If you can compute heating and cooling loads from room areas, why can't we calculate roof drain coverage by area as well. It may be trickier since multiple roof drains usually cover the roof but I know most architects add the sloping in anyway so you could find the area within individual sloped sections. The rainfall rate is then found for a given region (Re: International Plumbing Code) and then you use the table in the IPC to look up your roof drain size. You can also use rainfall rate and area to find GPM to get your size.
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