Green rooftops are officially en vogue with the French parliament expected to pass a law in the coming months requiring some new buildings in commercial zones to become more sustainable.
Under the new law, the roofs of newly constructed buildings that house retail space over 1,000 square meters must be partially covered in greenery – or alternatively have renewable energy installations such as solar panels.
It’s another small step in the growing global trend to make buildings ‘greener’ while also meeting the needs and delivering cost benefits to the occupants and the local communities.
Franz Jenowein, Sustainability Consulting Director at JLL, says: “This law is part of a larger set of laws on biodiversity with which France wants to reinforce its sustainability leadership in Europe – but as a concession of an earlier draft of the law these roofs now need only to be partially covered. Reinforcing this new policy, the Mayor of Paris introduced a plan with a goal to reach 1 million square meters of green roofs and facades in the city by 2020.”
Green roofs provide an environmentally chic form of insulation to keep buildings cool in summer and warmer in winter, thereby saving energy and reducing heating and air conditioning bills. Research from the Universidad Politécnica de Madrid and Università Politecnica delle Marche in Italy found that green roofs with high vegetation density are 60% more energy efficient than non-green roofs.
And when many buildings have green rooftops, this reduces the heat island effect – a phenomenon where temperatures in urban areas are higher than in surrounding regions due to buildings and asphalt absorbing heat – to keep cities cooler in hot weather.
Plants and soil also retain rainwater to cut the risk of run-off during flash flooding while helping to capture air pollution, improve air quality, and support local biodiversity, birdlife and even urban beekeeping.
Chris Nunn, Director of Sustainability at JLL Australia says: “Green roofs are becoming a common feature of many buildings around the world. In Sydney some central residential buildings have been constructed with green roof tops – providing amenity space for residents as well as delivering environmental benefits. In Brisbane the city council has included green roofs in its proposed action plan for dealing with climate change.”
Some countries go even further, says Jenowein. “The Austrian city of Linz requires green roofs on all new residential and commercial buildings with rooftops larger than 100 or 500 square meters, respectively, and Toronto in Canada also passed by-laws mandating rooftop vegetation,” he explains. “But they did not stop there. Both cities provide financial incentives with Linz covering 35% of green roof costs and Toronto chipping in with C$75 (US$60) per square meter in incentives.”
Thinking from the roof down
For all the pros of green rooftops there are also some cons – the weight of vegetation and even trees can require substantial reinforcements to standard roof tops. Waterproofing the base layer to avoid damage to the building is another big consideration along with the question of how to allow building residents safe access to the rooftop areas. And of course buildings have to make a choice between using vegetation or solar panels on limited rooftop space.
Placing movable planter boxes on a flat accessible roof is one solution to giving existing rooftops a green makeover. “These provide amenity space and food growing for building occupants, while leaving the roof membrane exposed so it can be maintained,” Nunn says. Green walls are another popular option – both inside and outside buildings to improve energy efficiency, provide structural protection, boost air quality and cut noise pollution.
It’s not just the environment that benefits: research shows that being around green space has a big impact on people’s physical, mental and emotional wellbeing.
Dr Mathew White, who led the study from the University of Exeter Medical School’s European Centre for Environment and Human Health, says: “We’ve found that living in an urban area with relatively high levels of green space can have a significantly positive impact on wellbeing, roughly equal to a third of the impact of being married.”