How much free water falls on your roof each year?
Rainwater harvesting collects precipitation from roof surfaces, storing it for irrigation, toilet flushing, or (with treatment) potable use. The collection formula is simple: Volume (L) = Roof Area (m²) × Rainfall (mm) × Runoff Coefficient. A 100 m² roof in an area receiving 800 mm of rain annually collects about 72,000 liters — enough for a family's garden irrigation needs year-round. Metal and tile roofs have the highest runoff coefficients (0.85-0.95), while flat gravel roofs lose 40% to absorption. Tank sizing typically aims for 1-3 months of storage to bridge dry spells. First-flush diverters discard the initial 1-2mm of rain that washes bird droppings and dust off the roof. In many countries, rainwater harvesting reduces stormwater runoff (decreasing flood risk and sewer overflows), lowers water bills by 40-50%, and reduces the energy used for municipal water treatment and pumping. Australia leads adoption, with 34% of households having rainwater tanks.