A green roof is a flat roof that may or may not be accessible to the public, and can accommodate pedestrian traffic and vegetation. It forms an ecosystem formed of mineral and organic elements (the substrate) and the fauna and flora that develop there. It is extensively employed in some countries. In Switzerland, the city of Basel is the best equipped in the world, with 30% of flat roofs covered with vegetation. Since the beginning of the 2000s, the municipality has made it compulsory for owners of flat roofs of more than 10 m² to plant vegetation.  

Ecosystem services provided by green roofs

Today, more and more professionals in the building and development industry are interested in these mechanisms, and are implementing them. Their multiple ecosystem services appear to be a driver for addressing the challenges of biodiversity decline, climate change and public health. From an environmental point of view, such innovations contribute to the retention of rainwater and the nurturing of biodiversity. By capturing some of the rainwater and delaying the evacuation of rainwater into local water systems, green roofs contribute to improving rainwater management. They can also provide places for social activity, movement and relaxation, and a breeding or feeding place for both bird and insect communities and native microfauna and wild flora. Green roofing is sometimes perceived as a risk to the waterproofing of the building. In fact, a green roof actually protects the building and the waterproofing layer by reducing temperature differences and protecting them from ultraviolet rays, assuming regular maintenance. Green roofs thus have a positive effect on the thermal insulation of the building. They are sometimes integrated into energy renovation projects for blocks of flats, such as the Desnouettes residence in the 15th arrondissement of Paris. The co-owners have chosen to treat the waterproofing of the roofs of the lower buildings with a vegetation-based protection covering an area of almost 600 m². There are also benefits for human health and well-being. In summer, green roofs help to cool buildings and their surroundings by contributing to the humidification of the air through evapotranspiration, the effect of which is enhanced when the roof is watered. The green vegetation cover also reflects the sun’s rays, thus limiting the absorption of heat by the roofs. Throughout the year, the presence of plants, visible from the street or neighbouring buildings, helps to address city dwellers’ growing demand for access to nature. This impact is increased considerably when the roofs are opened to the public or host urban agriculture projects, encouraging social interaction.    

Biodiversity varies according to several roof design parameters

In France, the market is estimated at around 1.5 million m² per year. The majority of green roofs are “extensive”; i.e. their substrate depth is less than 15 cm. Their success is based on their light weight, ease of installation, low cost and low maintenance requirements. The depth of the substrate increases for so-called “semi-intensive” roofs (15-30 cm) and “intensive” roofs (over 30 cm). A fourth category, the “wildroof”, is a single substrate of variable depth, with no planting. The benefits provided by these different types of roofs can vary, as suggested by the GROOVES (Green ROOfs Verified Ecosystem Services) study initiated by the Île-de-France’s Agence régionale de la biodiversité. 36 roofs in the Ile-de-France region of different types were analysed from 2017 to 2019 by means of plant and invertebrate inventories and substrate sampling. It appears that flora richness increases with substrate thickness up to 25 cm, and that pollinator diversity continues to increase beyond this threshold. Although hosting less diversity in species, “extensive” and “wildroof” roofs have a specific composition that differs from other roof types. But other design parameters cause variation in biodiversity. The composition of the substrate also plays an important role: a clay content of about 10% and sand content of about 60% provides maximum richness of plant life. With regard to water retention, roofs capable of regulating once-in-ten-year rainfall have “agricultural land”-type substrates, and are close to 30 cm deep. Building height is also a factor. According to the study, the effect on plant richness appears to be positive up to about 10 metres in height (i.e. 3 storeys), but does not increase thereafter. Like this study, feedback is increasing, providing valuable lessons for designers. The “Vegetation in existing buildings” guide by the Agence Qualité Construction and Ekopolis lists, with examples, twelve lessons based on the main regulatory references, and draws attention to the main areas of caution to be considered during the design, implementation and maintenance phases.  

Integrating green roof systems into a systems-driven approach

The number of projects is increasing, with some of them on a large scale – such as Biotope, a building developed by Linkcity Nord-Est, designed by the architects Henning Larsen Architects (Denmark) and Keurk Architecture (Lille) and built by Bouygues Bâtiment Nord-Est. The headquarters of the European Metropolis of Lille, this Biodivercity-labelled building includes more than 3,000 m² of green terraces. In total, 150 trees, 600 shrubs and 15,000 perennials have been planted. In addition to the impressive amount of green roof space, the innovation also lies in the pathway chosen for the project. Ecologists, nurserymen and landscapers worked together to identify the species best suited to this very specific situation. The study also took user well-being into account by working on how the vegetated spaces would be perceived by the five senses: the rustling of the leaves, the colours of the flowers, the smell of the herbs, and spaces for edible fruits.   While the increasing demand for green roofs is to be welcomed, particular attention needs be paid to the way they are produced: the use of agricultural soil has impacts on fertile land and on the sustainability of the soils concerned. It is therefore better to use recycled substrates, such as excavated soil from construction sites. Furthermore, green roofs are part of the toolkit for providing a better solution to the challenges of declining biodiversity and climate change in urban planning and building construction. However, these measures alone cannot provide an answer to these challenges, and need to be integrated into a more systemic approach. As ecologists Marc Barra and Hemminki Johan point out in a note from the Institut Paris Région, “the greening of buildings must not be used as a form of greenwashing for development projects that contribute to the artificialisation of the soil”.