Scupper Width for a Required Overflow Flow
The inverse of the overflow-scupper-sizing tile: the scupper width for a required overflow flow at a design head, from the rectangular weir Q = 3.33 L H^1.5. L = Q / (3.33 H^1.5) suppressed, or + 0.2 H for a contracted (narrow) scupper. 118 gpm at a 3.5 in head needs a 6.0 in opening (6.7 in contracted). Round the width UP; head is at the blocked-primary condition (IPC 1108 / FM Global). The code and roof-loading check govern.
Formula and source
L = Q / (3.33 H^1.5) (suppressed); L = Q / (3.33 H^1.5) + 0.2 H (contracted); Q_cfs = required_gpm / 448.8; L and H in ft, width reported in inches.
The rectangular (Francis) weir capacity of an overflow scupper (secondary roof drainage per IPC 1108 / FM Global), by name, solved for the width.
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