Actually, the correct term here is low-slope roof. A roof that is truly flat is a disaster as it will pond water. Ponding water is the greatest source of leaks in a membrane roof system. Most people tend to believe that as long as the pond is holding water, that section will not leak. That is simply not true. If you have a pond about 25′ in diameter over a 5000sf roof, the entire roof is feeding the pond. The pond will continue to leak while it is being fed and will not stop until the pond is empty. That assumes the hole in your membrane is at the deepest part of the pond.