Protecting buildings from extreme heat and heavy rain keeps them cooler and reduces run-off, writes Zhang Dongfang
Urbanisation and poor city-planning have also worsened these problems. Heat is absorbed by concrete surfaces and roads during the day and then radiated at night, which prevents temperatures from falling. Paved surfaces can also increase rainwater run-off, worsening the risk of flooding.
Green roofs offer a solution to these problems because they absorb rainwater and reduce radiant surfaces, helping to cool buildings internally and reduce the need for air-conditioning.
As a pioneer in the research and and construction of green roofs, Germany offers lessons that could be applied in China, says Zhao Dingguo, senior agronomist at Shanghai Institute of Agricultural Sciences and an expert on green roof technology.
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China is already experimenting with “sponge cities” to mitigate heavy rainfall and recycle water by adding green spaces, integrating ponds and filtration pools into city landscapes and laying permeable road surfaces so that 70% of rainfall is absorbed or reused.
A project to remodel 16 districts in major cities such as Wuhan and Chongqing began in 2015 at a cost of 600 million yuan (US$90m) for each area.
More widespread use of green roofs beyond these schemes could have a big impact. Researchers Michael Richter and Wolfgang Dickhaut of HafenCity University in Hamburg are part of a team monitoring the German city’s green roofs. They reported soils over six centimetres thick retain 60% of rainfall, while those that are 50 centimetres retain more than 90%. Rainfall typically seeps from the soil mat over four hours later so heavy rainfall enters drainage systems gradually.
The German model
The country’s expanse of green roofs has increased rapidly. Germany had 86 million square metres in 2014, with new additions of roughly eight million square metres a year, according to the European Federation of Green Roof and Wall Associations. Today, this has risen to between 100 and 150 million square metres, the German Roof Gardens Association estimates.
“Growth has been particularly fast recently as part of the response to climate change,” says Wolfgang Ansel, an official with the association.
A mixture of federal and local laws, backed by supportive subsidies, and punitive higher taxes on run-off from conventional roofs has driven the growth.
The first wave of green roofs appeared in the 1960s, followed by research into waterproofing technologies in the 1970s, and into environmental impacts in the 1980s, when local governments also started to issue supportive policies.
But it was only with the passing of federal laws that green roofs started to be built on a large scale, says Goya Ngan, a Canadian landscape architect and expert on Germany’s green roof policies.
He credits the 1992 UN Earth Summit’s Local Agenda 21 initiative, and EU directives, with shaping sustainable development strategies for European nations, and Germany’s legislation.
German federal law demands that each state produce a landscape plan, and federal nature conservation laws stipulate compensation for environmental damage in greenfield developments, which can be "paid" by installing green infrastructure, according to a 2011 report in Solutions Journal by German and US-based researchers.
“Munich and Stuttgart have become examples of what can be done,” says Mr Ansel. Munich has required green roofs on new buildings since 1996. Germany’s eighth largest city, Essen, has recently ruled that all new buildings and restoration work in its city centre must have green roofs.
Private householders also pay heavier taxes on conventional roofs, based on estimates of how much storm water run-off will enter local drainage systems.
For instance, in Hamburg the tax on a roof with five centimetres of planted soil is half that of an ordinary roof.
Out of 1,488 German urban areas with more than 10,000 people, 79% have financial incentives for reducing storm water run-off according to a 2016 survey by Germany’s Association of Green Building Companies. Six percent of them offer subsidies to install green roofs, and 51% have drawn up green roof building plans, an 11% rise on 2014.
Obstacles ahead
However, there are significant barriers facing the spread of green roofs globally, the most important being the lack of high-quality, lightweight systems suitable for different climates and ecosystems, as the European Federation of Green Roof and Wall Associations pointed out in a 2015 white paper.
Another barrier is high up-front installation costs. In Hamburg, green roofing costs three times more than a gravel alternative although costs equalise over a 40-year period. However, add in subsidies and the green roof is cheaper.
Early adopters have paid the most. Switzerland’s first modern green roofs, built in the 1990s, averaged US$20 per square foot more than an ordinary roof. In Zurich today, competition and economies of scale mean the cost premium is only US$3 per square foot, according to engineering and environmental consultancy firm Arup.
Next steps in China
Zhao says China has experimented with green roofs before, with some showpiece roofs dating back 40 years in Beijing and Shanghai. Green roofs are spreading thanks to more light-weight projects that use less soil. Zhao's own research is focused on developing and popularising these light-weight systems.
According to a report in Beijing’s Legal Daily newspaper, the municipal government started to promote green roofs in 2005, and the city now has over two million square metres of them, though there is still a need for more funding and greater awareness.
However, unclear ownership of roofs on many residential buildings is also a major problem, notes Zhao in an article in the China Building and Waterproofing journal.
To remedy this, the government needs to take a lead in promoting green roofs through legislation and additional funding, says Zhao.
Although there is still no legislative support for the promotion of green roofs, there has been some government funding: 800,000 square metres of Beijing’s two million square metres of green roofs had government funding, he told chinadialogue.
Green roofs were encouraged to improve air quality for the 2008 Beijing Olympics, and were used on many of the temporary international pavilions for the 2010 World Expo in Shanghai.
And with higher temperatures and heavier rains expected in the decades ahead, it is hoped that efforts to develop green roofs, together with the “sponge city” projects already underway, will develop further.
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