In short: a 5,000 sq ft flat roof with one drain, sized to a 4 in/hr storm, needs about 208 GPM of drainage capacity — more than a 4-inch drain can handle, so it needs a 5-inch drain (or a second drain to split the load). Bigger roofs, heavier design storms, or lower-drainage roof types like gravel and green roofs all shift the required size — use the calculator above for your numbers.
These figures are planning aids based on published sizing methods and code tables. For a permitted design, confirm sizing and the number of drains with a licensed plumbing engineer against your local code.
How to calculate roof drain size
Roof drains are sized with the Rational Method, the same runoff formula used for storm sewers and culverts, applied to one roof instead of a watershed. It converts roof area and a design rainfall rate into a flow the drain must carry, then you pick the smallest standard drain size that can carry it:
This calculator converts that flow to gallons per minute, then compares it against the fixed hydraulic capacity of each standard drain size — the same figures behind IPC Table 1106.2 — and returns the smallest size with enough headroom.
Worked example. Take a 5,000 sq ft flat membrane roof (C = 1.0) with one drain, designed for a 4 in/hr storm. Required flow = 1.0 × 4 × 5,000 × 0.0103896 = 207.8 GPM. A 4-inch drain caps out around 191 GPM — not quite enough — so the next size up, a 5-inch drain rated near 359 GPM, is the answer.
Where the 0.0103896 comes from. It converts cubic feet of rain per hour into gallons per minute: 1 ft³ = 7.480519 US gallons, and there are 720 minutes in the 12-hour ratio between inches and feet times 60 minutes per hour. The calculator carries the exact fraction rather than a rounded constant.
Why area, rainfall rate and roof type change the answer
Roof area per drain. Flow scales directly with area — double the tributary area on a drain and you double the flow it must carry. Splitting a large roof across more drains keeps each one's required flow, and therefore its required size, smaller and more standard.
Design rainfall intensity. This is the rate a storm dumps water on the roof, not the total rainfall. A short, intense downpour can demand far more instantaneous capacity than a long, gentle rain with the same total inches. Codes require sizing to a specific return-period storm (commonly 100-year, 60-minute) for your location, not a guess.
Roof runoff coefficient. A hard membrane or metal roof sheds essentially all the rain that lands on it (C near 1.0). A gravel-ballasted roof absorbs and slows some water; a vegetated green roof retains noticeably more, lowering C and, in turn, the required drain size for the same area and rainfall.
Roof drain capacity by size
The fixed hydraulic capacity this calculator uses for each drain size, and the maximum roof area a single drain of that size can serve at a few common design rainfall rates. Larger design rates shrink the area a drain can safely serve.
| Drain size | Capacity (GPM) | Max area @ 2 in/hr | Max area @ 4 in/hr | Max area @ 6 in/hr |
|---|---|---|---|---|
| 2 in | 30 | 1,440 ft² | 720 ft² | 480 ft² |
| 3 in | 91 | 4,400 ft² | 2,200 ft² | 1,467 ft² |
| 4 in | 191 | 9,200 ft² | 4,600 ft² | 3,067 ft² |
| 5 in (default example) | 359 | 17,300 ft² | 8,650 ft² | 5,767 ft² |
| 6 in | 561 | 27,000 ft² | 13,500 ft² | 9,000 ft² |
| 8 in | 1,205 | 58,000 ft² | 29,000 ft² | 19,333 ft² |
Worked examples
5,000 sq ft office roof, 1 drain, 4 in/hr
5 in
Required flow is about 208 GPM. A 4-inch drain (~191 GPM) falls just short, so the roof needs a 5-inch drain, rated near 359 GPM — about 42% headroom.
10,000 sq ft metal roof, 2 drains, 6 in/hr
5 in
Each drain serves 5,000 sq ft with C = 0.95. Required flow per drain is about 296 GPM, again calling for a 5-inch drain — about 18% headroom this time.
1,200 sq ft residential roof, 1 drain, 3 in/hr
3 in
Required flow is about 37 GPM. A 2-inch drain (~30 GPM) is too small, so a 3-inch drain, rated near 91 GPM, is the right size with plenty of margin.
What else affects a real design
Strainers and domes. Debris strainers and dome covers reduce a drain's effective throughput versus its bare-pipe rating, sometimes significantly if leaves or grit accumulate. Manufacturer capacity ratings, not the pipe size alone, should govern a final design.
Roof slope and ponding. Flatter roofs and low points away from drains slow water to the outlet and can create standing water even when the drain itself has capacity. Roof slope and drain placement are structural and architectural decisions alongside the hydraulic sizing here.
Secondary/overflow drainage. Codes require a separate overflow system — scuppers, overflow drains, or an overflow rim — sized to the same rainfall rate as the primary system, in case primary drains clog. This calculator sizes the primary system only.
Leader and horizontal pipe sizing. The drain outlet is one link in the chain — the vertical leader and any horizontal storm piping downstream need their own sizing check. See the downspout size calculator and storm drain size calculator.
Frequently asked questions
What size roof drain do I need?
It depends on the roof area each drain serves, your design rainfall intensity and the roof's runoff coefficient. Using the Rational Method (Q = C·i·A), a 2-inch drain handles roughly 30 GPM, a 3-inch about 91 GPM, a 4-inch about 191 GPM, a 5-inch about 359 GPM, a 6-inch about 561 GPM and an 8-inch about 1,205 GPM. Enter your numbers in the calculator above for an exact size.
How many roof drains does a flat roof need?
Enough that no single drain's tributary area exceeds its rated capacity at your design rainfall rate, and building codes also require a secondary (overflow) means of drainage in addition to the primary drains. Larger roofs are typically split across several primary drains so structural loads and pipe sizes stay reasonable and a single blockage cannot flood the whole roof.
What rainfall rate should I design for?
Plumbing codes size roof drainage to a local rainfall intensity for a specific return period, commonly a 100-year, 60-minute storm. Look up your site's rate from NOAA Atlas 14 or the value your local code references. Four inches per hour is a common placeholder for a rough estimate in much of the continental United States, but always confirm the code-mandated rate for your jurisdiction.
Do I need a secondary or overflow drain?
Yes. The International Plumbing Code and Uniform Plumbing Code both require a secondary means of roof drainage, sized to the same design rainfall rate as the primary system, using overflow drains, scuppers or an overflow rim height, in case the primary drains become blocked.
Does roof material affect drain sizing?
Yes, to a smaller degree than area or rainfall rate. A hard, impervious surface like a membrane or metal roof sheds nearly all the rain that falls on it, with a runoff coefficient near 1.0. A gravel-ballasted or vegetated green roof retains some water and releases it more slowly, which lowers the coefficient used in the sizing formula. That is the roof type setting in the calculator above.
Sources & references
- International Code Council. International Plumbing Code (IPC), Chapter 11 — Storm Drainage, Table 1106.2, Size of Roof Drains.
- IAPMO. Uniform Plumbing Code (UPC), Table 1101.1, Roof Drains.
- SMACNA. Architectural Sheet Metal Manual, Chapter 3 — Roof Drainage Systems.
- National Oceanic and Atmospheric Administration. NOAA Atlas 14, Precipitation-Frequency Atlas of the United States.
- Kuichling, E. (1889). The Relation Between the Rainfall and the Discharge of Sewers in Populous Districts. Transactions of the ASCE.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)