Flat roofs are normally found on Commercial buildings and rely on gravity to shed the water off. No roof is really 100 percent flat or else water would never go anywhere after a storm. There is a slight slope also known as low slope to shed the water off. These roofs are protected by a rubber membrane to keep mother nature out.
The process we use is called EPDM which is a rubber that is commonly used on flat roofs and the brand we trust is Rubber-All.