What impact do green roofs have on urban heat island mitigation?

Green roofs can ease urban heat, but their effect depends on how much roof area is planted, the local climate, and how well the vegetation is maintained.

Plants shade the roof and cool it through evapotranspiration, the release of water vapor from soil and leaves. This can lower roof surface temperatures and reduce the heat a building absorbs. Green roofs may also help keep indoor spaces cooler, which can reduce demand for air conditioning and the waste heat it releases outdoors.

The cooling is strongest at roof level. A single green roof usually has only a modest effect on nearby air temperatures; broader neighborhood benefits are more likely when green roofs cover a substantial area alongside trees and other vegetation. Results also vary with roof design, weather, and access to water, so a green roof is helpful but not a stand-alone fix for the urban heat island effect.