Why are flat roof rainwater calculations needed?

Rainwater calculations ensure that drainage schemes allow rainfall to drain from a roof within a set period of time. They take into account factors such as the location of the building, rainfall rates, roof pitch and roof area.

The rainwater calculations are used to determine the correct number and flow rate of outlets, leaf guards, downpipes and other components required for the specified drainage scheme. They are also used to calculate the capacity required for rainwater harvesting tanks.

Do I need a rainwater calculation?

Building Regulations state that rainwater must drain from a roof within a certain time to ensure that the roof does not contain more water than the structure can withstand.

Without rainwater calculations, it is easy to under-specify the roof drainage system by not installing enough drainage outlets or by using components with the wrong flow rate. This could lead to damage to the building structure due to the excess weight of the standing water or by causing leaks. It is also possible to over-specify the roof drainage system by installing more outlets than are required, leading to additional costs in labour and components.

Where can I get rainwater calculations from?

Reputable manufacturers’ technical departments will be able to carry out rainwater flow rate calculations on your behalf and provide you with a scheme in accordance with the UK design standard BS EN 12056 : 3-2000.

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