Estimate the scupper size a roof needs from its drainage area, design rainfall and water head, or check how much roof area a given scupper width can drain.
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Scupper Size Formula
The calculator has two modes: Required scupper width and Supported roof area. It uses the approximate rectangular-outlet equations G4-5 and G4-6 in the ASCE-authored 2020 Rain Loads guide, Chapter 4, page 53; it does not interpolate an IPC table or size water-feature pumps. These equations omit side-edge contraction losses, can overestimate flow and are not applicable to widths below 6 in (152.4 mm).
Open-top weir, or closed opening with head h ≤ clear height H: Q = 122 × W × (h ÷ 12)1.5.
Closed opening with h > H: Q = 1.87 × W × H × √[5.37 × (h − H ÷ 2)].
Runoff (US gpm) = area (ft²) × rainfall (in/h) × 4/385. Demand per scupper = runoff × user allowance ÷ scupper count. Supported area = total modeled capacity ÷ (rainfall × 4/385 × user allowance).
- Q = estimated discharge of one opening, US gallons/minute
- W = clear scupper width, in; model applicability requires W ≥ 6 in
- h = hydraulic head above the bottom of the opening, in
- H = clear closed-opening height, in; entered only for closed openings
- N = positive whole number of identical scuppers at the same head, with equal flow sharing
- A = designer-established contributing roof area, ft²
- i = site-specific design rainfall intensity, in/h
- User allowance = 1.00, 1.25, 1.50 or 2.00; blank means 1.00, with no claim of a code safety factor
Required width divides factored runoff equally among the scuppers and solves the active equation for width. It then selects a whole-inch US width or a 10 mm metric width whose modeled capacity meets demand. The equation lower bound is 6 in: metric sizing first enforces 152.4 mm, then rounds up to at least 160 mm. Supported area reverses the same model and rounds the displayed area down to whole units. US and metric inputs are separate entry sets; switching does not convert existing values.
Reference Tables
The first table is calculated from the open-top weir equation above for one clear opening, rainfall 5 in/h and allowance 1.00. It gives estimated supported area in ft², rounded down. It is illustrative equation output, not an IPC table, a tested product rating or code-approved sizing. For the same head, area scales inversely with rainfall and the allowance.
| Clear width W (in) | h = 1 in | h = 2 in | h = 3 in | h = 4 in |
|---|---|---|---|---|
| 6 | 338 | 958 | 1,761 | 2,711 |
| 8 | 451 | 1,278 | 2,348 | 3,615 |
| 12 | 677 | 1,917 | 3,522 | 5,423 |
| 16 | 903 | 2,556 | 4,697 | 7,231 |
| 20 | 1,129 | 3,195 | 5,871 | 9,039 |
| 24 | 1,355 | 3,835 | 7,045 | 10,847 |
| 30 | 1,694 | 4,793 | 8,806 | 13,559 |
Do not use broad regional rainfall ranges for a project design. Obtain a site-specific intensity, storm duration and recurrence criterion accepted by the authority having jurisdiction and the project’s qualified designer. The second table identifies inputs to confirm rather than recommending regional design values.
| Required design input | Source or check |
|---|---|
| Project location | Site and governing jurisdiction |
| Rainfall intensity | Approved site-specific rainfall source |
| Storm duration and recurrence | Applicable adopted code and project design criteria |
| Contributing area | Roof drainage layout established by the designer |
| Allowable water head | Structural ponding assessment and inlet elevation |
| Opening geometry and performance | Manufacturer data and inlet or downstream loss checks |
| Secondary drainage | Independent overflow design under the applicable code |
Worked Examples
Example 1, required open-top width. A 12,000 ft² roof has 2 identical scuppers at the same head. Use rainfall 4 in/h, head 2 in and allowance 1.00. Runoff is 498.7013 US gpm, or 249.3506 gpm per scupper. The weir equation gives a hydraulic minimum width of 30.0385 in per opening; whole-inch rounding selects 31 in per scupper. This preliminary hydraulic result still needs the structural, manufacturer and adopted-code checks.
Example 2, closed-opening capacity. Two identical closed scuppers are each 12 in wide and 4 in high, with head 6 in, rainfall 4 in/h and allowance 1.25. Since head exceeds height, the submerged equation gives about 416.0064 US gpm per opening, or 832.0127 gpm total. The model supports about 16,016.2453 ft²; the calculator reports 16,016 ft² after rounding down. This is not a rated or code-approved area.
FAQ
What head should you use? Use the hydraulic head above the opening bottom established by the project designer, within the roof’s structurally allowable ponding depth. Parapet height or the next overflow alone does not establish a safe head; primary and secondary inlet elevations require an independent design.
Closed scupper or open scupper? Open-top openings use the weir estimate. Closed openings use that estimate while h ≤ H and the submerged-orifice estimate when h > H. These two empirical formulas need not meet continuously at the boundary; the calculator does not invent a transition blend. Manufacturer-specific performance may differ.
Do you need a separate overflow scupper? Determine the secondary arrangement and design storm under the adopted local code with the project designer. The official 2018 IPC Section 1106.5 excerpt says primary-system flow is not credited when sizing secondary scuppers; it also links opening size and inlet elevation to structural water-depth limits. A calculator run is not a substitute for those checks.
Why does the result jump to the next standard size? The calculator uses a whole-inch US grid or a 10 mm metric grid, rather than a supplier catalog or IPC table. It enforces the 6 in (152.4 mm) equation range before rounding, so the smallest metric result is 160 mm. That equation limit is not a universal code minimum: the historical 2018 IPC excerpt separately specifies 4 in (102 mm) opening height and a width based on the circumference of an equivalent roof drain. Check the current adopted requirements.
