Rainwater yield
What your own roof actually collects in a year. The number is usually much larger than people expect.
The area it covers on the ground, not the sloped surface. Pitch does not change how much rain lands on it.
Look up your own. Britain runs from about 600 mm in Essex to over 2,000 in Cumbria.
29,200litres a yearabout 79.9 litres a day averaged out
- First-flush diverter
- 22.5 to 90 litres
- Runoff coefficient
- 0.80
- System efficiency
- 0.90
- People it could supply
- 19 at 4 L a day
An annual average hides the problem. Rain does not arrive evenly, and the gap between refills is what decides the tank size, not the yearly total. A dry British August can run three or four weeks.
This is not drinking water as it lands. Roof runoff carries bird droppings, moss, dust and whatever the roof is made of. It needs filtering and disinfecting first.
How this is worked out
1 mm of rain on 1 square metre is exactly 1 litre, so the yield is area x rainfall x runoff coefficient x system efficiency.
45 m2 of clay tile, 900 mm of rain: 45 x 900 x 0.80 x 0.90 = 29,160 litres a year, about 80 litres a day.
Coefficients: pitched tile 0.75 to 0.85, flat EPDM 0.90 to 0.95, green roof 0.35 to 0.60. The mid-point of each range is used here. System efficiency is taken at 0.90 for the losses in gutters, the diverter and the overflow.
The first-flush diverter is sized at 0.5 to 2 litres per square metre: the low end in clean rural air, the high end near a main road or under trees.
Every figure here comes from Size and build a rainwater system, which gives the reasoning and the sources.