How to use it
- Enter the area that drains to the pipe: roof plan area, paving or the whole yard.
- Choose the surface. Roofs shed almost all the rain (C 0.9), lawns much less.
- Enter the design rainfall intensity for your location and the pipe slope.
Worked example
A 150 m² roof (C 0.9) under a 150 L/s per hectare storm gives Q = 0.9 × 150 × 150 ÷ 10,000 = 2.0 L/s. A 15-minute storm then delivers about 1.8 m³ of water, which is a useful size for a soakaway or a rain tank.
Choosing the design rainfall
Short, intense storms govern small catchments such as roofs and yards. Local rainfall data give the intensity for a duration and a return period; 150–250 L/s·ha (54–90 mm/h) is common for 10–15 minute storms in temperate climates. Use a higher value for gutters and downpipes, where an overflow causes damage.
Frequently asked questions
What is the rational method?
Q = C × i × A: the runoff coefficient of the surface times the rainfall intensity times the area. It suits small catchments up to a few hectares.
How much water comes off a roof?
About 0.9 litres for every millimetre of rain on every square metre of roof. A 100 m² roof gives 900 litres from a 10 mm shower.
What size drain pipe do I need for a roof?
It depends on the area, the rainfall and the slope; for a house roof under 200 m² a 110 mm PVC drain at 1–2% usually has spare capacity.