ABSTRACT
Climate change is affecting urban areas through the extremes of flash flooding caused by more intensive rainfall and through heat islands, formed in the inner cities during summer months. Consequences are severe. Between 1998 and 2004, flooding catastrophies in Europe caused damage in excess of 25 billion EUR and the loss of 700 lives (Commission of the EC 2006). Severe summer heat has caused fatalities in major cities, such as in Paris. Both problems are made more severe by a particular factor affiliated with urban areas: impervious surfaces. Paved surfaces and impervious rooftops speed up the runoff of stormwater, causing urban drainage problems and flooding and excessive heat in summer. “Extensive” green roofs are an exciting new technology that permits retrofitting of existing urban areas to alleviate these problems. The common denominator is evaporation and evapotranspiration through plants, lessening the volume of runoff that causes flooding and permitting vegetation to live up to its role as the cooling system of the planet earth. Green architecture and green technologies have been a theme of the new millennium. In antiquity, the hanging gardens of Semiramis, built by Nebuchadnezzar II, were actually an early example of a green roof and were one of the Seven Wonders of the World (Wikipedia 2009a). Any roof in a temperate climate is actually a potential habitat for vegetation, such as the succulent houseleek (Sedum sempervivum). To date, green roofs were made possible by a technological breakthrough of the DuPont Company, which invented a woven plastic polymer that not only formed the basis for nylon stockings, but also for geofabrics invented to seal sanitary landfills. The use of geofabrics as a root blocking membrane for vegetation on roofs was quickly realized in Germany. Here a colleague in landscape construction, Professor Liesecke of the University of Hannover (Liesecke et al. 1989), conducted research concerning lightweight soils involving blown shale, making an important contribution to the special role Germany soon assumed in developing a green roof industry. Conventional roofs with a bituminous cover have a high shrink-swell ratio during the temperature extremes of the day, tend to crack and require regular replacement. The German contribution was the development of “extensive” green roofs, which have a thin layer of lightweight soil that can be installed over existing roofs without having to change or upgrade their structural support. This offers a tremendous opportunity to retrofit and green existing structures in cities, thereby realizing multiple benefits of green roofs, including: reduced runoff
and flood management; reducing energy demands in heating and cooling houses; improving the longevity and replacement costs of roofs; and benefiting city climate all while also providing for aesthetics and wildlife benefits in downtown areas. The latter includes ground nesting birds and various types of bees that feed on the nectar of blooming plants. Green roofs provide a new form of urban vegetation, as well as integrated, sustainable management of urban drainage. In this chapter we will explore pre-and post-development conditions of urban areas, showing what causes heat and urban drainage problems. We will further investigate a concept of maintaining pre-development conditions by sustaining the water cycle and energy budget of cities. A key to bringing about the use of green roofs, as a new technology, is to maximize performance aided through design principles of multiple benefits and multiple uses. Green roofs are of value to society and can rely on community action and support, involving institutional aspects of implementation. The wide extent to which this technology is already being used will be demonstrated through examples of installations.
