Reduction of the urban cooling effects of an intensive green roof due to vegetation damage

Green roofs are increasingly being utilised as urban climate adaptation measures given good evidence of their passive cooling effects. Shading of conventional building surfaces, higher albedo and latent cooling from evapotranspiration mean that vegetated surfaces can be used to significantly lower surface and air temperatures in cities. The extent to which cooling properties may be affected by poor maintenance, green roof damage and the general health of the vegetation is not as well known. This study investigates the ability of an intensive green roof in Manchester, UK, to cool the air above it relative to an adjacent conventional, concrete roof. The monthly median air temperature was found to be 1.06 °C lower at 300 mm over the green roof. Diurnal trends were apparent, with strongest cooling of 1.58 °C, occurring at night when Urban Heat Islands are most common. A section of green roof was damaged by mismanagement in a drought period, which allowed an investigation into the impairment to the cooling effect when large areas of bare substrate are present. Daytime temperatures were higher over this section of the roof than over an adjacent healthy green roof, being a maximum 0.63 °C warmer than the bare roof on average, in the late morning. Cooling still occurred at night, albeit with a lower maximum average relative cooling of 0.78 °C. Implications of the spatial influence of green roof cooling are discussed along with the need to undertake careful maintenance and irrigation of the roofs for the benefits to be maintained.

► Green roofs cool overlying air masses relative to a conventional roof.
► The cooling effect is strongest at night with an average difference of 1.58 °C.
► Cooling effects are reduced when the green roof vegetation is damaged.