Rainwater harvesting
Litres collected by a roof or ground surface from its area, the rainfall, the material and the first-flush diversion.
Tool
Tools › Water and food
How it is calculated
1 mm of rain on 1 m² is 1 L. Litres = area (m²) × (coefficient × rainfall (mm) − diversion (mm) × rain events). The diversion is subtracted from the runoff once per rain event; if it is larger than the runoff, nothing is collected.
Annual example. A 50 m² (in plan) metal roof, 600 mm per year, 30 rain events and a 1 mm diversion: 50 × (0.9 × 600 − 1 × 30) = 50 × (540 − 30) = 25 500 L = 25.5 m³, about 25 500 ÷ 365 = 69.9 L per day on average. 50 × 30 = 1500 L are diverted.
Single-event example. A 100 m² tiled roof and a 20 mm storm: 100 × (0.8 × 20 − 1) = 1500 L.
Keep in mind
- Coefficients are approximate and depend on slope, rain intensity and gutter losses: metal sheet 0.8-0.95; tiles 0.6-0.9; smooth membrane 0.8-0.9; gravel 0.5-0.6; green roof 0.1-0.5; compacted ground 0.1-0.5 (DIN 1989-1; Gould and Nissen-Petersen; Pacey and Cullis).
- The annual average says nothing about the dry season: size the tank with month-by-month rainfall and the consumption of the months without rain (tool “Tank capacity”).
- A small gutter overflows in heavy rain, losing water that this calculation counts as collected.
- Rainwater is not safe to drink by default: even from a clean roof, filter and disinfect it (chlorine, SODIS or boiling) before drinking. Do not collect from roofs with lead paint, tar or treated timber, and avoid old, deteriorated fibre-cement roofs (asbestos).
See also
Related articles
- Chemical disinfection of water with chlorine and iodine — Bleach, calcium hypochlorite and iodine doses per liter and per tank, contact times, residual chlorine, limits and safe handling.
- Slow sand filter and charcoal biofilter — Household (biosand) and community slow sand filter, with biological layer, sizing and cleaning; granular charcoal bed.