Scupper Width for a Required Overflow Flow
The scupper width for a required overflow flow at a design head.
Example
You enter
- Required overflow flow (gpm) 118
- Head above scupper invert (in) 3.5
You get
- Width suppressed (in) 6.02
- Width (contracted / narrow scupper) 6.7 in
Checked against published worked examples from: North Carolina Building Code Council (International Code Council base text).
Details, formula, and sources
Scopefrom 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 or tableL = Q / (3.33 H^1.5) (suppressed); L = Q / (3.33 H^1.5) + 0.2 H (contracted); Q_cfs = required_gpm / 448.83; L and H in ft, width reported in inches.
Source and editionThe rectangular (Francis) weir capacity of an overflow scupper (secondary roof drainage per IPC 1108 / FM Global), by name, solved for the width.
Free accessThe Francis rectangular-weir formula is public; the secondary-drainage requirement is in the published IPC (Section 1108) and FM Global data sheets.
What governsEstimate. AHJ and licensed professional govern.
API field namesrequired_gpm, head_in, width_suppressed_in, width_contracted_in
- Weir flow L = Q / (3.33 H^1.5) suppressed, + 0.2 H contracted (cfs, ft)Francis rectangular weir
- Blocked-primary head H evaluated with the primary drainage assumed pluggedIPC 1108 / FM Global
- Structural check separate the ponding depth and roof load are the structural engineer'sscope of this tile