Flat roofs, also known as Built Up Roofs (B.U.R.), require a water-tight seal like no other roofing system because they need to support water ponding until the water evaporates. Flat roofs are common over patio terraces like the examples below, but can also be installed over the entire roof.
One of the most effective roofing systems is a flat/B.U.R. roof combined with a slight slope for draining. The downside to these roofs is the little aesthetic options when it comes to colors and styles.