Green roofs: when nature reclaims the city

18 October 2021

Soil sealing in towns cities and its impact

It’s no secret that our towns and cities are made of tarmac and concrete. These waterproof materials are widely used to create our roads, car parks, buildings and other constructions. They are nevertheless the cause of many problems, including the undeniable loss of biodiversity in urban and semi-urban areas.

ensemble de toitures vertes
un sol forestier

Soil’s role in biodiversity

Soil fulfils many essential roles and is very sensitive to human activity:

  • It is a source of biodiversity: indeed, it provides a home for many animal species (insects, worms etc.), as well as many species of bacteria and fungi that are essential to the process of decomposition and the recycling of organic matter.
  • It acts as a support for plants (trees, hedges etc.) while providing them with the nutrients they need to grow.
  • It is essential to animal life: the plants that grow in it provide shelter, food and resting places for many species of insects, birds and mammals.
  • It absorbs, filters and stores rainwater: when it rains, the plants absorb some of the water directly. The rest seeps into the soil, which acts as a reservoir on drier days.
  • It absorbs and stores CO2 from the atmosphere by burying organic matter and transforming it using micro-organisms.

Soil sealing modifies and significantly restricts these different roles. Living organisms in the soil are deprived of water, oxygen and food. Plants disappear and animals move to more hospitable areas. Rainwater is no longer absorbed and its runoff causes other problems, such as an increased risk of flooding, lower groundwater levels and the pollution of our waterways.

Green roofs, a solution for recreating permeable areas in towns and cities

Green roofs are an interesting solution for reducing the impact of the artificialisation of soil and helping to maintain biodiversity in towns and cities. They are living environments in their own right, with very specific conditions that help certain plants grow, playing a key role in the development of animal species such as invertebrates. They helps us bring nature back to built-up areas, whilst also providing us with ecosystem services.

imperméabilisation des sols en ville
différents types types de toitures végétalisées

Different types of green roofs

The classification of green roofs is mainly based on the thickness of the substrate installed to serve as a support for the growth of plants.

Extensive green roofs

The depth of the substrate is less than 15cm (usually between 5 and 8cm) and the vegetation is planted. In Belgium, as well as throughout Europe, these are the most common types of roofs. They are easy to install, and also very simple to maintain, as rainwater is all they need to flourish (except in extremely dry areas). They also have the advantage of being low cost and lightweight.

As a rule, the plants found on these roofs are: mosses and lichens, succulents (sedums) and xerophilous plants. They host quite a specific, but limited, biodiversity.

Semi-intensive green roofs

The substrate is between 15 and 30cm and the vegetation is planted by humans. This type of roof is perfect for creating a roof terrace, but it nevertheless requires more maintenance and regular watering.

The plants most often used are similar to those found in a traditional garden. The right combination of ground cover and climbing plants might include lavender, thyme, iris, stipa grass etc.

Intensive green roofs

The substrate is at least 30cm and the plant species are planted by hand. This is a real roof garden: grass, plants, shrubs etc. can all be planted on it. However, it requires a lot of maintenance and the roof must be able to support a heavy load.

How can we reintroduce biodiversity with green roofs?

Although green roofs can be found all over the world, extensive green roofs make up the vast majority. They are easier to implement thanks to their standardised procedures and are therefore very successful.

However, the solution for bringing diversity back to towns and cities seems to be a combination of the different types of existing roofs. Indeed, each one has its own specific characteristics in terms of animal and plant species.

The GROOVES study by the Île-de-France Regional Agency for Biodiversity (ARB)

Between 2017 and 2019, the ÎdF ARB conducted a study of 36 green roofs in Greater Paris. The selection of roofs was based on the distribution of the different types of vegetation identified (extensive, intensive or semi-intensive).

un papillon Belle Dame sur un sedum
une toiture extensive

Generally speaking, the study shows that despite the inhospitable environment in which they evolve (pollution, limited space, heat island etc.), the species present on these roofs show a great capacity for adaptation and colonisation. It also raises some interesting points about the specific features of each roof, such as:

  • Extensive roofs (also known as sedums) support more endangered species and fewer invasive neophytes.

  • Intensive roofs allow a greater diversity of flora.

  • In terms of rainwater retention, it has been shown that intensive roofs are particularly effective.

  • In terms of fauna, it has also been found that intensive roofs were more conducive to the establishment of animal species.

Why is this so important?

In addition to the aesthetic appeal of these green roofs and their value in terms of biodiversity, they are also useful for the many ecosystem services that they provide. Here are some that are particularly noteworthy:

  • A green roof helps retain rainwater more effectively, especially on flat roofs: water is retained and released slowly, thus reducing the risk of flooding due to the saturation of drainage systems.

  • Islands of coolness: plants are particularly effective in cooling the air during hot weather (up to 8°C). In the event of a heatwave, they therefore help to provide cooler areas in urban environments.

  • Pollination: a green roof is a popular destination for insects, including pollinating insects, which are finally finding their place in the city.

  • Pollution: a green roof also helps to reduce CO2 emissions and improve air quality.

  • Complementary habitat: these roofs offer a refuge for various animal species, such as invertebrates, as well as birds, insects etc.

Although green roofs are a way to increase biodiversity in urban and semi-urban areas, they are not the only solution. Find out more in our article dedicated to urban biodiversity.

des arbres sur un toit