Drainage systems for flat roofs are an important part of a flat roofing system. Mobley Brothers Roofing and Renovation list the most popular drainage systems for flat roofs below. They are a leading provider of outstanding commercial roofing services since 2007. We have an A+ Better Business Bureau rating. Compared to sloped roofs, flat ones can’t fully capitalize on gravity to drain water effectively and efficiently. Although they are pitched to some extent, they usually require extraordinary drainage systems to prevent damaging ponding and dry themselves out with little human intervention.