Deck Membranes and Downspouts
The dominant solution for waterproofing areas below decks is to drape plastic membranes within joist spaces below decking to form troughs. The Trex RainEscape deck waterproofing system, shown below, is an example of a membrane-and-downspout system.
Exterior Decking Membrane
A number of recently introduced under-deck ceiling systems use corrugated aluminum or plastic panels designed to catch water that drains through decking and channel it away from the area below. Among the newest is the RainTight system, introduced at Deck Expo 2014.
RainTight panels are made of heavy-gauge aluminum. The seamless panels are custom-fabricated, based on the size of the deck. They lock together along their long edges to form leak-proof joints.