Install an under-deck ceiling using corrugated fiberglass panels. Use 2x4 lumber and spacers to ensure the panels slope downward and away from the house. Adding gutters and downspouts to the lower edge of the ceiling eliminates any splashes. With a waterproof ceiling on an elevated deck, you do not change the appearance of the decking boards while creating a dry patio at a lower level.
Attach pressure-treated 2x4s so they are perpendicular to the joists that support the decking and spaced about 3 feet apart. Cut the spacers so the panels drop 1/4 inch for every foot. For a 12-foot deck, the lower edge of the ceiling should be 3 inches below the upper edge.
Connect the panels to the 2x4s using special hex-head screws with a rubber washer under the head that prevents leaks. Insert these screws at the highest point of the bend due to the corrugation and tighten them enough that the washer flattens slightly. Keep the screws aligned in the same ridge along the panel. Attach the gutter at the end of the panels, inside the support beam.
Panels are available in various lengths, but most are 26 inches wide. This allows a small overlap to prevent leaks. Vinyl or galvanized metal panels are other options for this project and give different ceiling results.