No matter the square footage or gpm of a roof drain design, a 2-inch water dam is provided standard. Scuppers are really the only time any consideration is taken with the height of ponding before cascading off the side of the building.
Some AHJs, Oklahoma State for instance, safeguard their cities by requiring a 10.2-inch rainfall rate to be used for sizing overflows as a response to past roof collapses. The same increased protection can be seen when designing a project to FM Global Standards, requiring the overflow to be sized twice the 100-year, 60-minute rainfall intensity; however, neither Oklahoma nor FM Global take the overflow water dam height into account.