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What are “green roofs” and how can they save you money?
Perhaps the most remarkable feature of the shopping centre is not what you can find in its plethora of fashionable Parisian stores, but instead what lies above it. The centre is home to what’s known as a ‘green roof’ covering 7,000 square metres, roughly the size of a football stadium.
Green roofs, or living roofs as they are sometimes known, are quite simply roofs that are partially or completely covered by vegetation, presenting a number of potential benefits. Green roofs are typically installed with a number of different intentions in mind – though the aim of the Beaugrenelle installation is to provide a haven for biodiversity and limit extreme temperatures within the complex. The earliest green roof was developed as far back as the 1880s when roofer H. Koch developed a new material that was composed of tar covered in sand.
Green roofs are nothing new. The earliest green roof was developed as far back as the 1880s when roofer H. Koch developed a new material that was composed of tar covered in sand. As a side benefit of the strong, durable roofing, the combination seeded and plants grew naturally – the benefits of the ‘greening’ were discovered as a happy accident! It was a long time before green roofs took off – when plant ecology expert Reinhard Bornkamm resurrected the idea in 1960. Bornkamm built the first modern large-scale green roof project in Berlin at the Free University. From there it spread fast worldwide, with one million square metres of green roof installed in France alone in 2013.
So why are they taking off now?
Public policy undoubtedly has had a huge impact with many cities benefitting from green housing initiatives and sanctions on managing storm water. In 2008, tax breaks were given to developers and building owners who installed green roofs in New York, providing significant savings on annual property bills. Installation costs have also fallen in New York, from $30 per square foot in 2002 to less than $20 per square foot in 20141.
Many cities around the world are seeing the effects of fast-paced urbanisation and the phenomena of “urban heat islands” are one problem that’s cropped up. An urban heat island occurs when a metropolitan area becomes significantly warmer than its surrounding rural area due to the concentrated human activity – cars, industry and simple heating of buildings for example. Conventional black rooftops are also major contributors to this phenomenon, by absorbing and re-radiating the sun’s energy as heat. The urban heat island effect could be responsible for up to two thirds of New York’s localised warming over the last Century. Green roofs could be an answer.
High rise cities such as Tokyo and New York are a particular challenge. Stuart Gaffin, a researcher at Columbia University suggested the urban heat island effect could be responsible for up to two thirds of New York’s localised warming over the last Century. Green roofs could be an answer – the researchers at Columbia University have demonstrated that a layer of plants and earth can cut the rate of heat absorption through the roof of a building in summer by 84% (for the technically minded amongst you, this works as the plants regulate their temperatures through evapotranspiration)2.
A similar Canadian study conducted in Toronto in 20013 found that greening 6% of city roofs could reduce temperatures by one or two degrees centigrade, meaning that green roofs could play an important part in bringing ‘heat island’ temperatures under control.
The insulating effect of the roofs also brings down energy costs for the buildings they are located on.
Green roofs act as a filter for the air – another method helping to reduce the urban heat island effect, which is exacerbated by air pollution. In 2005, an American study4 estimated that 800 tonnes of waste nitrogen dioxide could be trapped if just 20% of the roofs in Detroit, Michigan were covered in sedum plants – a big result for a relatively small investment. In Mexico City, green roofs are being heavily invested in to assist the metropolis with its smog issue, with nearly US$1 million being spent in 2013 on green roofs in the city alone. This figure is set to increase further in 2014. The roofs feature crops which can withstand Mexico City summers while producing oxygen and filtering carbon dioxide and heavy metal particles from the air – helping the environment while feeding the population (an excellent example of a Social Innovation approach).
Limiting rainwater runoff is another key benefit of green roofs. The roof can retain as much as 90% of rainwater5 meaning runoff is dramatically reduced – helping beleaguered drainage systems. In areas experiencing issues with storm water (such as Bondorf in Germany) green roofs are a key part of storm-proofing the city.
Green roofs are a valuable habitat for city wildlife with a number of species attracted to the biodiverse environments they create. ‘Urban meadow’ green roofs are a key part of strategies designed to aid declining bee populations.
Quality of life
As well as acting as a noise and heat buffer, green roofs can provide a valuable space for relaxation and recreation – particularly in large cities where green spaces are at a minimum. The installation of a green roof at St Lukes Hospital in Tokyo was found to decrease stress and lead to shorter patient recovery times.
So what’s next for green roofs?
With more widespread investment across the globe and increasing government incentives to invest, green roofs have become an increasingly familiar sight. Time will tell whether cost comes down to make them a viable proposition on a smaller scale.