Waterproof Rooftop Decks the waterproofing system. ➢ “Flat” roofs must slope at least ¼” per ... A deck more than 30” above the floor or grade ... flashing that integrates with the roof's drainage plane. ... tile. Asphalt, shake, wood shingle, clay, concrete, aluminum, ... the shingles or tiles. The supports, of course, have to bear on an adequate load path.