The average British terraced roof (30m²) receives between 19,000-55,000 litres of rainfall each year, though the total volume depends on the annual rainfall depth. 

Capturing this runoff holds the potential to store large volumes of non-potable water, reducing the burden on existing drainage networks and potentially decreasing flooding. 

The degree to which flooding can be reduced depends on the rainfall amount and pattern, the tank size and roof area. Even a small tank (210 litres, which is roughly equivalent to the size of a small bath) can significantly decrease the impact of significant rainfall events. A tank of this size can hold 7 mm of rainfall from an average terraced roof. This may not seem like a lot, but it can substantially reduce the impact of a storm. 

To put this in context, in the English city of Hull, a storm that deposits 22.3 mm of rainfall is sufficient to cause flooding. This amount of rain typically falls once every ten years. If we can hold back 7 mm of that rain in many household water butts, then the amount that is required to cause a flood rises to 28.6 mm. A storm that results in this amount of rainfall occurs only once every 30 years. 

However, this flood reduction strategy is only effective if the whole community acts together. Large numbers of householders need to install water butts for it to work. It is also not a stand-alone solution in its own right: but combined with sustainable drainage systems and other natural flood management measures, water butts can make a small but meaningful contribution. 

The tanks also need to be empty when substantial amounts of rain fall.  There are several solutions to this problem: tanks can be designed so that they slowly leak or they can be connected to the internet so that they can be emptied automatically before a storm. Another alternative is to set up an alerts system that would send a text to water butt owners advising them to empty their tank a certain number of hours before a forecast rainfall event.