BUILDING & PLANNING · FIELD TOOL
Roof runoff Q = C × i × A ÷ 3600 (L/s). A 200 m² roof in a 75 mm/h storm with C = 0.95 sheds 3.96 L/s: two downpipes at 2.5 L/s each. With 800 mm of rain a year and 85% harvest efficiency, it collects about 129 m³ a year.
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| Downpipes needed | 2 at 2.5 L/s each |
| Rainwater harvest per year | 129.2 m³ |
| Covers demand for | 431 days a year |
| Suggested tank (one month of demand) | 9.0 m³ |
Gutters, outlets and overflows must be sized to your drainage code (e.g. EN 12056-3, IPC Chapter 11 or local rules).
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Design runoff Q = C × i × A ÷ 3600 (L/s)
Downpipes = ⌈Q ÷ capacity of one downpipe⌉
Annual harvest = A × annual rainfall × C × efficiency (L)200 m² roof, C 0.95, 75 mm/h, 2.5 L/s per downpipe, 800 mm/year, 85% efficiency:
| Roof | C |
|---|---|
| Metal, tile, smooth membrane | 0.90–0.95 |
| Flat roof with gravel | 0.80 |
| Green roof | 0.30–0.50 |
Multiply the runoff coefficient by the rainfall intensity (mm/h) and the roof plan area (m²), then divide by 3,600 for litres per second.
Divide the design runoff by the capacity of one downpipe from the maker's tables. Spread them so no gutter run is too long.
Roof area × annual rainfall × runoff coefficient × an efficiency of about 80–90% for first-flush and filter losses. 1 mm on 1 m² is 1 litre.