BAU-R-2: Green roofs in cities | Umweltbundesamt

BAU-R-2: Green roofs in cities

Resilient roof and façade greenings can have many beneficial effects – both on the climate in an urban district and on the building itself, also on rain retention, the air quality and on biological diversity – thus mitigating local impacts of climate change. Densely built-up and ‘sealed’ cities depend on this, and they are in a position to provide targeted funding with the purpose of counteracting the impacts of climate change.

Green roofs in cities – good for adaptation to climate change and for biodiversity

The greening of urban areas is not restricted to parks and park-like zones or to the greening of roadsides and private gardens. Façades and rooftops of buildings also provide a lot of space for greening. For façades there is a range of greening options, from climbing plants to densely planted vertical gardens. Building statics permitting, roofs can be planted with a great variety of largely self-maintaining vegetation, ranging from mosses, herbs and grasses to dense planting schemes with crop species, shrubs or trees. Resilient greening schemes on roofs and façades can have multiple effects which are capable of mitigating adverse impacts of climate change thus benefiting individual buildings and properties. In urban areas these beneficial effects can also – at least partly – be transmitted to the environment of the buildings concerned.

By reducing the influx of sunlight and the fact that plants evaporate water through their leaves, greened roofs and façades effectively cool down buildings as well as their surrounding air¹⁴⁵. On one hand, this is beneficial to users of buildings on hot summer days or during periods of great heat. On the other hand, the greening of buildings – thanks to the absorption of irradiated energy and transpiration – reduces urban heat island effects, especially in densely populated and densely built-up towns and cities.¹⁴⁶ NB: The evaporation effect and any associated urban climate effects can only materialise provided the substrate of ‘greened’ roof vegetation can absorb and retain moisture. This has to be taken into account especially, where greening is done extensively, because as a rule, this type of greening is maintenance-free; however, in hot summer months it may require watering. The greening of façades – in addition to the general urban climate – is an essential component in the endeavour to achieve a beneficial effect on the immediate environment at street level. Another beneficial consequence of greening is its air-quality effect. The vegetation surface slows down the air current so that particulate matter and pollutants can precipitate more easily.^{147, 147}

Green roofs also serve a useful purpose in downpours. Depending on their design and technical layout, green roofs can potentially store considerable amounts of rainwater for gradual evaporation once precipitation stops. With a view to potentially frequent and more intensive heavy rainfall, greening systems provide a buffer which absorbs rainwater like a sponge thus slowing down the water cycle. In this way, green roofs can contribute to lightening the burden on drainage systems for properties in entire districts as well as reducing water build-up and flooding-related damage.

Furthermore, greened roofs and façades contribute to the protection of components of buildings and entire modules. For example, they can reduce damage from heavy rain and hailstones to façades and roofs, and they slow down or prevent the weathering of roof seals. As a ‘bonus’, green roofs and façades also provide habitats for flora and fauna. Such greening systems provide nesting and foraging spaces for birds, wild bees, butterflies and ground beetles thus increasing biodiversity in urban areas.

For this indicator, 15 cities with population sizes in excess of 500,000 were evaluated in combining satellite images in a 10 metre resolution with data from 3D building models. To put it in a simplified manner – by using satellite images, the vegetation areas were located within cities and tailored to buildings with flat or monopitch roofs, with the aid of 3D building models. It was not possible to consider any greened basement car parks on the basis of these data. In order to avoid misinterpretations, the evaluation included – in view of the spatial resolution of the satellite images – only roofs of a minimum size in excess of 400 m² and a minimum greening proportion of 20 %. This was to avoid the mistaken inclusion of, for instance, tall trees projecting above a flat roof space, in the classification. Restricting this process to the inclusion of flat and monopitch roof sizes in excess of 400 m² meant that just about 60 % of flat and monopitch roof areas or indeed 30 % of the entire roof area were examined in the 15 cities covered by this process.

Regarding the roof area examined in 15 cities with an average population density in excess of 500,000, approximately 10 % was classified as greened (equivalent to an area of approximately 0.8 m² per inhabitant). The range amounted to approximately 5 % to 19 % (equivalent to an area of 0.3 to 1.3 m² per inhabitant). Regardless of the data-technical limitations mentioned, the baseline situation from 2020 demonstrates that to date only a comparatively small part of flat and monopitch roofs have been greened.

In order to increase the proportion of greened roof areas, many cities subsidise the greening of roofs and façades either directly or indirectly, for instance by means of reducing the sewage fees charged to compliant proprietors of buildings with greened roofs. Besides, many town or city administrations lay down specifications on the greening of buildings in their building / development planning. The objective is to drive forward a successive expansion of greened roof spaces thus opening up the potential of multifarious beneficial impacts on a particular town or city, also in terms of adaptation to climate change.

Apart from individual municipalities, the greening of buildings is also incentivised at the level of the Federal and Länder (Federal States) governments. As far as Federal Government is concerned, the programmes for the promotion of urban development support both the subsidisation of roof and façade greenings and various funding programmes offered by individual departments in order to facilitate for instance the adaptation to climate change or to improve the efficiency of buildings. For its part, the Federal Government has adopted an objective for real estate under its own remit, to explore the potential for and – where appropriate – implement more greening systems in future development projects. In doing so, the Federal Government has also adopted the important role model function for other stakeholders in its capacity as developer and proprietor.¹⁴⁸

^{145 - Sieker H., Steyer R., Büter B., Leßmann D., Tils R. von, Becker C., Hübner S. 2019: Untersuchung der Potentiale für die Nutzung von Regenwasser zur Verdunstungskühlung in Städten. Umweltbundesamt (Hg.). Texte, 111/2019, Desdsau-Roßlau, 112 pp. https://www.umweltbundesamt.de/sites/default/files/medien/1410/publikationen/2019-09-16_texte_111-2019_verdunstungskuehlung.pdf.}

^{146 - Mann G., Fischer B., Fischer S., Gohlke R., Mollenhauer F., Wolff F., Köhler M., Pfoser N. 2022: Förderrichtlinie Dach- und Fassadenbegrünung – Machbarkeitsstudie. Bundesinstitut für Bau-, Stadt- und Raumforschung am Bundesamt für Bauordnung und Raumwesen (Hg.). 105 pp. https://www.bbsr.bund.de/BBSR/DE/forschung/programme/weitere/gruen-in-der-stadt/machbarkeitsstudie-gebaeudegruen/endbericht.pdf.}

^{147 - BBSR (Hg.) 2015: Überflutungs- und Hitzevorsorge durch die Stadtentwicklung – Strategien und Maßnahmen zum Regenwassermanagement gegen urbane Sturzfluten und überhitzte Städte. Ergebnisbericht der fallstudiengestützten Expertise „Klimaanpassungsstrategien zur Überflutungsvorsorge verschiedener Siedlungstypen als kommunale Gemeinschaftsaufgabe“. Bonn: 37, 43. https://www.bbsr.bund.de/BBSR/DE/veroeffentlichungen/sonderveroeffentlichungen/2015/UeberflutungHitzeVorsorge.html.
147 - Tröltzsch J., Görlach B., Lückge H., Peter M., Sartorius C. 2012: Kosten und Nutzen von Anpassungsmaßnahmen an den Klimawandel – Analyse von 28 Anpassungsmaßnahmen in Deutschland. Climate Change 10/2012, Dessau-Roßlau, 221 pp. https://www.umweltbundesamt.de/publikationen/kosten-nutzen-von-anpassungsmassnahmen-an-den.}

^{148 - BMUB – Bundesministerium für Umwelt, Naturschutz, Bau und Reaktorsicherheit(Hg.) 2017: Weißbuch Stadtgrün – Grün in der Stadt – Für eine lebenswerte Zukunft. Berlin: 42ff. https://www.bmwsb.bund.de/SharedDocs/downloads/Webs/BMWSB/DE/publikationen/wohnen/weissbuch-stadtgruen.html.}