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Green Roofs and Brighter Futuresby Brian BurtonMarch, 2003--Green roofing system development involves the creation of specially designed "contained" horticultural elements and infrastructure components on top of buildingss. Typically, a green roof system (GRS) will consist of four basic layers: plants and vegetation; lightweight growing medium and rooting zone; drainage layer; and a waterproof and root repellent protective layer. A GRS creates a component of the building "envelope" that is alive and growing. This green space, also referred to as a "biomass," can be below, at or, above grade, but in all cases the plants are not planted in the "ground." A GRS is actually an extension of the existing roof, not merely potted plants. These systems involve installation of waterproof and root-repellent membranes, drainage systems, filter cloth, lightweight growing media, plants, and other components based, to a substantial degree, on the structural loading capacity and the objectives of the green roof design. A GRS may be modular, with drainage layers, filter cloth, growing media, and plants already prepared in movable, interlocking grids. In other cases, each component of the system may be installed separately. There are several different types of green roof systems: intensive, extensive, and semi-extensive, which are designed to be either accessible or inaccessible. Intensive systems employ engineering solutions to enable standard landscape components of lawns, planting beds, paving, and water to be supported at roof level. These systems are usually accessible, require high maintenance, irrigation, and feeding, and may differ little in appearance from ground level landscapes. Extensive systems, on the other hand are comprised of lightweight layers that sit on top of the roof and are tolerant of the extreme conditions of temperature, wind, and drought. They are relatively low in maintenance and require little fertilizing or irrigation. Extensive roofs are lightweight ,composed of thin layers of growing medium (10cm or less), which means the plants that grow on them need to be robust. As a result a restricted range of plants, mostly sedum and grass species, are used, which can create a monotone effect. A new concept, however, is beginning to emerge that combines some of the characteristics of the two. "Semi-extensive" systems use the same lightweight materials and layers of the extensive system--and have the same low input philosophy behind them--but employ slightly greater depths of growing material. (The use of semi-extensive landscaping is often a function of having areas of the roof where additional load bearing capacity exists, like over pillars, to reduce the overall costs, and for aesthetic reasons). Although this increases the loading on the roof, it does enable a far greater and more interesting range of plant material or vegetation types to be grown. However, once the substrate (the substance upon which a plant grows) depth exceeds 10cm there will be a greater need for maintenance--especially with weed growth. Intensive, extensive, and semi-extensive, systems regardless of whether they are accessible or inaccessible, are always constructed, as one might expect, to provide accessibility at least for roof maintenance activities. A GRS can vary from small-scale and simple designs that utilize a single plant species, to very large and extensive covers of landscaped gardens with numerous species. The intended function of the roof space determines the GRS design, resulting in designs requiring different growing medium depths to accommodate various plants, shrubs, and trees. That additional structural support may be necessary to accommodate higher roof loads must also be taken into consideration. They can be installed on flat as well as sloped roofs, are low maintenance, and, depending on the climate and the amount of rainfall, can grow a variety of hardy grasses, wildflowers, mosses, and sedums in a soil layer as thin as eight centimeters. "Green roof infrastructure investment addresses many key challenges facing cities across North America, such as how to grow our cities up rather than sprawling to accommodate new growth, and still maintain a very high quality of life," said Steven Peck, Executive Director of Green Roofs for Healthy Cities, (GRHC) "This research will help us to develop appropriate public incentives to encourage widespread private sector GRS installation and build a green roof industry in North America." "Green roofs would allow developers and architects to exploit the largely wasted rooftop spaces in urban environments," said Peck. "Green Roof technology could help us address the main challenges facing cities into the 21st century--like how to accommodate new population growth while maintaining a high quality of life; how to generate new investment and new employment; and how to reduce air pollution and protect our precious water resources." Green Roofing Creates Long-Term Business OpportunitiesAccording to Gayle Bertrand, Director of Membership Development for GRHC, the benefits of green roofing systems and technologies are just beginning to gain our attention in North America and the market is in its early stages.In Europe, however, green roofing systems have become very well established as a direct result of government legislative and financial support that recognizes the many tangible benefits of rooftop greening. This support, reports Bertrand, has led to the creation of a vibrant, multi-million dollar market for GRS products and services across Europe. "In Germany for instance, the industry continues to experience rapid and sustained growth with well over 12 million square meters, (140 million square feet), of green roofing systems installed in 2002, up by nearly 70% since 1994." According to the experts, if green roofing activity in North America (with a total population nearly 5 times greater than Germany) were to reach levels comparable to those we now see in the Republic of Germany over the next 10 years, explains Bertrand, "We would see over 1 billion square meters of extensive, intensive, semi-intensive, and retro-fit green roof installations completed in the US and Canada every year! "This market in green roofing design, installation and maintenance will create significant and long-term business opportunities for design, specification & inspection professionals, engineers, consultants, nurseries, manufactures, suppliers, and landscape contractors. In fact," reports Bertrand, "in a recent survey completed for Green Roofs for Healthy Cities, most experts predicted the growth rate for green roofing would be greater than any other landscaping market sector." BackgroundAccording to Architect Monica Kuhn a leading authority on the subject, "Sod has been used as a roofing material almost as long as man has been constructing buildings. Scandinavia and Iceland have outstanding examples of houses that are over four hundred years old, still roofed with wood, bark, and one or more layers of living turf; in Tanzania, grass roofs kept homes cool in the hottest of temperatures; and even Canada, not to be outdone, boasts the reconstruction of a Viking village on the Northern tip of Newfoundland, built entirely of stone and green turf, as well as a grass-covered fisherman's hut from the 1600s, at the fortress of Louisburg in Nova Scotia. Early settlers in the prairies built some of their first homes with walls of sod, and wooden roofs covered with prairie grasses."Regardless of the type, the environmental benefits of adding to the amount of biomass growing in an urban area are tremendous. Through photosynthesis, plants take in carbon dioxide and carbon monoxide and give off oxygen. Studies have shown that one hundred and fifty square metres of "plant surface area" (area of roof x height of the plant x surface area of the leaves and stem) produce enough oxygen for one person for twenty-four hours. At the same time, the plants, through evapo-transpiration, filter out airborne particulates, hydrocarbons, and sulfur, among others, thereby cleaning the air around them. Cities, with all of their reflective surfaces and mechanical exhausts, are often referred to as heat islands; a layer of plants would absorb that heat and eventually have an positive effect on temperatures, and wind speed, during both the summer and winter seasons. Economic BenefitsAccording to Dr. Karen Liu of the NRC, GRS can provide benefits to urban areas by reducing the energy required to heat or cool buildings-thereby reducing greenhouse gas emissions--through shading, evapo-transpiration of plants, and improved insulation values. "Other benefits, adds Dr.Liu, "include additional green space and higher property values. These GRS can also play a role in storm water management by delaying input and reducing the rate and amount of runoff into the sewage system, which in turn reduces the possibility of sewage overflow.""Results from the first year of the study NRC conducted showed that GRSs reduce the heat flow across the roofing system and lower the energy required for space conditioning during the warmer months. In the spring and summer of 2001, the green roof reduced the overall heat entering the building (during the day) by more than 85%, and that leaving the building (at night) by about 70%." reported Dr Liu. "In addition to reducing heat flow, GRS significantly moderated the daily temperature fluctuations experienced by the roofing membrane, especially in spring and summer. Median daily temperature fluctuations in the green roof membrane we tested were reduced from 46°C (as measured on the reference roof membrane) to 6°C, thereby reducing the thermal stress in the membrane and possibly extending its life." "Vegetation, primarily forests, has been identified as an important component of any strategy to reduce greenhouse gas emissions, through the sequestration of carbon in the woody biomass of trees," adds Dr Liu. "Given the limited space available for additional trees in many North American metropolitan cities, new adaptation strategies such as placing the vegetation directly on building roofs become especially attractive." According to Dr Liu, "GRSs are found throughout Europe. Germany, in particular, has carried out a significant amount of technical research to improve the various roofing components. Here at home, Canada has agreed to reduce GHG emissions by six per cent relative to 1990 levels by 2008-2012. GRS may be a part of the solution." Community Cost Savings OpportunitiesGRSs save money for communities through:
A GRS will not only absorb heat, decreasing the tendency towards thermal air movement, but will also filter the air moving across it.
Shaping the Future: Green Roofs for Healthy CitiesMany cities across North America are realizing the benefits of GRS. The new Vancouver Public Library, designed by architect Moshe Safdie, has a GRS. In New York City, an alliance of more than 75 private companies, conservationists, public officials, scholars, and community activists announced plans this summer to blanket the city with green roofs. Portland, Oregon, Chicago, Seattle, and the state of Maryland offer tax credits for installing green roofs.These types of incentives are encouraging commercial applications to blossom. For instance, the US$60 million Gap headquarters in San Bruno, California, features a spectacular series of undulating green roofs. Architect William McDonough has won a number of awards for his ecological designs.
Seattle WA is one of the first cities in the U.S. to adopt the voluntary U.S. green building counsels rating system for its municipal buildings. |
