According to Deutsche Welle (DW), researchers in Leipzig, Germany, are studying how rooftops covered with plants can help reduce heat while also improving the way cities manage rainwater.
Green roofs use vegetation and growing material instead of conventional bare roofing surfaces. They can absorb part of the rainfall that would otherwise flow directly into drainage systems and can also cool their surroundings as water evaporates from the soil and plants.

Researchers at Germany’s Helmholtz Centre for Environmental Research (UFZ) have been testing different types of green roofs to understand how effectively they perform during hot and wet weather.

A UFZ-led study published in Science of the Total Environment compared a wetland green roof, an extensive green roof and a conventional gravel roof over a period of 31 months.
According to the study, the wetland green roof retained around 82.4% of stormwater, significantly higher than the 63.4% recorded for an extensive green roof and 29.9% for a conventional gravel roof.
The researchers also found that the wetland roof generally provided stronger cooling during summer, suggesting that roof design and the availability of water can make a significant difference to performance.

Dry weather can reduce cooling benefits

Scientists, however, caution that green roofs are not equally effective under all conditions.

According to UFZ, prolonged dry weather can reduce their cooling effect because the soil loses moisture. With less water available for evaporation and plant transpiration, the roof becomes less effective at lowering temperatures.

This creates a particular challenge during extended heatwaves, when cities need cooling the most.Researchers say irrigation could therefore play an important role in maintaining the benefits of green roofs during dry periods.

A separate UFZ study examined the use of intelligent irrigation systems that combine weather forecasts with information about water conditions on the roof. Researchers found that smarter irrigation can help maintain cooling while using available water more efficiently.

A tool for hotter cities

The findings suggest that green roofs could help cities tackle two climate-related challenges at the same time. During heavy rain, they can capture and temporarily store water, reducing pressure on drainage systems. During hot weather, moisture stored in the roof can contribute to cooling through evaporation and vegetation.

According to DW, such nature-based solutions are becoming increasingly important as cities search for ways to protect residents from rising temperatures and more extreme weather.
Scientists do not consider green roofs a complete solution to urban heat. However, the research indicates that, when properly designed and supplied with sufficient water, they could become an important part of wider climate-adaptation strategies.

For densely populated cities with limited space for new parks, transforming otherwise unused rooftops into green spaces could provide an additional way to reduce heat, manage rainfall and improve resilience as temperatures continue to rise.