finished. They show up two, five, or ten years later as heave, rot, or sag, which is why the unglamorous structural decisions made before the first board goes down are what actually determine how long a deck lasts.