Rapid growth of cities around the world have unfavorable impact on the environment. In developed countries, the level of urbanization is expected to reach 83 % in 2030 (Mentens, 2006). Parks and other green districts in the cities, are replaced by concrete and asphalt surfaces, to provide sufficient space for new buildings. Since, these surfaces are impermeable to water and heat up very fast, they are responsible for few environmental problems (Wong, 2008).
Water Runoff
In urban areas, stormwater runs directly off impermeable surfaces, back into rivers and other water reservoirs. However, only 15 % of water that falls in the cities, is able to reach plants and soil. It means that 75 % of water runoff (Mentens, 2006). As it runs through metropolitan areas, it is getting more polluted with residues of chemicals, making it very hard to clean.
Air pollution is caused by introduction of chemicals and particulate matter into atmosphere from anthropogenic and natural sources. Pollutants are introduced to environment in the form of gases, liquid droplets or solid particles. Bad air quality in the metropolitan environment is a major threat to human health, which may lead to respiratory infections, heart disease and even lung cancer according to the World Health Organization (Ardron, 2006; Wong, 2008).
Urban Heat Islands
Conventional rooftops are usually construct with impermeable materials without any slope, making them exposed to heat and light. Because of that, conventional roofs temperature may be raised up to 70 °C, forming urban heat islands (Wong, 2008) . This effect refers to the fact that cities can be up to 5 - 7 °C hotter than the surrounding rural areas (Liu, 2003).
Idea of Green Roofs
Green roofs are an innovative technology that can help communities diminish urban heat islands and improve air quality. Regarding to urban heat islands, vegetation on a green roof, shades rooftops and eliminate heat from the air through evapotranspiration (Liu, 2003).
Shading
Shading decrease surface temperatures below the plants. Sunlight is blocked by all of the plants growing on green roofs, with help of specially engineered soil which protects underlying roof membrane against exposure to wind and ultraviolet radiation. Since sunlight is blocked, these cooler surfaces, reduce the heat circulation. Decreased surface temperatures helps building and urban areas stay cooler (Huang, 1990; Akbari, 1997).
Evapotranspiration
In principle evapotranspiration cools down the air by re-using heat from the air to evaporate water. Evapotranspiration consists of transpiration, which is movement of water that plants absorb from roots to leaves and evaporation, which is conversion of water from liquid to gas. Green roof temperatures depend on the roof's composition, moisture content of the growing medium, geographic location, solar exposure, and other site-specific factors (Liu, 2003).
1roof.jpg
Figure . Shading and Evapotranspiration. Shading decreases level of sunlight that reaches roof when evapotranspiration cools a green roof by using heat to evaporate water from the growing medium (Liu, 2003).
Benefits of Green Roofs in Urban Areas
Green roofs the same as parks, woods and other ground level green areas provide many of the same benefits. However, huge advantage of green roofs is fact that they can be set up in any urban area which lacks space for planting at the ground level. Changing conventional rooftops to green, throughout a city can help reduce surface urban heat islands and cool the air. The surface of a vegetated rooftop can be cooler than the ambient air, while conventional rooftop surfaces can exceed ambient air temperatures by up to 50°C (Wong, 2008).
23 °C
66 °C
2roof.jpg
Figure . Temperature differences between a green and conventional roof. On a typical day, green roof of building showed on a picture (Chicago City Hall) is almost 40 °C cooler than the neighboring conventional roof (Wong, 2008).
Reduced Energy Use
Green roofs can save energy needed to cool and heat the buildings they cover. When green roofs are dry, they act as insulator which reduce flow of heat through the roof, thus decreasing the cooling energy needed to reduce building interior temperatures. Depending on water storage throughout the year, insulting properties of green roofs vary as they behave as dynamic systems. Moreover, when green roofs are wet, they absorb and store large amounts of heat, reducing temperature fluctuations (Liu, 2003; Akbari, 1997).
Reduced Air Pollution and Greenhouse Gas Emissions
Plants take in carbon dioxide from the atmosphere and release oxygen in process of photosynthesis. During photosynthesis, not only carbon dioxide is absorbed but also other air pollutants are taken up. The reduced energy demand from green roofs also decreases air pollution and greenhouse gas emissions associated with energy production. Moreover plant surfaces can remove certain pollutants from the air through dry deposition. Dry deposition is the process by which particles gather themselves on solid surfaces, reducing their concentration in air. A green roof can remove from the air particulate matter and gaseous pollutants such as: nitrogen oxides, sulfur dioxide, carbon monoxide and ground level ozone. Only 1 m2 of green roof is able reduce 0.2 kg of air particulates per year (Ardron, 2006; Wong, 2008).
Improved Stormwater Management
Another advantage of green roofs is that they can decrease and slow down stormwater runoff in the urban environment. As any other natural surfaces, plants and soil of a green roof, absorb water that normally would be wasted. Research have shown that extensive roofs typically capture between 50 and nearly 100 % of incoming rain. This value depends on the amount of used soil, the density of vegetation, the intensity and frequency of local rains (Vanwoert, 2005; Wong, 2008).
Improved Human Health and Quality of Life
Green roofs, by decreasing heat transfer through the roof of a building, can improve indoor comfort and reduce heat flow associated with heat waves during summer. This means that green roofs are able to improve people comfort particularly in buildings without air conditioning. Moreover green roofs provide many of the same quality of life benefits as parks or gardens, if they allowed for public access (Wong, 2008).
