This is set to become the defining summer weather pattern in Europe: extreme heat followed by violent thunderstorms. A foretaste is already here. Yet solutions have long been available. The shift to renewables and solar-electric buildings will help curb the further accumulation of such extreme weather events, and urban planning has a role to play too. Against the backdrop of the current heatwave and storms, the German Federal Association for Building Greening (BuGG) is calling for the systematic greening of roofs and façades, both regarded as proven but still underused strategies for preparing urban districts for climate change.
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The benefits have long been understood. Green roofs absorb more water during heavy rainfall, preventing streets from flooding, and in hot weather that water evaporates and cools the surroundings. Green roofs and façades also shield the building envelope from direct solar radiation.
Solar power straight from the cool roof
That same solar radiation can be put to excellent use for on-site power generation, for instance to run air conditioning or ventilation systems inside the building. The concept becomes particularly efficient when a green roof is combined with a photovoltaic system: because the vegetation lowers roof temperatures, the solar modules lose less efficiency than they otherwise would in hot conditions, and can deliver more electricity precisely when it is needed for cooling, namely during the day. Supplying these systems with fossil-generated power instead would only worsen the already strained climate balance of inner cities.
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Studies confirm measurable effects
Greening buildings is more than cosmetic. The BuGG points to a model calculation carried out by the German Weather Service for the city of Essen, which shows that air temperatures in areas with a high share of green roofs are around 0.7 kelvin lower than in comparable urban areas without building greenery.
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A study by the University of Natural Resources and Life Sciences in Vienna (BOKU) is more specific still. It found that 850 square metres of façade greening provide an evaporative and oxygen-generating contribution equivalent to 45 air conditioning units, each rated at 3 kW and running eight hours a day. The daily energy demand of such units would amount to roughly one MWh, a quantity that could comfortably be covered by a photovoltaic system on the corresponding roof, with self-consumption also making the system economically viable without any feed-in tariff.
Longer service life for roof waterproofing
Alongside climate protection and power generation, structural considerations also speak in favour of greening. A joint survey by the BuGG and the Central Association of the German Roofing Trade (ZVDH) found that roof waterproofing beneath a vegetation layer lasts longer than 20 years in around 55 percent of cases without major repairs being required, while for solar green roofs, 45 percent still reach this mark.
Unprotected flat roofs perform considerably worse: only 25 percent of them last more than two decades without significant damage, and around 40 percent need refurbishment within 16 years. Plant cover and substrate buffer UV radiation, temperature swings and hail impact, so the roof membrane ages more slowly. (su)
