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Green Roofs

A green roof, or rooftop garden, is a vegetative layer grown on a rooftop (USEPA, 2010a)(see Figure 1). According to the United States Environmental Protection Agency (USEPA), "green roofs" provide shade and remove heat from the air through evapotranspiration as shown on Figure 2. Green roofs are solutions to prevent urban heat islands. Urban heat island means that a metropolitan area is much warmer than the neighboring rural areas. Typical section view of a green roof is illustrated on Figure 3.

Content Table

Green Roofs

Introduction

Green roofs alleviate the reduction of storm water runoff. They also reduce spreading of contaminants and able to uptake contaminants through vegetation. Individuals can use green roofs on their houses and sheds. It is part of the LOD technique (local disposal of water)(Swedish Board of Housing, Building and Planning, 2010; Pasche E. 2009). One can distinguish between intensive and extensive roofs. Extensive green roof is when there is a thin layer of soil, the plants are draught tolerant and it requires little maintenance. Intesive green roof has a thick layer of soil and is like a normal garden with e.g. trees. This type needs more maintenance and a roof that can bear heavy loads (Scandinavian Green Roof).

Figure 1 Frankfurt, Germany as the Leader of Intensive Green Roofs (Source: USEPA 2010b)

Figure 2 Evapotranspiration and Shading on a Green Roof (USEPA, 2010b)

Figure 3. Green Roofs in Vancouver Canada (Source: Pacwest Roofing 2011)

Figure 4. Typical Section View of a Green Roof (Source: USEPA 2010b)

Green Roofs on Sustainable Building

Any building that utilizing green roofs can be classified as a sustainable building, which means that this type of a structure is environmentally responsible and efficiently using resources to reduce the impact on human health and the environment through reduction of energy use, air pollution, stormwater and heat stress. In the United States, the cost of installing green roofs is in the range of $10 to $45 per square foot depending on the type of vegetation and material to be used.

Costs for green roofs vary very much between different countries, and are generally higher in countries where there are few entrepreneurs that install green roofs. An extensive green roof is normally less expensive than an intensive one. Site-built green roofs are often cheaper than prefabricated mats. The following factors are affecting the price per meter (Prokop et al., 2011): 1. Size of roof – the larger the area the cheaper per square meter the roof will be 2. Height of the roof. This will affect the price in terms of the cost of raising the elements to roof level 3. Type of green roof 4. Initial maintenance and establishment costs 5. Type of waterproofing and insulation used (difference in labor costs) 6. Other factors (Roof elements that intrude above the roof such as outlets, roof lights and industrial plant and other additions such as access hatches, safety lines can lead to increase in price per meter squared.) 7. Involvement of manufacturers and contractors.

Figure 5 Weilburg, Germany (not too far from Frankfurt) a Sustainable Building with Green Roofs (Source:Meinhold, 2009)

Figure 6. Featuring more Green Roofs on Mixed-Used Development (Source: Meinhold, 2009)

Green Roofs Reduce Storm Water

The effect of green roof on the retention of storm water increases as information soil medium depth increases, up to a point. Probably little effect over 12 cm compared to increased building cost. (ii) As roof slope decreases (Snodgrass and McIntyre, 2010).

Green Roof can Mitigate Heat Island Effect

The annual mean air temperature of a city with 1 million people or more can be 1.8-5.4^oF (1-3^oC) warmer than its surrounding and in the evening (see Figure 7), the difference can be as high as 22^oF(12^oC) (USEPA 2011). On hot summer days, the surface temperature of a green roof can be cooler than the air temperature, whereas the surface of a conventional rooftop can be up to 90^oF (50^oC warmer) (Liu and Baskaran, 2003). Green roofs absorb heat and act as insulators for buildings, reducing energy needed to provided cooling and heating (USEPA, 2010a). It also, improved human health comfort by reducing heat transfer through building roof, can improve indoor comfort and lower heat stress associated with the heat waves (USEPA, 2010a).

Figure 7. Urban Heat Island Effect (Arch1design.com)

Pro and Con to use this Measure

Pros

Water retention: depending on their design green roofs can retain a significant amount of rain water
Substitute for lost areas of landscape, increase green space in cities.
Longer life for the roof membrane; due to less UV and less daily changes in temperature.
Marketing: A better surroundings gives higher rent, better marketing and better working environment.
Lower cost from a life-cycle standpoint.
Binding dust and toxic particles: Dust from the air are filtered
Improved noise protection: reduction of sound reflection and sound proofing.
Improved building thermal performance (insulation): Reduction in summer cooling needs and reduction in winter heat losses when a green roof is used. This reduced demand for energy would also mean a reduction of CO₂ production.
Reduction of the urban heat island effect
Biodiversity: refuge for wildlife and “red listed” species in urban areas
Intensive green roofs could include urban agriculture/horticulture, and bring food production close to people.

Cons

Higher initial cost
Maintenance costs: usually minor for extensive roofs, and as a garden for intensive roofs.
Restrictions involving climate and weather conditions (e.g.: rooftop gardens are inappropriate in very windy places; plants are fragile and can be blown away).
If “black listed” plant materials are used, new species can spread from roofs (no reference).
More costly repairs and fixings (finding and repairing eventual leaks is more expensive and difficult).
Plants can be unreliable and affected by the surrounding environment. Therefore their life length is uncertain and they require some maintenance.

Green Roofs in Asia

The Japanese government has therefore given green roofs a high priority at national and city level as a primary means to reducing building energy use (Arch1design 2008). In 1996, a technology transfer agreement was signed to bring the German green roof concept into Singapore, and assess its suitability to tropical conditions (Arch1design 2008). In 2003 the first large-scale extensive green roof was installed at HDB’s Pilot Project in Edgefield Plains, Punggol (Arch1design 2008). In Hong Kong Intensive Green Roofs are already well-established in the form of podium gardens as they provide what Hong Kong needs most – valuable functional open space for human use (Arch1design 2008). Extensive Green Roofs, on the other hand, are better-suited to retro-fitting projects which have their own technical constraints, and are not yet well-established in Hong Kong (Arch1design 2008).The following figures illustrate the application of Green Roofs and Green Buildings in Asia:

Figure 8. Green Roof in Japan (Source: Arch1design 2008)

Figure 9. Botanical Building in Seoul, South Korea (Source: Mobile Network 2011)

Figure 10. Botanical Building in Seoul, South Korea (Source: Mobile Network 2011)

Figure 11. Green Towers in Seoul, South Korea (Source: Mobile Network 2011)

Figure 12. Green Towers in Seoul, South Korea (Source: Mobile Network 2011)

Figure 13. Green Towers in Seoul, South Korea (Source: Mobile Network 2011)

Figure 14. Green Towers in Seoul, South Korea (Source: Mobile Network 2011)

Figure 15. Proposed Green Roofs Development in Seoul, South Korea (Source: Cleanairforcities.blogspot.com 2010)

References

Arch1design. 2008. Green Roofs in Tokyo, Japan, and Asia. http://arch1design.com/blog/eco-design/green-roofs-in-tokyo-japan-and-asia/ (Accessed on November 7, 2011).

Cleanairforcities.blogspot.com. 2010. An Enormous Green Roof for Seoul. http://cleanerairforcities.blogspot.com/search/label/%22Garak%20Wholesale%20Market%22 (Accessed on November 7, 2011).

Liu, K. and B. Baskaran. 2003. Thermal Performance of Green Roofs Through Field Evaluation (PDF) (11 pp, 401K). National Research Council of Canada. Report No. NRCC-46412.

Meinhold, B.. 2009. Rathaus Terraces: Mixed Used Development for Medieval German City.

Mobile Network. 2011. Covered Botanical Building. http://www.wayfaring.info/category/countries/south-korea/page/2/ (Accessed on November 7, 2011)

Pacwest Roofing. 2011. Vancouver Green Roofing. http://www.pacwestroofinginc.com/Rooftypes-Vancouver/Green-Roofing-Vancouver.html

Pasche, E. (2009). TUHH, Wasserbau. Interreg IVb, SAWA, Concept for a transnational Strategy in Developing a Flood Risk Management Plan. Interreg IVb, SAWA.

Prokop, G., H. Jobstmann and A. Schönbauer (2011). Overview of best practices for limiting soil sealing or mitigating its effects in EU-27 . Study contracted by the European Commission, DG Environment. http://ec.europa.eu/environment/soil/pdf/sealing/Soil%20sealing%20-%20Final%20Report.pdf

Snodgrass E.C. and L. McIntyre (2010). The green roof manual: a professional guide to design, Installation and Maintenance. Timber Press, Inc., Portland and London; 295 pages.

Swedish Board of Housing, Building and Planning. 2010. Mångfunktionella ytor, Klimatanpassning av befintlig bebyggd miljö I städer och tätorter genom grönstruktur, Kapitell 5.

U.S. Environmental Protection Agency. 2010a. Heat Island Effect and Green Roofs.

U.S. Environmental Protection Agency. 2010b. Reducing Urban Heat Islands: Compendium of Strategies.

U.S. ENvironmental Protection Agency. 2011. Heat Island Effect.