Waterproof Roof Deck

Q: I’m planning to replace a 24-foot x 24-foot walkout deck over the flat roof over our garage. What’s the best way to build a waterproof floor?

A: The industry standard for the slope of a walkout like you're planning is 1/4-inch of vertical drop for every 12 inches of horizontal run. This provides the run-off you'll need for good drainage, without a sense of incline. As for waterproofing, the best option is something called EPDM. (Thank goodness for acronyms. These letters stand for ethylene propylene diene monomer). This rubber-like sheet material is used in flat roof situations like yours. It goes down first against a wooden deck, then wood strapping is applied every foot or two across the top, to support deck boards. There's more to success than I have room to explain here, so you’ll need more information. The best source I've seen is an article from the Feb/Mar2001 issue of Fine Homebuilding magazine (800-888-8286). On page 104 you'll find an excellent illustrated article showing all the important flashing details necessary for watertight performance.