Green roofs for the urban climate

Reducing roofs’ surface temperature can play a key role in improving ambient temperatures in urban areas. Replacing conventional roofs with green ones, which are a lot less hot in summer months, is one way to do it. Because of their evaporation capacity, green roofs play a key role in the urban climate.

Retention roofs drive evaporation performance

Urban climates deteriorate from year to year, but the heat island effect is increasing at the same time. Greening roofs is a major strategy towards mitigating the impact of heat islands and compensating for the lack of green spaces in towns and cities at the same time. Greened roofs can turn the hottest urban spaces into the coolest ones in the truest sense of the word.

Numerous thermal images of urban areas show that roofs are the hottest places. In the summer months, conventional roofing materials can reach temperatures of up to 80°C. The sun’s rays are absorbed, stored and only released back slowly into the air at night through convection. Therefore, urban roofs store a lot of heat and intensify the urban heat island effect all the more. This is where green roofs can make a difference because they cut the temperature due to evaporative cooling.

To promote evaporation, there must be a sufficient supply of water, as the green roof’s performance depends on the substrate’s water content.

The study by Gössner et al. (2021) compared various roof systems with each other and found that the retention roof had the highest substrate moisture with peak figures of over 35%. Soil moisture only fell below 10% on just a few hot summer days when the water storage layer on the retention roof was depleted. This high level of moisture on retention roofs is required for greater evaporation capacity.

Evaporative cooling improves the urban microclimate

By changing the local and micro climates, green roofs temper the urban heat island effect. This is due to the phenomenon of evapotranspiration. The vegetation on green roofs absorbs water through its roots and releases it into the air as water vapour via the leaves. The plants use the energy gained from the sun’s rays to evaporate water. Therefore, the energy required for evaporation is not applied to increase the air temperature or make the ground hotter.

The resulting evaporation processes have a direct ambient impact. As a result, the green roof directly influences urban weather or temperatures in small urban areas by sustainably reducing heat and improving the microclimate.

A study by Gössner et al. (2021) carried out in Germany showed that the average drop in air temperatures due to green roofs was 1.34°C. What’s more, the retention roof in the study had the lowest substrate temperature of 22°C at midday in June.

Fig. 1: Evaporative cooling due to green roofs; our own graphic 2022

Green roofs improve air quality

Alongside a reduction in heat islands, green roofs also improve the air quality in urban spaces. This is because the environment cools down due to the absorption of the sun’s rays and evaporation.

According to Nguyen et al. (2022), green roofs have a positive impact on air quality in not just one but two ways. Firstly, by cutting carbon emissions due to the energy saved in the building. And secondly, because of the reduction in greenhouse gases via the vegetation itself. The ability to cut air pollution varies depending on the solution concerned. It was found that herbs, spices and aromatic plants performed better than grasses.

Calculations by the Bundesverband GebäudeGrün e.V. show that a roof planted with sedum can absorb 1.4 – 9.8 g/m² of particulate matter annually. Their study also revealed that a 19 m² green roof can absorb up to 190 g of particulate matter per year (Bundesverband GebäudeGrün e.V. 2012).

Bundesverband GebäudeGrün e.V. (2012): Feinstaubbindungsvermögen der für Bauwerksbegrünung typischen Pflanzen, p. 28.

Gössner, Dominik; Mohri, Milena; Krespach, Justine Jasmin (2021): Evapotranspiration Measurements and Assessment of Driving Factors: A Comparison of Different Green Roof Systems during Summer in Germany. In: Land 10 (12), p. 1334. Retrieved here: Link

Nguyen, Cuong Ngoc; Muttil, Nitin; Tariq, Muhammad Atiq Ur Rehman; Ng, Anne W. M. (2022): Quantifying the Benefits and Ecosystem Services Provided by Green Roofs—A Review. In: Water 14 (1), p. 68. Retrieved here: Link