Overflow scupper sizing

Size one secondary (overflow) scupper: the hydraulic head dh from ASCE 7-22 Table C8.2-3, the overtopping and capacity checks against your parapet, and the rain load R — every step cited, and the same engine the paid module runs.

Enter the scupper

The intensity is always yours — nothing is looked up here, and the paid module does not look one up either. If you already have the design flow, put it below instead of the area; supply one route or the other, never both.

A height on a channel (open-top) scupper is refused rather than ignored: the two are different devices and pass different flows at the same head.

The invert height is ds. Without a parapet height there is no capacity basis, so no pass/fail check can run — the head and the load are still computed, and the result says so rather than reporting a pass.

Secondary-system details, ponding, and the code basis

ASCE 7-22 §8.2(b) assumes a secondary system blocked when its inlet is less than 2 in above the primary inlet — in which case ds is measured to the next unblocked system, not to this one. Leave it empty and the result says it could not be confirmed.

dp comes out of a structural analysis, and this calculator performs none. Leave it empty and R is reported as the 5.2(ds + dh) lower bound and labelled as one — which is the honest answer, not a smaller one.

The risk category picks the design storm's return period (ASCE 7-22 Table 8.2-1). The state decides which plumbing code sets the minimum opening size — the head, the rain load and the capacity check are ASCE 7-22 and are the same everywhere.

What this calculator does

  • dh from the flow tests, not from a formula. The hydraulic head is read off ASCE 7-22 Commentary Table C8.2-3 — the FM Global flow-test data — and interpolated under the table's own footnote b. The closed-form weir/orifice model is run as well and reported beside it, so you can see the gap: in the drowned (orifice) regime the closed form passes 2–3 % more flow than the tests measured, which is the unconservative direction for dh, for R and for overtopping.
  • Both checks, when your parapet allows them. Overtopping — ds + dh against the parapet less freeboard — and scupper capacity at the head actually available above the invert. Without a parapet height neither can run, and the result says ANALYSIS rather than pretending to a pass.
  • The rain load, or an honest refusal to state one. R = 5.2(ds + dh + dp), ASCE 7-22 Eq. (8.2-1). Supply dp from your structural analysis and R is determined; leave it out and what you get is the 5.2(ds + dh) lower bound, labelled as a lower bound.
  • §8.2(a)–(d) moves the measurement, it does not just warn. A secondary inlet less than 2 in above the primary is assumed blocked, and ds is then measured to the next unblocked system — so the answer changes, and the reason is printed.
  • Cited on every line. IPC 2021 §1106 · FPC 2023 · ASCE 7-22 Ch. 8 and Table C8.2-3, each against the number it produced.

What this calculator does not do

  • One overflow scupper. One scupper. The paid module takes a whole drafted roof plan, several drainage areas, and the primary/secondary interaction between them.
  • No ponding analysis. No ponding analysis is performed. ASCE 7-22 puts dp inside Eq. (8.2-1) and §8.3 separately requires low-slope bays to be shown free of progressive deflection; both need a structural model. Without dp the rain load reported is the 5.2(ds + dh) LOWER BOUND and is labelled as one.
  • No rainfall lookup. The rainfall intensity is always yours. Nothing is looked up — ASCE 7-22's 200-yr and 500-yr rates are not in standard NOAA Atlas 14 output and this engine will not invent them.
  • No document. No stamped report, no PDF, no saved project. The number is whole; the document is what the paid module is for.
  • Not a primary drain, a gutter or a roof drain. This page is the secondary (overflow) scupper only. The paid module also sizes gutters and leaders from the IPC tables, and reports where the roof-drain head data it would need is not installed rather than estimating it.

Draining a whole roof?

The paid module takes the whole drafted roof: several drainage areas, gutters and roof drains as well as scuppers, the primary/secondary interaction, the ponding handoff to the SER, the DXF and the sealed report.

See the plans