When I build a deck with more than 4 ft. of space between the framing and the grade, I install an underdeck-drainage system. Consisting of a membrane installed under the decking, the system collects and channels water into a perimeter gutter system and away from the building. The area beneath the deck is kept dry and can be used for storage or, with enough headroom, a covered patio.
Several commercial underdeckdrainage systems are available for installation during deck construction or as a retrofit. I prefer to fabricate my own system from EPDM rubber-roof material and an aluminum gutter. It’s relatively fast and easy to install before the decking is laid, and this method costs less than commercial systems.
EPDM (ethylene propylene diene monomer) rubber is a roofing material used on flat and low-slope roofs. It’s available from roofing-supply houses in two standard thicknesses, 0.45 in. and 0.60 in., and in 10-ft.- or 20-ft.-wide rolls 50 ft. to 100 ft. in length. I usually use the thicker material,thinner grade works fine for this application. When working alone, I find 10-ft.-wide rolls easier to handle.
I figure that it costs about $1 per sq. ft. in material for 0.60-in. EPDM, gutter, and accessories. The slightly thinner 0.45-in. EPDM costs 90¢ per sq. ft. For a finished ceiling under the deck with vinyl soffit material and accessories, add $1 per sq. ft.