The urban sprawl of cities cover only 0.4% of the world’s surface, yet they generate the majority of the carbon emissions responsible for global warming (State of the World 2007, Worldwatch Institute, January 2007). As more and more of the global population migrate from rural areas into burgeoning cities of concrete and bitumen, the issue of green spaces become ever more critical. The urban roofscape of growing cities that displaces the natural environment is a hellish place hot bituminous surfaces, extreme temperature contrasts, high winds and an antipathy to water. Some cities have turned to innovative approaches, transforming city rooftops into living gardens in the sky.
Sod houses on the American prairie commonly had living roofs, as did the log houses of Northern Europe. But it has only been recently that the trend has caught on in the modern cities. On commercial buildings, a green roof is typically made up of four layers a layer of water proof membrane, followed by storage cups to store excess water, a growing medium of soil composite and vegetation. With new and improved technology, the membranes spread across rooftops can now capture water for irrigation, allow drainage, support growing plants, and resist invasion of roots into building structure. In some cities such as Germany, Switzerland and Austria, living roofs are required by law if roofs are of a suitable pitch. In other places, architects and builders are given fee reductions to encourage the use of green roofs.
The Benefits of Green Roofs
These roofs mitigate environmental extremes that are the norm on conventional roofs. In many cases they are self sustaining, capturing water from rains and reducing storm water runoff. When rain falls on normal roofs, it runs off the city in floods into storm drains, unabsorbed and undeterred. A living roof absorbs the water, filters it, slows it down and stores some of it for later use. This reduces the risk of sewer overflows during flash floods and returns cleaner water to the surrounding watershed.
Although the cost of installing a green roof can be two to three times that of a normal one, it is likely to be cheaper in the long run due to energy savings. A living roof acts as insulation against the hot rays of sun beating on a normal rooftop. Cities tend to be warmer than the surrounding areas due to the high concentration of glass and concrete. The soil mixture and vegetation atop a green roof reduces heating and cooling costs of the building it grows on by as much as 20 percent. (Green Roofs, Verlyn Klinkenborg, National Geographic Magazine, May 2009).
The beauty of these green roofs improves the urban landscape. It provides a sanctuary for animals and plants in concrete cities. In New York City, the High Line, once an abandoned railway line used for delivering cattle, has been completely transformed. More than 100 species of plants, many of them native, line the first phase of the promenade, now open to the public. A rusting, dilapidated eyesore is now a haven of wilderness, 30 feet above the streets of Manhattan (After Years of Advocacy, Newly Renovated High Line Opens, Robin Pogrebin, New York Times, June 2009).Other unconventional spaces are being utilized, anything from canal banks in London to bus shelters in downtown San Francisco to sheds in Oregon.
As the global food crisis escalates and more farmland is given over to bio fuel crops, green roofs can offer an alternative source of food. In Vancouver, the Fairmont Waterfront Hotel grows fruits, vegetables, herbs and honey worth about $16,000 annually to be used in the kitchens. On the other side of the world, rice grown atop a sake company’s head office in Tokyo is used to brew the popular drink (Green Roofs). The 2012 Olympics held in London could see athletes being fed off harvests of potatoes and brussels sprouts from local rooftops, which are thought to comprise an area 24 times that of Richmond Park, a 1,000 hectare Royal Park to the west of London. (London rooftops could be turned into allotments, Chris Irvine, The Telegraph, November 2008).
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