Covering the roofs of buildings with plants provides many benefits to urban environments including, reducing storm-water runoff, saving energy and providing habitats for wildlife, says a team of international scientists.
A review of the history and research on "green roofs", published in the journal Bioscience, concludes that while the technology has been widely adopted in some European countries and Japan, more research and governmental support is needed to encourage greater use across the rest of the world.
People have planted gardens on their roofs since ancient times, and the urban roof garden is a welcome patch of green for many city dwellers. The modern, more functional, green roof originated in Germany at the beginning of the 1900s as a way of preventing damage to roofs from solar radiation and to make buildings more fire retardant.
Green roofs can be added to flat and sloping roofs of existing buildings or during construction. They consist of an insulation layer, waterproof membrane, and a layer of growing medium up to 20 cm thick. This "soil" is commonly composed of lightweight clay granules and crushed up bricks. Green roofs require little maintenance and watering, but because of their exposed positions are challenging environments for plants to survive in. From research carried out since the 1980s, low-growing, drought-resistant plants were found to be the most successful colonisers, but the new report emphasises the need for further investigations on suitable plant species, especially for different climates.
Although green roofs are more expensive to construct than conventional roofs, they are more cost effective over time, say the researchers, because of their energy saving and protective properties. For example, the plants and soil protect the roof's waterproof membrane from damage caused by ultraviolet solar rays, extending its life by more than 20 years. Some green roofs in Berlin have survived for 90 years without needing major repairs, says the report.
Green roofs also reduce the roof surface temperature. An experiment in Canada found a green roof reached only 30 degrees Celsius in summer compared to a regular roof which reached 70 degrees Celsius. The amount of heat passing into the building during the summer is also reduced by green roofs, so lessening the use of air-conditioning and saving energy. By reducing the amount of dark heat-absorbing surfaces in urban areas, the overall temperature of the city can also be reduced.
One of the biggest benefits of green roofs, say the researchers, is their water-capturing and evaporative capacity. This reduces the amount of rainwater runoff from buildings, so eases the strain on storm drains and sewage systems and helps to relieve flooding. In tests in North America it was found that green roofs reduced building runoff by 60–79 percent.
Other benefits cited in the article include sound insulation and possible improvement of air quality. The only reported negative effect of green roofs was an increase in leached nitrogen and phosphorous in water runoff. Reducing fertilisation, using different soils and selecting certain plants species which absorb these nutrients and other pollutants could solve this problem, suggest the researchers.
Conservation groups have promoted green roofs as they are a haven for urban wildlife including beetles, spiders, ants, bees, nesting birds, and some rare plants. But much more research on green roof ecosystems is needed concludes the report.
Feeds