Climate change consequences such as increased precipitation and higher surface temperatures are imminent if these effects are not mitigated (Pachauri et al., 2014). Furthermore, since human populations are continuing to grow, resource demands per individual will also increase. Thus, larger urban communities will need to be developed (Gill et al., 2007). If these expansion trends transpire without effective sustainable habits by municipal governments and communities, urban expansion will occur at the expense of an ecologically-friendly future for subsequent generations. A proposed solution is to convert the roofs of urban buildings into ecological spaces, known as green roofs (Vijayaraghavan, 2016; Berardi, GhaffarianHoseini and GhaffarianHoseini, 2014; Rowe, 2011; Oberndorfer et al., 2007).
By general design, there appear to be two classifications of green roofs: extensive (Figure 1) and intensive (Figure 2) (Vijayaraghavan, 2016; Berardi, GhaffarianHoseini and GhaffarianHoseini, 2014; Rowe, 2011; Oberndorfer et al., 2007). It is apparent that extensive green roofs tend to require fewer resources and maintenance efforts (Table 1). For this reason, extensive green roofs are more commonly used.
Table 1. A comparison of defining characteristics between extensive versus intensive green roofs (Vijayaraghavan, 2016; Berardi, GhaffarianHoseini and GhaffarianHoseini, 2014; Rowe, 2011; Oberndorfer et al., 2007).
| Characteristic | Extensive | Intensive |
| Substrate layer thickness | <20 cm | 20 to 200 cm |
| Accessibility | Inaccessible (due to fragile root systems) | Accessible (for public recreational purposes) |
| Total Weight | 60 to 150 kg/m² | >300 kg/m² |
| Plant diversity | Low (typically moss, herbs, and grass) | High (lawn or perennials, shrubs, and trees) |
| Construction | Relatively simple | Relatively complex |
| Irrigation | Usually unnecessary | Requires drainage and irrigation systems |
| Maintenance | Simple | Complicated |
| Cost | Low | High |
There are three main components of green roofs: vegetation, growth substrate, and membranes (Figure 3). The diversity of plant species composing the vegetation layer depends on the depth of the substrate, local climatic conditions, and ability to transpire or store water (Vijayaraghavan, 2016; Oberndorfer et al., 2007). Sedum species is the most common plant used due to its ability to regulate excess water through evapotranspiration or storage (Vijayaraghavan, 2016; Berardi, GhaffarianHoseini and GhaffarianHoseini, 2014; Rowe, 2011; Oberndorfer et al., 2007). The growth substrate layer is comprised of a mix of several plant growth materials at particular ratios which influence water retention capabilities (Vijayaraghavan, 2016). The membranes of a green roof include the filter fabric, drainage material, root barrier, insulation, and waterproofing layer (Vijayaraghavan, 2016). All of which are specifically engineered to prevent leakage and damage to the building infrastructure below the green roof.
Due to the vegetation and substrate layers, water is retained in the plant stomata and in the pores of the substrate mixture. This capability to store excess water results in delayed peak flow and reduces the risks of flooding (Vijayaraghavan, 2016; Berardi, GhaffarianHoseini and GhaffarianHoseini, 2014; Rowe, 2011; Oberndorfer et al., 2007). Energy demands from buildings also decrease due to the vegetation layer’s ability to shade and protect the infrastructure from UV radiation (Vijayaraghavan, 2016; Rowe, 2011; Oberndorfer et al., 2007). The vegetation layer also provides a visually pleasing scene to improve mental health for the community (Vijayaraghavan, 2016; Rowe, 2011; Oberndorfer et al., 2007).
Since green roofs replace a partial proportion of the ecological space destroyed to construct that particular urban building, environmental remediation can potentially occur through stormwater management, decreased energy consumption, and increased city aesthetics. These effects alleviate impacts from extreme weather patterns like increased precipitation and surface temperatures. Overall, the implementation of green roofs in all urban communities will likely mitigate the consequences of climate change and urban expansion.
References
Berardi, U., GhaffarianHoseini, A. and GhaffarianHoseini, A., 2014. State-of-the-art analysis of the environmental benefits of green roofs. Applied Energy, 115, pp.411–428.
Gill, S.E., Handley, J.F., Ennos, A.R. and Pauleit, S., 2007. Adapting Cities for Climate Change: The Role of the Green Infrastructure. Built Environment, 33(1), pp.115–133.
Oberndorfer, E., Lundholm, J., Bass, B., Coffman, R.R., Doshi, H., Dunnett, N., Gaffin, S., Köhler, M., Liu, K.K.Y. and Rowe, B., 2007. Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services. BioScience, 57(10), pp.823–833.
Pachauri, R.K., Allen, M.R., Barros, V.R., Broome, J., Cramer, W., Christ, R., Church, J.A., Clarke, L., Dahe, Q. and Dasgupta, P., 2014. Climate change 2014: synthesis report. Contribution of Working Groups I, II and III to the fifth assessment report of the Intergovernmental Panel on Climate Change. IPCC.
Rowe, D.B., 2011. Green roofs as a means of pollution abatement. Environmental pollution, 159(8–9), pp.2100–2110.
Vijayaraghavan, K., 2016. Green roofs: A critical review on the role of components, benefits, limitations and trends. Renewable and Sustainable Energy Reviews, 57, pp.740–752.