To waterproof your roof, you must create an unbroken barrier over the roof structure. This is typically done by installing a solid surface (like plywood) and then applying a waterproof membrane, such as a peel-and-stick underlayment or a liquid-applied coating. For proper drainage, your roof must have a slope, even if it looks flat. A slight pitch of at least 2% (or a quarter-inch of fall per foot) will guide rainwater towards gutters and downspouts, preventing water from pooling and causing leaks.