Design Standards & Dimensions
Storm Drainage Sizing: Roof Drains, Leaders and Secondary Drains (IPC Chapter 11)
IPC Chapter 11 sizes roof drains, leaders and storm drains from the 100-year, 1-hour rainfall rate converted to gallons per minute, and requires a separate secondary drain or scupper system wherever a parapet or curb would trap water.
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Last updated October 27, 2026 · Rainfall basis, flow conversion, pipe and leader capacities and secondary drainage rules are taken from the [2021 International Plumbing Code, Chapter 11, Storm Drainage](https://codes.iccsafe.org/content/IPC2021P2/chapter-11-storm-drainage) (Sections 1101 through 1108, Tables 1106.2, 1106.3 and 1106.6 and Equation 11-1). The roof-area tables and worked example are calculated by Harth from those capacities; areas are rounded down. These are typical model-code values; verify the rainfall rate, pipe capacities and secondary drainage method against the edition adopted by the authority having jurisdiction, the IBC roof drainage and rain load provisions it adopts, and the roof drain manufacturer's rated flow.
Direct answer
The 2021 International Plumbing Code sizes storm drainage in Chapter 11 from the 100-year, 1-hour rainfall rate shown in Figures 1106.1(1) through 1106.1(5) or approved local weather data. Equation 11-1 converts that rate to flow: gallons per minute equal 0.0104 times the rainfall rate in inches per hour times the roof area in square feet, with half of any wall that sheds water onto the roof added under Section 1106.4. The flow is checked against the roof drain manufacturer's rated capacity, vertical leaders are sized from Table 1106.3, where a 4-inch leader carries 192 gpm, and horizontal storm drains from Table 1106.2 by slope, where an 8-inch drain at 1/8 inch per foot carries 714 gpm. Section 1108 requires secondary roof drains or scuppers wherever the roof edge would trap water, discharging separately above grade and sized for the same rainfall rate without counting the primary system. Scupper openings must be at least 4 inches high.
Content and cited inputs reviewed by Harth team · Published 2026-10-27 · Updated 2026-10-27.
What are the key takeaways?
- Design rainfall: 100-year, 1-hour rainfall rate
- Flow conversion: gpm = 0.0104 x rainfall (in/h) x roof area (sq ft)
- 4-inch vertical leader: 192 gpm
- Secondary drainage: Separate system sized for the primary rainfall rate
How do you size roof drains and storm drain piping?
Chapter 11 of the 2021 International Plumbing Code sizes storm drainage from one design storm: the 100-year, 1-hour rainfall rate shown in Figures 1106.1(1) through 1106.1(5), or a rate taken from approved local weather data (Section 1106.1). The same rate sizes the roof drains, the vertical leaders and the horizontal storm drains. The sequence is:
- Find the rainfall rate in inches per hour for the site from the IPC maps, Appendix B or local data the code official accepts.
- Measure the roof area each drain serves. Section 1106.4 adds one half of the area of any vertical wall that diverts rain onto the roof.
- Convert to flow with Equation 11-1: flow in gallons per minute = 0.0104 x rainfall rate (inches per hour) x roof area (square feet). The constant converts one square foot of roof under one inch per hour of rain into gallons per minute.
- Check the roof drain. Section 1106.2 requires the calculated flow to be checked against the manufacturer's published flow rate for the drain model and size, and Section 1105.2 bases that rate on the head of water expected at the drain.
- Size the leader from Table 1106.3 and each horizontal storm drain from Table 1106.2 at its slope. Pipe size can never be reduced in the direction of flow (Section 1101.5).
- Size gutters from Table 1106.6 where the roof drains to gutters rather than internal drains.
What are the IPC 2021 storm drain pipe capacities?
Table 1106.2 gives the maximum flow for storm drain piping, vertical and horizontal, by pipe size and slope.
| Pipe size (inches) | Vertical drain (gpm) | Horizontal at 1/16 in/ft (gpm) | Horizontal at 1/8 in/ft (gpm) | Horizontal at 1/4 in/ft (gpm) | Horizontal at 1/2 in/ft (gpm) |
|---|---|---|---|---|---|
| 2 | 34 | 15 | 22 | 31 | 44 |
| 3 | 87 | 39 | 55 | 79 | 111 |
| 4 | 180 | 81 | 115 | 163 | 231 |
| 5 | 311 | 117 | 165 | 234 | 331 |
| 6 | 538 | 243 | 344 | 487 | 689 |
| 8 | 1,117 | 505 | 714 | 1,010 | 1,429 |
| 10 | 2,050 | 927 | 1,311 | 1,855 | 2,623 |
| 12 | 3,272 | 1,480 | 2,093 | 2,960 | 4,187 |
| 15 | 5,543 | 2,508 | 3,546 | 5,016 | 7,093 |
Vertical leaders that carry flow from gutters or roof drains use the separate leader capacities in Table 1106.3: 30 gpm for a 2-inch leader, 54 gpm for 2-1/2 inches, 92 gpm for 3 inches, 192 gpm for 4 inches, 360 gpm for 5 inches, 563 gpm for 6 inches and 1,208 gpm for 8 inches, with equivalent rectangular sizes such as 2 x 4 inches (92 gpm) and 3 x 4-1/4 inches (192 gpm).
When are secondary (emergency overflow) roof drains required?
Section 1108.1 requires secondary roof drains or scuppers wherever roof drains are required and the roof perimeter, such as a parapet, would trap water if the primary drains became blocked. Three rules shape the design:
- Separate system. Secondary drains discharge separately from the primary system, above grade, where building occupants or maintenance staff would normally see the flow (Section 1108.2). That visible discharge is the signal that the primary drains are blocked. Where primary and secondary drains are one manufactured assembly, each must have an independent inlet and outlet.
- Same rainfall rate. The secondary system is sized under Section 1106 for the rainfall rate used for the primary system, and the primary system's flow is not counted (Section 1108.3).
- Scuppers. A scupper opening must be at least 4 inches high and at least as wide as the circumference of the roof drain required for the area it serves, and the scuppers must keep ponding below the depth the roof was designed for (Sections 1106.5 and 1108.3). Section 1101.7 requires the roof structure to be designed for the maximum possible water depth with all primary drains assumed blocked, including the head of water needed for the secondary drains to flow.
Which other Chapter 11 rules catch designs out?
- Storm water may not drain into a sewer intended for sewage only (Section 1101.3), and floor drains may not connect to a storm drain (Section 1104.2).
- Leaders and storm drains connected to a combined sewer must be trapped, either individually or with one main trap before the connection (Section 1103.1). Connections to a separate storm sewer need no trap.
- Conductors cannot double as soil, waste or vent pipes (Section 1104.1).
- One- and two-family dwellings may discharge storm water onto lawns or streets where approved, provided it flows away from the building (Section 1101.2).
- Siphonic roof drainage is designed to ASME A112.6.9 and ASPE 45 rather than the gravity tables (Section 1107.1).
- Roof drains are tested and rated to ASME A112.6.4 or ASPE/IAPMO Z1034 (Section 1102.6), and the roofing membrane cannot obstruct the drain opening (Section 1105.1).
Maximum roof area per vertical leader by rainfall rate
Calculated by Harth from IPC 2021 Table 1106.3 leader capacities and Equation 11-1 (area = capacity / (0.0104 x rainfall rate)), rounded down to the nearest 10 square feet. Area is horizontal projected roof area, including half of any wall that sheds water onto the roof. Typical model-code values; verify against the AHJ-adopted edition and the roof drain manufacturer's rated flow.
| Leader size (inches) | Capacity (gpm) | 1 in/h | 2 in/h | 3 in/h | 4 in/h | 5 in/h |
|---|---|---|---|---|---|---|
| 2 | 30 | 2,880 sq ft | 1,440 sq ft | 960 sq ft | 720 sq ft | 570 sq ft |
| 2-1/2 | 54 | 5,190 sq ft | 2,590 sq ft | 1,730 sq ft | 1,290 sq ft | 1,030 sq ft |
| 3 | 92 | 8,840 sq ft | 4,420 sq ft | 2,940 sq ft | 2,210 sq ft | 1,760 sq ft |
| 4 | 192 | 18,460 sq ft | 9,230 sq ft | 6,150 sq ft | 4,610 sq ft | 3,690 sq ft |
| 5 | 360 | 34,610 sq ft | 17,300 sq ft | 11,530 sq ft | 8,650 sq ft | 6,920 sq ft |
| 6 | 563 | 54,130 sq ft | 27,060 sq ft | 18,040 sq ft | 13,530 sq ft | 10,820 sq ft |
| 8 | 1,208 | 116,150 sq ft | 58,070 sq ft | 38,710 sq ft | 29,030 sq ft | 23,230 sq ft |
Maximum roof area per horizontal storm drain at 1/8 inch per foot
Calculated the same way from the 1/8-inch-per-foot column of IPC 2021 Table 1106.2.
| Drain size (inches) | Capacity (gpm) | 1 in/h | 2 in/h | 3 in/h | 4 in/h | 5 in/h |
|---|---|---|---|---|---|---|
| 3 | 55 | 5,280 sq ft | 2,640 sq ft | 1,760 sq ft | 1,320 sq ft | 1,050 sq ft |
| 4 | 115 | 11,050 sq ft | 5,520 sq ft | 3,680 sq ft | 2,760 sq ft | 2,210 sq ft |
| 5 | 165 | 15,860 sq ft | 7,930 sq ft | 5,280 sq ft | 3,960 sq ft | 3,170 sq ft |
| 6 | 344 | 33,070 sq ft | 16,530 sq ft | 11,020 sq ft | 8,260 sq ft | 6,610 sq ft |
| 8 | 714 | 68,650 sq ft | 34,320 sq ft | 22,880 sq ft | 17,160 sq ft | 13,730 sq ft |
| 10 | 1,311 | 126,050 sq ft | 63,020 sq ft | 42,010 sq ft | 31,510 sq ft | 25,210 sq ft |
| 12 | 2,093 | 201,250 sq ft | 100,620 sq ft | 67,080 sq ft | 50,310 sq ft | 40,250 sq ft |
Worked example: a 12,000-square-foot parapet roof at 3 inches per hour
| Step | Code basis | Calculation | Result |
|---|---|---|---|
| Total roof flow | Equation 11-1 | 0.0104 x 3 x 12,000 | 374.4 gpm |
| Flow per drain, four equal 3,000 sq ft areas | Equation 11-1 | 0.0104 x 3 x 3,000 | 93.6 gpm each |
| Roof drain check | Section 1106.2 | Manufacturer's rated flow at the expected head must reach 93.6 gpm | Select drain model to suit |
| Vertical leader per drain | Table 1106.3 | 3-inch leader carries 92 gpm, below 93.6 | 4-inch leader (192 gpm) |
| Horizontal branch per drain at 1/8 in/ft | Table 1106.2 | 4-inch carries 115 gpm | 4-inch branch |
| Horizontal main at 1/8 in/ft | Table 1106.2 | 6-inch carries 344 gpm, below 374.4 | 8-inch main (714 gpm) |
| Horizontal main at 1/4 in/ft | Table 1106.2 | 6-inch carries 487 gpm | 6-inch main |
| Secondary drains | Section 1108.3 | Same 3 in/h rate, separate piping, primary flow ignored | Four secondary drains and leaders sized as above, discharging above grade |
| Secondary scupper alternative | Sections 1106.5 and 1108.3 | Opening at least 4 inches high; width at least the circumference of a 4-inch drain (3.14 x 4 in) | 4 x 12.6 inches minimum per area served, subject to the roof ponding check |
The 3-inch leader in this example misses by 1.6 gpm, which is why the leader table should be read against calculated flow rather than rounded rules of thumb.
Where can you explore related guidance?
- Explore all design standards guidance
- Traps, Interceptors and Separators: IPC Chapter 10 Requirements
- Minimum Plumbing Fixture Requirements by Occupancy (IPC Table 403.1)
- Building Drain: Building Code Definition
- Drain: Building Code Definition
Sources and method
Rainfall basis, flow conversion, pipe and leader capacities and secondary drainage rules are taken from the 2021 International Plumbing Code, Chapter 11, Storm Drainage (Sections 1101 through 1108, Tables 1106.2, 1106.3 and 1106.6 and Equation 11-1). The roof-area tables and worked example are calculated by Harth from those capacities; areas are rounded down. These are typical model-code values; verify the rainfall rate, pipe capacities and secondary drainage method against the edition adopted by the authority having jurisdiction, the IBC roof drainage and rain load provisions it adopts, and the roof drain manufacturer's rated flow.
Source links
Frequently asked questions
How do you calculate roof drain size?
Multiply the 100-year, 1-hour rainfall rate by the roof area each drain serves and by 0.0104 to get gallons per minute (IPC Equation 11-1). Check that flow against the drain manufacturer's rated capacity, then pick the leader from Table 1106.3 and the horizontal drain from Table 1106.2 at its slope.
What rainfall rate does the plumbing code use?
IPC Section 1106.1 uses the 100-year, 1-hour rainfall rate from Figures 1106.1(1) through 1106.1(5), which cover the western, central and eastern United States, Alaska and Hawaii, or rates from approved local weather data. Appendix B lists rates for various cities where the jurisdiction adopts it.
How much roof area can a 4-inch leader drain?
A 4-inch vertical leader carries 192 gpm under IPC Table 1106.3. Dividing by 0.0104 and the rainfall rate gives about 9,230 square feet at 2 inches per hour, 6,150 square feet at 3 inches per hour and 4,610 square feet at 4 inches per hour, subject to the drain's own rated flow.
When are overflow drains or scuppers required?
IPC Section 1108.1 requires secondary roof drains or scuppers where roof drains are required and parapets, curbs or other construction would trap water if the primary drains backed up. The secondary system must discharge separately, above grade, in a location occupants or maintenance staff would normally see.
How are emergency overflow scuppers sized?
Scuppers must keep ponding below the depth the roof structure was designed for. IPC Sections 1106.5 and 1108.3 require openings at least 4 inches high and at least as wide as the circumference of the roof drain required for the same area, and flow through the primary system is ignored when sizing them.
Can storm water connect to the sanitary sewer?
Not to a sewer intended for sewage only (IPC Section 1101.3). Where the public sewer is a combined sewer, leaders and storm drains must be trapped individually or with one main trap before the connection (Section 1103.1). Floor drains may not connect to a storm drain (Section 1104.2).
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