When editorial adviser Mike Guertin builds a deck with more than 4 ft. of space between the framing and the grade, he installs 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. Guertin explains how to make an underdeck-drainage system from EPDM rubber-roof...