Greening the City

Roofs in full bloom

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A flower bed is a lot more pleasant to look at than tarpaulin or gravel. “But planted roofs aren’t just beautiful, they also play a useful environmental role,” insists Raphaël Lamé, a director at Le Prieuré, a specialized French horticultural company, who is in charge of the technical commission of ADIVET, an association of professionals working in the sector. “First of all, they are able to store 40 to 60% of a city’s rainwater: part of it is used by the plants, while the rest evaporates.” In contrast, water running off waterproof surfaces picks up everything from heavy metals and hydrocarbons to dog feces, before entering the distribution system where it will, hopefully, be cleaned up. “Another environmental advantage lies in the fact that excess water only flows out after a certain time, thus spreading out the load on the distribution system. In this way, in the event of violent storms for instance, planted roofs can help to prevent overflows at sewage plants, which lead to pollution.” According to preliminary studies carried out by the Centre Scientifique et Technique du Bâtiment in partnership with ADIVET, they also appear to have an ‘air-conditioning’ effect. During a heat wave, the temperature of the top floor in the building may fall by 1 to 3°C compared with an unplanted roof terrace. “Some builders have also measured a soundproofing effect,” Lamé adds, “with the volume of noise falling by 2 to 5 decibels.”
The plants generally used are not in the least exotic, and include Sedum, thyme, carnations, lavender, iris, etc. The roofs are subsequently colonized by wild flora and fauna. Surveys carried out in Switzerland, Germany, the UK and Canada have shown the positive impact of planted roofs on urban biodiversity. “They act as a refuge for dozens of invertebrate species,” says Tanegmart Redjala-Ounnas, an R&D engineer at Le Prieuré. “In Switzerland, birds such as the little-ringed plover, the lapwing and the skylark, whose habitat is endangered, even nest there!”