Make a Second-Story Deck Waterproof With Gaskets Between Grooved Boards

The Problem

The space under a second-story deck — or the storage under a dock deck — gets rained on through every board gap, which is why it's usually dead space. The conventional fixes are under-deck ceiling systems or membrane-and-sleeper builds: effective, but a whole additional system hanging below your deck.

The Technique

Seal the deck surface itself, at the board gaps, with an inter-board gasket (DexerDry is the product shown).

1. The gasket is an extrusion with small spines along its sides, made to seat into the grooves of grooved deck boards — the same grooves hidden-fastener systems use. On solid boards, route the groove yourself; any router with a slot cutter does it.
2. Run your deck boards away from the house, so the sealed surface sheds water outward.
3. Set the gasket into one board's groove — the spines let it slip in and lock — then bring the next board up and squish the two together. The compressed gasket makes the joint watertight.
4. Board by board, the whole surface becomes a shedding roof: water hits the deck, can't pass the gaskets, and runs off the outboard edge.

The space below stays dry — a real ceiling for a patio underneath, or dry storage under a dock.