Storm Drain Sizing Chart
Sizing roof drains, vertical leaders, and horizontal storm drains by the roof area they serve and the local rainfall rate, per IPC Section 1106. The tables are normalized to a 1 inch-per-hour rainfall rate; for any other rate, divide the allowable roof area by the rate. Higher rainfall means larger drains for the same roof.
Vertical leaders & conductors
| Leader size | Max roof area at 1 in/hr (ft²) |
|---|---|
| 2" | 2,880 |
| 3" | 8,800 |
| 4" | 18,400 |
| 5" | 34,600 |
| 6" | 54,000 |
| 8" | 116,000 |
Horizontal storm drains
| Pipe size | 1/8" per ft | 1/4" per ft | 1/2" per ft |
|---|---|---|---|
| 3" | 3,228 | 4,640 | 6,576 |
| 4" | 7,520 | 10,600 | 15,040 |
| 6" | 21,400 | 30,200 | 42,800 |
| 8" | 46,000 | 65,200 | 92,000 |
Roof area and rainfall, together
Storm sizing is the product of two inputs. The roof area sets how much water arrives; the rainfall rate sets how fast. The tables fix the rate at a convenient 1 inch per hour, so the numbers are the roof area each pipe would drain in that reference storm. To use them for a real location, you divide by the actual rainfall rate — which is why the same 4-inch leader that covers 18,400 square feet in a 1 in/hr climate covers only a quarter of that where storms hit 4 in/hr. The steeper you slope a horizontal drain, the more it carries, which is the second table.
These are IPC values; the UPC has its own storm tables with the same rainfall logic but different numbers. Confirm the edition and the local rainfall data, and slope the horizontal runs with the pipe slope calculator.
Common questions
How do you size a roof drain or leader?
By the roof area it serves and the local rainfall rate. The tables give the maximum roof area each pipe size can handle at a 1 inch-per-hour rainfall rate — a 4" leader handles 18,400 sq ft at 1 in/hr. For your actual rainfall rate, divide that area by the rate: at 4 in/hr, the same 4" leader handles 18,400 ÷ 4 = 4,600 sq ft.
What rainfall rate do I use?
The 100-year, 1-hour rainfall rate for the location, from the IPC rainfall figures or approved local data. It varies widely — the Gulf Coast can be 4 inches per hour or more while an arid region might be 1 to 2 — and it directly divides the allowable roof area, so a high-rainfall area needs much larger drains for the same roof.
What is the storm drainage flow formula?
IPC Equation 11-1 converts roof area and rainfall to flow: Q (gpm) = (rainfall in/hr × roof area ft²) ÷ 96.23. The sizing tables are the practical form of that relationship, pre-solved for each pipe size at 1 in/hr.
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