Roof Drain Sizing Calculator
Size roof drains and leaders based on area and rainfall
Roof Information
Rainfall Data
Use local 100-year, 1-hour rainfall data
Drain Configuration
Typical maximum: 50-75 feet
Roof Drainage Requirements
Number of Drains
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Drain Size (each)
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Total Roof Area
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Adjusted Area
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Total Flow Rate
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Flow per Drain
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Leader Size
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Overflow Drains
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Note: Roof drain strainer dome should have 1.5-2x the area of the leader pipe.
Drain Capacity (at 4 in/hr)
| Drain/Leader Size | Max Area (sq ft) | GPM |
|---|---|---|
| 2" | 720 | 23 |
| 3" | 2,200 | 67 |
| 4" | 4,600 | 144 |
| 5" | 8,650 | 261 |
| 6" | 13,500 | 424 |
| 8" | 29,000 | 913 |
Rainfall Adjustment
Adjusted Area = Actual Area × (Design Rainfall / 4)
Code tables based on 4 in/hr; adjust for local rates
Example: 5,000 sq ft at 6 in/hr = 5,000 × (6/4) = 7,500 sq ft equivalent
Horizontal Storm Drain Sizing (1/4" slope)
| Pipe Size | 1 in/hr | 2 in/hr | 4 in/hr | 6 in/hr | GPM |
|---|---|---|---|---|---|
| 3" | 3,288 sf | 1,644 sf | 822 sf | 548 sf | 26 |
| 4" | 7,520 sf | 3,760 sf | 1,880 sf | 1,253 sf | 59 |
| 5" | 13,360 sf | 6,680 sf | 3,340 sf | 2,227 sf | 105 |
| 6" | 21,400 sf | 10,700 sf | 5,350 sf | 3,567 sf | 168 |
| 8" | 46,000 sf | 23,000 sf | 11,500 sf | 7,667 sf | 362 |
| 10" | 82,800 sf | 41,400 sf | 20,700 sf | 13,800 sf | 651 |