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Green roofs for “resilient” cities

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If Paris is legendary symbolized as the most beautiful city in the world, the City of Light is far from being the greenest capital in the world, and does not even appear in the top ten most ecological cities in France. To work in favor of “ecological resilience” (to regain normal functioning after disturbance), the law has been promoting, since the beginning of the 2010s, the installation of ecological roofs.

Paris, far from the greenest major cities in the world

Paris, a green capital with its beautiful historic parks? Not really, compared to many world capitals, and even big French cities. According to a ranking of the greenest cities in France carried out by The United Nations Environment Program (UNEP) in 2014, Paris is not even in the top ten places. Angers, Nantes and Limoges occupy the podium, followed by Lyon in fourth place. The north-east of France wins the rest of the ranking, with in particular Metz, Reims, Nancy and Strasbourg.

Globally, Iceland is the leader in terms of “eco-sustainability”, as the greenest country in the world. Its capital Rejkavik draws its energy only from electricity produced by water (hydroelectricity) and by the heat of the earth's crust (geothermal energy). Copenhagen stands out for its network of offshore wind turbines and the behavior of its inhabitants, who use bicycles much more than cars; the capital of Denmark has also acquired the most ecological and modern metro in Europe.

90% of Vancouver's energy needs in Canada are covered by green energies. In Europe, London took advantage of the Olympic Games to embark on a vast ecological program aimed at reducing 60% of its greenhouse gas emissions in the coming years. Finally, by way of comparison, Berliners have an average of 21 m² of green space per inhabitant, against only 14 m² for Parisians ... And 68 m² for the happy inhabitants of Yaounde.

The ecological interests of vegetated terraces

To try to improve the situation in Paris, all means are carefully considered. Reduction of speed on the ring road, alternating traffic, prohibition of circulation for older vehicles… But the fight also and above all involves the ecosystem and the soil. And as the creation of green spaces is not an easy thing in the capital, an alternative has been favored for a few years, failing to be able to be sufficient on its own: the greening of roofs and facades. The Grenelle 2 Law of 2010, which translated into action the commitments of the famous Grenelle de l'Environnement, considers that a building permit " cannot oppose the use of renewable materials or construction materials or processes allowing 'to avoid the emission of greenhouse gases, to the installation of devices promoting the retention of rainwater or the production of renewable energies '.

An encouragement therefore, in favor of green roofs which effectively prevent the too rapid flow of rainwater in the gutters, and thus flooding. Even to the point of passing in force if necessary: ​​" the planning provisions when they are opposed to the installation of green roofs, must not be applied ". In Germany, Austria and Switzerland, some cities even require the construction of green roofs on roofs with adequate angles of inclination.

In addition to this safety interest, green roofs also have the advantage of seriously lowering the temperature of buildings, by developing a thermal regulation effect. These roofs therefore make it possible to lower energy consumption, and also provide effective sound insulation. Plants also capture CO2 while producing oxygen, while the substrate fixes dust and harmful particles such as lead and carbon.

Green roofs also promote urban biodiversity by providing anchoring points for insects (bees) and birds, which naturally supply these places with seeds of new species. The most famous Parisian example is the Quai Branly museum inaugurated in 2006, with its green walls. Works by botanist Patrick Blanc, these creations cover an area of ​​800 m², rich in 15,000 species that take root in a vertical PVC slat constantly supplied with water rich in nutrients.

Build a green roof at home

Building a green roof and walls at home is possible, even if compared to Germany and the Scandinavian countries, the French market remains confidential: only 1% of new roofs are green, i.e. 200,000 m² installed per year (against 13 million in Germany). However, there are serious rules to follow, especially in terms of security. A vegetated terrace can be very heavy, in particular because of its beneficial role in retaining rainwater. It is therefore better to provide an ultra solid roof and an impeccable seal against water and also the formidable roots of plants; at least 14 centimeters of waterproofing layers (often bituminous materials) are necessary to avoid the risks.

Even with a solid roof, it's not about just laying the earth just any way and starting to grow tomatoes. Here again, there are very specific parameters. Less than 15 cm of substrate, the roofs are said to have “semi-extensive” greening (and “extensive” below 7 cm). They therefore require little maintenance, but can only support small plants. Beyond 30 cm ("intensive" revegetation, it is possible to start growing vegetables and even planting shrubs, which of course requires special care as for a normal garden. But the comfort and aesthetics "Green" have a price: 120 to 150 euros per square meter for a roof of fifteen square meters, plus 5 to 10 euros for maintenance per square meter every year.

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