Calculate roof scupper size, drainage capacity, or water feature pump flow from roof area, weir length, rainfall, head, and scupper count.
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Scupper Size Formula
The calculator uses three formulas, one for each mode. All are based on the IPC weir flow table that lists drainage area handled by a rectangular roof scupper at a given head.
A_per_scupper = A_total / N L_required = smallest tabulated L where (C(H, L) * 5 / i) >= A_per_scupper
A_capacity = L * k(H) * 5 / i
Q_total = L * f * N
- A_total = total roof area drained, sq ft
- N = number of scuppers
- A_per_scupper = roof area assigned to one scupper, sq ft
- L = scupper weir length (clear width), in
- H = design head over the weir, in (1, 2, 3, or 4)
- i = local design rainfall rate, in/hr
- C(H, L) = tabulated drainage area at 5 in/hr rainfall, sq ft
- k(H) = per-inch capacity factor at 5 in/hr (57.67, 160.25, 288.46, 448.71 for H = 1 to 4 in)
- Q_total = water feature pump flow, GPM
- f = spillway flow factor, GPM per inch (1.00, 1.25, or 1.50)
The Roof size mode divides total area by the number of scuppers, then steps through standard weir lengths and returns the smallest that handles the per-scupper area at your rainfall and head. Capacity mode runs the same equation in reverse to tell you how much roof one scupper of a known size can handle. Water feature mode ignores rainfall and sizes pump flow from spillway length.
Reference Tables
The first table is the IPC-style drainage area in square feet that one rectangular scupper handles at 5 in/hr rainfall. Scale by 5 / i to adjust for other rainfall rates.
| Weir length L (in) | H = 1 in | H = 2 in | H = 3 in | H = 4 in |
|---|---|---|---|---|
| 4 | 230 | 641 | 1,153 | 1,794 |
| 6 | 346 | 961 | 1,730 | 2,692 |
| 8 | 461 | 1,282 | 2,307 | 3,589 |
| 12 | 692 | 1,923 | 3,461 | 5,384 |
| 16 | 923 | 2,564 | 4,615 | 7,179 |
| 20 | 1,153 | 3,205 | 5,769 | 8,974 |
| 24 | 1,384 | 3,846 | 6,923 | 10,769 |
The second table lists typical 100-year, 1-hour design rainfall rates used in scupper sizing across U.S. regions. Always confirm with the local code or weather service.
| Region | Design rate (in/hr) |
|---|---|
| Pacific Northwest | 1.5 to 2.5 |
| Southwest desert | 2.5 to 3.5 |
| Mountain West | 2.0 to 3.0 |
| Midwest and Plains | 3.0 to 4.0 |
| Northeast | 3.0 to 4.0 |
| Southeast | 4.0 to 5.0 |
| Gulf Coast and South Florida | 4.5 to 6.5 |
Worked Examples
Example 1, roof scupper. A 12,000 sq ft warehouse roof drains through 2 scuppers. Local rainfall is 4 in/hr and the design head is 2 in. Each scupper handles 6,000 sq ft. From the table, a 12 in weir at 2 in head covers 1,923 sq ft at 5 in/hr. Adjusting for 4 in/hr: 1,923 × 5 / 4 = 2,404 sq ft. That is too small. A 24 in weir gives 3,846 × 5 / 4 = 4,808 sq ft, still short. You need either a third scupper or a higher allowable head. At 3 in head, a 24 in weir gives 6,923 × 5 / 4 = 8,654 sq ft, which works.
Example 2, water feature. A pool spillway is 18 in wide and you want a standard sheet at 1.25 GPM per inch. Q = 18 × 1.25 = 22.5 GPM per scupper. For 3 matching spillways, total pump flow is 67.5 GPM, or about 256 L/min.
FAQ
What head should you use? Use the maximum head allowed before water reaches the next overflow point or the top of the parapet. 1 in to 2 in is common for primary drainage. Overflow scuppers are typically sized at higher heads above the primary level.
Closed scupper or open scupper? The IPC table is for open-top channel scuppers. Closed (fully enclosed) scuppers behave more like orifices at higher heads and need a different calculation. The calculator assumes open weir flow.
Do you need a separate overflow scupper? Most codes require a secondary or overflow drainage path sized for the same design storm, with its inlet set above the primary. Run the calculator twice, once for primary and once for overflow at a higher head.
Why does the result jump to the next standard size? The calculator picks from standard weir lengths of 4, 6, 8, 12, 16, 20, and 24 in to match the code table. If your area falls between two sizes, it returns the next size up.