Overflow Scupper Sizing (Weir Flow)
Secondary (overflow) scupper as a rectangular weir: Q = 3.33 L H^1.5 (cfs, feet), contracted 3.33(L - 0.2H)H^1.5. A 6 in scupper at 3.5 in head passes 118 gpm (104 contracted); a wide 12 in / 2 in scupper passes 102 gpm. Head is at the blocked-primary condition (IPC 1108 / FM Global). The code and roof-loading check govern.
Formula and source
Q = 3.33 L H^1.5 (cfs, L and H in ft); contracted Q = 3.33 (L - 0.2 H) H^1.5; gpm = cfs x 448.8.
The rectangular (Francis) weir capacity of an overflow scupper, secondary roof drainage per IPC 1108 / FM Global, by name.
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