Overflow Scupper Sizing (Weir Flow)
Secondary (overflow) scupper as a rectangular weir.
Example
You enter
- Scupper opening width (in) 6
- Head above scupper invert, 6 ft back from the opening (in) 3.5
You get
- Capacity (suppressed: only an opening spanning the full channel width) 117.7
- Capacity (contracted, the IPC Figure 1106.5 basis) 104 gpm
Checked against published worked examples from: North Carolina Building Code Council (International Code Council base text).
Details, formula, and sources
ScopeQ = 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 104 gpm on the contracted form the IPC sizing figure uses (118 only if the opening spans the full channel width); a wide 12 in / 2 in scupper passes 98 gpm. Head is measured 6 ft back from the opening at the blocked-primary condition (IPC 1108 / FM Global). The code and roof-loading check govern.
Formula or tableQ = 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 448.83.
Source and editionThe rectangular (Francis) weir capacity of an overflow scupper, secondary roof drainage per IPC 1108 / FM Global, by name. The IPC's scupper sizing figure (Figure 1106.5 in the 2015 IPC, as adopted in the 2018 NC Plumbing Code) prints the contracted form Q = 3.33 (L - 0.2 H) H^1.5 with H measured 6 ft back from the opening; the suppressed value is the full-channel-width case.
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 nameslength_in, head_in, q_gpm, q_gpm_contracted
- Weir flow Q = 3.33 L H^1.5 (cfs, ft); contracted 3.33 (L - 0.2 H) H^1.5Francis 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