Harvesting rainwater from rooftops could help cities stay cool and cut the number of heatwave days.

The University of Manchester has used AI-assisted simulations to show that roof-based rainwater cooling could reduce energy demand, lower urban temperatures and help cities adapt to intense heatwaves.

Rainwater collected from rooftops would be used to spray roofs during hot weather. The cooler roofs transfer less heat into buildings, while lower air conditioning use means less waste heat was released. Together, these effects helped reduce urban temperatures, cutting the number of heatwave days overall and lessening the intensity of extreme heat events.

The study suggests that when the sprinklers switched on, for example, when the roof reached a certain temperature, had a greater impact than either the size of the tank or the amount of water applied. The researchers also found that bigger is not always better. Very large tanks delivered only modest additional reductions in energy use and extreme heat, while applying extra water did not always lead to more cooling because some of it remained on the roof instead of evaporating.

Lead author Dr Zhonghua Zheng, co-lead for environmental data science and AI at Manchester Environmental Research Institute (MERI) said: "Our study shows that harvesting rainwater from roofs and using it strategically for cooling could provide a practical way to reduce both energy demand and urban temperatures. What is particularly encouraging is that the benefits become even greater during hotter years, suggesting this approach could become increasingly important as the climate continues to warm.”

The researchers say the approach could help local authorities and urban planners evaluate how rainwater-based cooling systems might work in their own regions while balancing practical considerations such as cost, water availability and local regulations.

Junjie Yu, PhD researcher at The University of Manchester, added: “The rainwater tank also provides an additional co-benefit on reducing the extreme urban runoff. This approach exemplifies a ‘natural solution to natural challenges’, in which rainwater serves as a natural resource to mitigate both thermal stress and hydrological extremes.”

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