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