Can you tell us more about your job, why green roofs have become the solution to bring nature into towns and cities, and what benefits they offer?

At Elan, we assist on real estate projects by ensuring that greater attention is paid to the biodiversity and biochemical quality of the buildings. These urban projects may concern housing, the tertiary sector, public facilities, etc., and we also help companies to obtain different labels, including Biodivercity, or to implement a biodiversity strategy. Elan is a specialist in all the green structures that can be associated with urban buildings, such as green walls and roofs favouring biodiversity which is an important new demand expressed by project owners and public institutions. Green roofs have traditionally been used for many years in northern Europe, notably in the Faroe Islands, as a solution to improve thermal comfort thanks to the insulating properties of the layer of earth. Other advantages were rapidly identified and are now widely recognised: • Protecting nature and strengthening biodiversity. Green roofs are very dry environments with poor earth which means that the biodiversity that develops in them is often wild and threatened. For around fifteen years now, researchers have been looking into “wasteland roofs” where the vegetation develops spontaneously which leads to better development of the biodiversity. • Combating the urban heat island effect produced by the solar radiation absorbed by buildings and asphalted surfaces. We even talk about “cool roofs” thanks to the evaporation produced by the plants and the substrate. And, when a green roof is coupled with solar panels, the presence of the plants enhances solar panel efficiency. • Protecting the waterproofing materials of roofs. • Improving air quality. • The aesthetic and landscape benefits ensure better living conditions for the inhabitants. Lastly, and this is an important advantage, a green roof is capable of retaining water and reducing runoff, which avoids the risk of network overload in the event of heavy rainfall. For towns and cities, this solution offers huge potential for alternative management of rainwater: on average 40% of the surface area in urban areas are roofs which could potentially be planted with greenery. Indeed, the origin of the development of green roofs in urban zones is linked to a study launched by the city of Portland to calibrate its water evacuation networks. To limit the considerable investments, the authorities decided to implement an ambitious green roof policy, and this strategy proved to be very conclusive. In France today, the ALUR Act and the Biodiversity Law strengthen the integration of water management in the local urbanism plan by imposing control of the flow of rainwater, which leads to the creation of green roofs in many urban projects.

In partnership with Open Innovation, Linkcity and Bouygues Bâtiment IDF Habitat Social, you are launching an experiment at Challenger alongside the company Natural Grass. What does it entail and what is the aim?

The main difficulty with green roofs is the significant extra weight that they place on the roof. This implies the design of a structure capable of supporting a heavier weight, which really pushes up the costs. To limit this extra weight, one of the main solutions currently applied is to reduce the thickness of the substrate which is detrimental to the development of biodiversity. The substrate is the layer in which the plants take root and obtain the nutrients required for growth. It is made up of 20% organic matter (compost, peat, etc.) and 80% mineral matter (such as volcanic rock). The substrate composition has a significant effect on the natural environments. The use of peat leads to the destruction of natural environments and of exceptional carbon sinks. However, there are now new circular substrates made of recycled materials which are kinder to the natural environment, but for the moment little is known about their qualities. Various different types of roofs exist. Intensive roofs have more than 50 centimetres of substrate which allows the creation of rich and diversified gardens. Semi-intensive roofs, with between 15 and 50 centimetres of substrate, allow good development of the biodiversity and average water retention. Extensive roofs, with a shallow layer of substrate of less than 15 centimetres, offer limited water retention. Around 50% of rainwater is retained at best, and the biodiversity that develops in them is also very limited. Creating a good roof depends on finding a compromise between the thickness of the substrate, the volume of water retained, and the biodiversity that you want to develop on it. Natural Grass proposes a new type of substrate which stores the same volume of water in 30 centimetres as that stored by a classic substrate of 50 centimetres thanks to its circular composition using a lot of wood and cork. The aim of the experiment is to verify these hypotheses. So, Challenger has put in place two roofs with classic substrate and one roof with Natural Grass substrate as well as a control garden, to measure the volume of water retained by each of them. The objective is also to qualify the benefits of this new substrate for the biodiversity. These roofs will all be subject to the same meteorological conditions and we will be able to measure the volume of water runoff and observe the evolution of the plants throughout the four seasons. Other tests could be envisaged concerning the durability of this new substrate, such as measuring the compaction of the substrate over time, or the evolution of the plant life after three years.

How was this experiment organised and what are the expectations within the Group?

After the Matching Up organised on housing, Linkcity, HAS and Natural Grass have strengthened their relations, and Elan is involved as an expert. Linkcity is very interested in this solution notably due to the new constraints of the local urbanism plan and the development of the Biodivercity label. HAS is interested in it to optimise and master construction costs while favouring biodiversity, which is a strong professional theme at Bouygues Construction and corresponds to the very high expectations of public bodies, the company, and architects. Finally, this new substrate is part of a drive to support a circular economy and reduce our impact on the natural environment. It remains to be seen whether this project will last over time and how advantageous it is for biodiversity. The Group is also studying other ideas for green roofs and walls, such as BY OTOPE concrete, one of the finalist projects of the Innovation competition.