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| Fig. 1: The new British Horse Society HQ, which has a green roof. (Source: Wikimedia Commons) |
Many cities favor the replacement of natural greenery with urban-friendly infrastructure, such as buildings, sidewalks, and roads. To remedy this while preserving urbanization and maximizing the amount of usable surface area in cities is the introduction of green roofs, or living roofs. [1] As the name might suggest, green roofs take advantage of common unused space that almost every building has: roofs! Green roofs consist of layering plants and soil to rooftops not only to push urban cities in the direction of sustainability, but to increase aesthetics, allow for natural cooling, and roof protection. [2,3] An example of a green roof is provided in Fig. 1.
Green roofs are composed of vegetation, substrate (soil, filters, drainage fabrics), and insulation and waterproofing materials. [1,3,4] There are two types of green roofs, depending on the size and extent of the substrate layer: extensive roofs have shallow soil layer (<15 cm) and intensive roofs have deeper substrate layers (approximately 20-200 cm). [3,4] Fig. 2 depicts the layers of intensive and extensive green roofs.
While a deeper soil layer can accommodate more varieties of plants, such as shrubs and small trees, extensive roofs can only accommodate grasses, mosses, and succulents. [4] Extensive green roofs are typically cheaper to install and maintain than intensive roofs, which need to be irrigated and - depending on the buildings architecture - structurally supported.
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| Fig. 2: Diagram of extensive and intensive green roof designs. (Source: Wikimedia Commons) |
If properly maintained, green roofs can reduce the heat flux (amount of heat transferred per unit area per unit time) through the roof in the summer months, acting as an insulating layer. [2,4] Proper maintenance includes choosing good shadowing foliage (low radiation transmission of solar energy) and lighter soils (reduction in thermal conductivity, large field capacity). [2] It is demonstrated that green roofs provide many environmental benefits, including decreased building energy consumption, improved air and water quality, reduction of noise pollution, and reduced heat-island effect. [3]
The implementation of plants and grasses to cities helps to eliminate harmful air contaminants, however, the extent to which the air quality is improved varies based on the type of green roof implemented and the vegetation species - larger plants can capture more air pollutants than thin foliage (such as grasses, who have a small surface area). [4] Plants can filter these pollutants directly through their stomata structures or indirectly by decreasing the need for additional energy consumption - by cooling the ambient temperature, less energy (such as air conditioning and power consumption) is used by companies and industries. [5] Since intensive roofs allow for plants with more surface area to grow, they are superior to extensive roofs in collecting and filtering harmful air particulates.
The urban heat island (UHI) effect is the process in which densely populated cities experience higher temperatures than rural areas. [4] Green roofs can combat UHI by decreasing the surrounding air temperature and albedo of urban areas. [3] The warmer ambient temperature of UHIs is caused by the absorption of radiation from the sun by buildings. [4] An albedo is a numbered index (from 0 to 1) describing how reflective a surface is, with 0 being a perfect absorber and 1 being a perfect reflector. [4] Despite its popularity and prevalence among the literature, estimates of the albedo for green roofs varies drastically, ranging from 0.7-0.85 to no greater than 0.30, signaling that the albedo depends on a variety of variables and is not the most reliable variable for analysis. [1,4] For comparisons, the albedo of other roofing types is given in Table 1 (sourced from the GSA [4]). The albedo of green roofs can be further increased by adding high-albedo coatings to green roof structures. [6]
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| Table 1: Albedo values for various roof construction materials. [4] |
The main influence of water quality from green roofs comes primarily from runoff; in particular, through two ways: by increasing the amount of water retained by the roof and by slowing the flow of water from roof to sewer systems. [4] These two factors are dependent on the type of green roof (vegetation, substrate layers, drainage layers etc.). For example, in some green roofs, water retention can be improved by having a deeper substrate layer, as it can accommodate plants with higher evapotranspiration capabilities. [4] The drainage layer also influences how water retentive a green roof is. Common drainage materials include coarse aggregate materials (low transmissivity) and synthetic geocomposites, which store less water. [4]
In addition to water retention, green roofs also improve the quality of water runoff. [4] Some species of plants (sedum) have higher metal retention rates. [5] Substrate layers can also absorb water pollutants. One study compared the amount of ammonia nitrogen present in green roof runoff versus rainwater and found that green roofs decreased this percentage. [5] On the other hand, this same research group found that some green roofs retain more metals (depending on the season and climate) such as copper, iron, and zinc. [5]
There is also debate about how sustainable the materials used to construct green roofs are. Waste can be reduced by using recycled materials (wood, plastics, crushed bricks, glass) for the insulating, support, and substrate layers. [6,7] Numerous case studies on the implementation of recycled materials in green roof construction have been conducted, ranging from using recycled glass (which increases the water quality), crushed porcelain and foamed glass (neutralizes acid rain), crushed brick (increases number of species able to grow) and construction waste material (supports plant growth and helps control erosion). [8-13]
The implementation of green roofs in urban areas is heavily reliant on politics and local government policies. In particular, the expansion of green roofs is limited by both the construction and maintenance costs and is often referred to as "having long-term investments with short-term returns". [3] Initial costs of green roofs are relatively high and widely range based on the type of green roof (whether intensive or extensive, or a mix of the two) and local costs (labor, materials, equipment). [2]
Compared to regular roofs (which need infrequent maintenance), green roofs need to be constantly maintained: watering, fertilization, upkeep, etc. [3] This also limits the types of vegetation - plants with cheaper maintenance costs have lower environmental benefits and can change aesthetics.
Research on the lifetimes of green roofs is extremely limited, as there are many factors to consider. Cost-to-benefit analyses typically focus solely on the installation and energy savings costs while ignoring the benefits (for example, water and air quality enhancement). [5] A measure of how much an investment is worth over the course of its lifetime is the Net Present Value (NPV). The formula for how to calculate this is given in Teotoacute;nio et al. as [14]
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(1) |
where t indexes the year, Bt and Ct are the positive and the negative cash flows in year t, respectively, and r is the discount rate. The reported NPV of green roofs ranges, with some groups estimating 25% lower (based on a predicted 40-year lifespan) and others recording a 10-14% increase compared to traditional roofs. [3] The GSA breaks down the national NPV of green roofs compared to conventional (black) roofs for various roof sizes, as shown in Table 2. [4]
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| Table 2: Breakdown of NPV values for green roofs. [4] | |||||||||||||||||||||||||||||||
The reported total national NPV is small, yet positive, with the highest gains coming from stormwater and, as expected, the highest losses coming from installation and maintenance costs.
There is a very limited amount of research evaluating the influence of green roofs on real estate and the reportings heavily fluctuate. Table 2 reports that the returns on homes with green roofs are larger than those with conventional black roofs, about a $100 per square foot difference. [4] There is a reported increase in the property value of green-roofed homes, giving rise to incentives for building and property owners to invest. [6] In contrast, one independent study implementing hedonic regression modeling found that the rent in New York apartments with green roofs were 16% higher than regular-roofed apartment buildings. [15] Due to the lack in research, the conclusions drawn by these studies are not clearly understood and varies on a case-by-case basis.
© Madison Singleton. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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[2] E. Paloma Del Barrio, "Analysis of the Green Roofs Cooling Potential in Buildings," Energy Build. 27, 179 (1998).
[3] K. Vijayaraghavan, "Green Roofs: A Critical Review on the Role of Components, Benefits, Limitations and Trends," Renew. Sustain. Energy Rev. 57, 740 (2016).
[4] "The Benefits and Challenges of Green Roofs on Public and Commercial Buildings," U.S. General Services Administration, May 2011.
[5] M. Shafique, R. Kim, and M. Rafiq, "Green Roof Benefits, Opportunities and Challenges - A Review," Renew. Sustain. Energy Rev. 90, 757 (2018).
[6] D. Perivoliotis et al., "Sustainable Urban Environment Through Green Roofs: A Literature Review with Case Studies," Sustainability 15, 15976 (2023).
[7] S. Cascone, "Green Roof Design: State of the Art on Technology and Materials," Sustainability 11, 3020 (2019).
[8] C.-F. Chen, S.-F. Kang, and J.-H. Lin, "Effects of Recycled Glass and Different Substrate Materials on the Leachate Quality and Plant Growth of Green Roofs," Ecol. Eng. 112, 10 (2018).
[9] M. Eksi and D. B. Rowe, "Green Roof Substrates: Effect of Recycled Crushed Porcelain and Foamed Glass on Plant Growth and Water Retention," Urban For. Urban Green. 20, 81 (2016).
[10] J. M. Matlock and D. B. Rowe, "The Suitability of Crushed Porcelain and Foamed Glass as Alternatives to Heat-Expanded Shale in Green Roof Substrates: An Assessment of Plant Growth, Substrate Moisture, and Thermal Regulation," Ecol. Eng. 94, 244 (2016).
[11] C. J. Molineux et al., "Using Recycled Aggregates in Green Roof Substrates for Plant Diversity," Ecol. Eng. 82, 596 (2015).
[12] A. J. Bates et al., "Effects of Recycled Aggregate Growth Substrate on Green Roof Vegetation Development: A Six Year Experiment," Landsc. Urban Plan. 135, 22 (2015).
[13] S. B. Mickovski et al., "Laboratory Study on the Potential Use of Recycled Inert Construction Waste Material in the Substrate Mix For Extensive Green Roofs," Ecol. Eng. 61 C, 706 (2013).
[14] I. Teotónio, C. M. Silva, and C. O. Cruz, "Economics of Green Roofs and Green Walls: A Literature Review," Sustain. Cities Soc. 69, 102781 (2021).
[15] K. Ichihara and J. P. Cohen, "New York City Property Values: What Is the Impact of Green Roofs on Rental Pricing?" Lett. Spat. Resour. Sci. 4, 21 (2011).
