Introduction

Stormwater runoff has caused significant water quality and availability issues in North America. The rate of urban development has resulted in water quality impairment and quantity management issues in North America. Greening of rooftops by incorporating plants into the design of roofing systems has been suggested as a method to reduce the impacts of storm water runoff. This can be achieved by reducing the amount of impervious surfaces within a developed area.  Replacing the impervious surface of a traditional roof with green roofs will also protect water resources from being contaminated and reduce the impact of climate change.

A green roof is a vegetative layer grown on a rooftop. It consists of a waterproof membrane, growing medium (soil) and vegetation (plants) overlying a traditional roof. The current technology for green roofs was developed in Germany about thirty years ago (Blackhurst et al., 2010). At present, green roofs are prevalent throughout Europe, but their availability into the United States urban environment is recently gaining popularity (Blackhurst et al., 2010; MacDonagh, 2005). As explained by McIntosh (2010) a green roof is a roof that is partially or completely covered with vegetation and soil, or a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and drainage and irrigation systems. Container gardens on roofs, where plants are maintained in pots, are not generally considered to be true green roofs, although it is an area of debate (McIntosh, 2010). Green roofs can be installed on a wide range of buildings, from industrial facilities to private residences. They can be as simple as a 2-inch covering of hardy groundcover or as complex as a fully accessible park complete with trees. Green roofs can help mitigate the problems that cities create by bringing the natural cooling, water-treatment and air filtration properties that vegetated landscapes provide to the urban environment. Architects and planners can use green roofs to help solve environmental problems by bringing nature back to the city in key ways (GSA, 2012). Green roofs have many advantages including:  stormwater management, air quality, noise insulation, social, economic, psychological and thermal benefits. Green roofs provide shade and remove heat from the air through evapotranspiration, reducing temperatures of the roof surface and the surrounding air. The surface temperature of a green roof during summer season can be cooler than the air temperature, whereas the surface of a conventional rooftop can be 50°C warm.

Cross Section of a Green Roof

A green roof generally consists of five constituents, viz: a roof support, a roofing membrane (membrane protection and roof barrier), isolation, a drainage layer, a growing media and vegetation (Ouldboukhitine, et al., 2011). A typical cross-section of a green roof is illustrated in Figure 1 below.

There are typically two types of green roof, viz: extensive and intensive green roofs. Intensive green roofs are often designed as public places and generally require substrate depths between 150 and 1200mm.  Therefore, they may comprise of trees and shrubs used for landscaping found at ground level (Snodgrass and McIntyre, 2010). Intensive roofs are more expensive than extensive roofs because of the need for more structurally sound buildings to support the weight. In contrast, an extensive green roof needs a growing medium between 50 and 150mm to support plant life.  This limits the size of plants that can be used on the roof, thus, confining the weight of the green roof on the building structure (Molineux et al., 2009; Bianchini & Hewage, 2012).  In comparison with intensive roofs, extensive green roofs are more widely used due to their low costs, light weight, shallow soil layer and independence from slight maintenance. The Evergreen State College won the 2005 Green Roofs for Healthy Cities Award of Excellence in the Extensive Institutional category. The seminar II building was the first academic building in Washington to achieve LEED® Gold certification. The roof of the 153,000 square feet Evergreen State College Seminar II Building, completed in 2004 is 40% green roof. The building is 70% ventilation cooled and the energy used for cooling and heating was predicted to be 40% less than that of a traditional building. Green roofs exist on 13 separate roof areas. The main objective of the project was to create an environment that optimized student learning by maximizing the facility’s connection to the environment through sustainable building features that support studying outside the classroom. Therefore, storm water management and other measures to protect the site and create learning opportunities through sustainable design became important on this project. The initial cost, as well as running costs of the green roof were estimated to be lower than conventional construction.

Fig 2.1 The SEMINAR II Building green roof top

 

Fig 2.2 The SEMINAR II Building green roof top

 

Fig 3.1: Green roof in Olympia

Fig 3.2: A tiny sized green roof in a chicken house in Olympia

 

Fig 4.1: Maintenance of green roof in Tacoma, Washington

Fig 4.2: Maintenance of green roof in Tacoma, Washington

Fig 4.3: Green roof in Tacoma, Washington

Fig 5: Green roof at bus stop

Fig 6: Extensive green roof in Singapore

Fig 7: Intensive green roof in Singapore

Roles of Green Roof for Urban Sustainability

Urban development has increased the number of impervious surfaces (rooftops, roads, parking lots, and other paved surfaces) in cities resulting in decrease in storm water infiltration (Garrisson et al., 2012). These impervious surfaces collect pollutants such as oil, heavy metals, salts, pesticides and animal wastes. Rainfall can wash these contaminants into waterways, thus reducing the quality of streams, lakes, rivers and so on. Greening the roofs will reduce the washing of these pollutants into waterways and enable sustainability of the urban areas. The roles of green roofs for urban sustainability include the following:

Storm water management

Green roofs can help preserve a building’s roof surface while reducing storm water runoff in the urban environment (Lallanilla, 2017). Pollutants present in the rainfall is captured by trees and plants on the roof, filtered through the soil, before they are released unto the land surface. Green roofs are ideal for storm water management as they can reduce the volume of storm water runoff (Getter and Rowe, 2006). They store water during rainfall and delay runoff until after peak period. Excess precipitation is returned into the atmosphere through evapotranspiration. The rainfall pattern, slope and type of green roof determines the rate of runoff. Generally, extensive green roofs and steeper slopes increase total runoff. In comparison, while conventional roofs merely    shed rainfall, green roofs use most incident water and slowly release the remainder. For instance, in a typical year in the Midwestern United States, 75% of the water is retained on a green roof, stored in plants and the soil layer, and 25% becomes runoff. Also, green roofs control runoff even in the winter season, with average water absorption rate of 40-50% while the average summer water absorption rate is 70-100% (MacDonagh, 2008).

Mitigation of Urban Heat Island Effects

The replacement of vegetation by expansively built surfaces with high heat absorbing properties in the urban area contributed tremendously to the urban heat island. The urban heat island is typically warmer than its surrounding rural areas due to human activities. The change in urban surfaces result in high solar radiation and increased heat and thermal energy storage. This increase the ambient air temperature of these urban areas (Getter & Rowe, 2006). Even at night, the air temperatures are warmer because the built surfaces have absorbed heat and radiate it back during the evening hours. Green roofs have been proven to significantly reduce urban heat island effect through the process of evapotranspiration. Plants take water in through their root systems and release it through their leaves in a process called transpiration, while the plant surfaces and growing medium enhance the evaporation process. This process allows the green roof to cool down. Energy from incoming solar radiation that would have heat the roof surface and increase ambient air temperatures is used in the evapotranspiration process. This result in latent heat loss that lowers surrounding air temperatures (Garisson et al., 2012).

Reduced energy use: 

Green roofs were initiated to replace the vegetated footprint that was destroyed when the urban areas started developing. Green roofs can preserve the roof surface of a building while providing substantial environmental benefits. The plants and growing medium of a green roof shade and protect the underlying roof structure from sunlight, thereby reducing its temperature. Also, green roofs have high albedo (ranging, from 0.7 – 0.85), similar to other roof technologies, such as white roofs (albedo = 0.8) developed to enhance cooling of roofs. The reflectivity of white roofs can decline up to 11% due to accumulation of dust and debris (Getter & Rowe, 2006).  Conventional roof surfaces have much lower albedos, ranging from 0.05 to 0.25 (USEPA, 2005). Thus, green roofs improve the energy efficiency of buildings by reducing the energy needed to provide cooling. Likewise, they form a layer of insulations for homes, thereby reducing the amount of energy needed by home owners to insulate and heat their buildings

Reduction of Noise Pollution

The insulation provided by green roofs helps to reduce noise pollution which is a major characteristic of urban areas (Mihai, 2009).  The building surface, soil, plants and the air layer trapped between the green roof assembly provide sound insulation. The substrate blocks lower frequencies, while the plants block higher frequencies of noise. This provide a reduction in indoor sound levels up to 40 decibels. This reduction in noise level is well appreciated in by people that live near industries, airports and major highways., or other forms of industrial-related noise pollution. In addition, wind moving through the stems and leaves on green roofs can mask noise or create a beneficial soundscape (MacDonagh, 2008). Therefore, converting conventional roofs to green roofs could reduce sound pressure from roads and other sources in urban areas.

Increased biodiversity and Habitat Preservation

Green roofs can provide food, shelter and nesting opportunities for many species of animals such as spiders, beetles, butterflies, birds and other invertebrates with food, habitat, shelter, and nesting opportunities (Brenneisen, 2003; Gedge, 2003). Most green roofs are inaccessible to the public; hence they can provide a peaceful habitat for microorganisms, insects and birds. Some researchers are evaluating roofs as a potential way to restore native plant species to an area. Some studies were conducted on green roofs to establish their viability to provide feasible habitats for endangered species.  The studies explained that since green roofs are located on elevated portion of the urban area, they can provide a unique ecosystem protected from ground level predators, traffic, noise, and human intervention (Federal Technology Alert, 2004). These findings have encouraged local and national conservation organizations to develop the use of green roofs further.

Mitigation of air pollution

The assemblies of vertical building which characterize downtown urban areas often inhibits ventilation, reduces wind speed and trap pockets of heat. This makes gaseous pollutants remain suspended for long periods of time in the atmosphere (Getter and Rowe, 2006). Green roofs can absorb these gaseous pollutants such as particulate matter, nitrous oxides or carbon dioxide through the plant’s foliage, thereby cleansing the air through natural process. Particles will eventually be washed away into the soil through rainwater movement, and some of the pollutants will be absorbed into plant tissues.

In a year, green roofs can remove about 0.2 Kg of dust particles per square meter of grass (Peck and Kuhn, 2001). Particulate air pollution can pose adverse health effects such as increased respiratory problems, decreased lung function and increased hospitalizations due to respiratory and cardiovascular disease. Mitigation of air pollution is also essential to get funds from government. Some municipalities such as Washington, D.C., will not have access to federal funds because they did not meet federal air quality standards for particulate matter (Getter and Rowe, 2006). Researchers explained that if 20% of all existing ‘‘green roof ready’’ buildings in Washington, D.C., implemented the technology, it would remove the same amount of air pollution equivalent to using 17,000 street trees (Deutsch et al., 2005). An air quality model for greening all rooftops in Chicago predicts a reduction of 417,309.26 kg of nitrogen oxide and 517,100.61 kg of sulfur oxide emissions per year (Laberge, 2003). At the University of Michigan, Clark et al. (2005) estimate that if 20% of all industrial and commercial roof surfaces in Detroit, Michigan, were traditional extensive Sedum green roofs, more than 800,000 kg per year of nitrous oxide would be removed.

Social Benefits

Green roofs are unique and beautiful, they add aesthetic value to buildings and improve serenity of urban areas. Looking at these green plants and nature on the roof has some beneficial health effects such as releasing muscle tension, reducing stress, lowering blood pressure and increasing positive feelings. These health benefits also improve productivity and health of workers who had a view these green roofs (USEPA., 2009). They are less stressed, experience greater job satisfaction and reported fewer headaches These benefits improve the values of buildings in the cities. Some landlords often increase tenancy rates based on access to aesthetic view of green roofs compared to conventional barren roof scenery (Getter & Rowe, 2006).

Economic Benefits

Green roofs are of economic benefit to building owners because they do not have to worry about leaking roofs. Conventional roofs are exposed to sun, wind, snow and rain, thereby experiencing large variations in temperature (Garrison et al., 2012). These extreme temperatures cause the roof membrane to shrink in cool weather and expand in hot weather. Thus, shortening its lifespan. However, gardening on the roof prolongs the life span of roofs. As clearly stated earlier, there is big savings on energy required to cool or heat buildings. In addition, green roofs have the capacity to capture waste and convert it to useful product. For instance, a Belgian factory that manufactures biodegradable laundry products used treated by-products of their manufacturing process to irrigate and fertilize two acres of native grasses and wildflowers on its roof (MacDonagh, 2008).

Cost Implications of Green Roofs

Green roof plays a significant role in achieving a more environmentally sustainable city. This role is limited to regionally specific commercial and multifamily buildings. Private buildings are discouraged from installing green roofs due to high cost of setting it up. Typically, a conventional roof costs $10 to $12 per square foot, while the initial cost for a green roof cost twice as much (MacDonagh, 2008). Studies suggest that green roofs are more effective in regions with high cost of electricity, multistory building stock and climates that show reductions in urban heat island with the introduction of green roofs (Blackhurst et al., 2010). Similar, research has demonstrated that the environmental benefits of green roofs may not exceed their costs. Carter and Keeler (2008) demonstrated that cost of green roofs installed in a watershed near Atlanta are approximately 10% higher than the environmental benefits of storm water management, energy reductions and improvements to air quality over a forty-year period. Carter also confirmed that the social benefits exceed the private benefits   However, a life cycle analysis reveals that in the long run, the life expectancy of the green roof is three times greater than that of a conventional roof. For instance, in England’s wet climate, most conventional flat roofs have an average lifespan of only ten to fifteen years. A London department store that installed green roof fifty years ago, still found the roof membrane in excellent condition.

In conclusion, green roofs are of ecological and economic importance for sustainability of urban areas. The use of green roofs is a potential solution to the destruction of natural habitat for urban development. They promote a better quality of life for all members of the society by absorbing greenhouse gases and particulates in the atmosphere. Commercial building owners and institutions are able to make big savings on energy required to cool and heat their buildings while providing habitat for endangered species and improving aesthetic value of the urban areas.

 

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