Green roofs are vegetated rooftops, also known as eco-roofs or living roofs (Carson et al., 2013), and are complex layered structures designed to operate as an ecosystem (Hopkins and Goodwin, 2011). Depending on the intended purpose of a green roof and the depth of the soil layer, green roofs can generally be classified into two categories: extensive and intensive. Extensive green roofs are featured by a thin substrate (< 200 mm deep); relatively lightweight; low cost;
inaccessibility for human recreational activities; and requiring minimal or no maintenance (Wolf and Lundholm, 2008, Czemiel Berndtsson et al., 2009, Hopkins and Goodwin, 2011).
As a result of the shallow substrate, periodic drought and rapid fluctuations in soil moisture has led to the key selection criteria of plants being short rooting, drought-tolerant and drought avoidant (Wolf and Lundholm, 2008). Plants planted on extensive green roofs, usually range from 50 mm to 130 mm in height (Hathaway et al., 2008), including low growing succulents, herbs, grasses and mosses (Czemiel Berndtsson et al., 2009). Extensive green roofs may be constructed in the three major ways: prefabricated vegetation mats, built-in-place (shot planting, seed sowing or spontaneous self-established vegetation), and modular tray system (Banting et al., 2005, Oberndorfer et al., 2007, Czemiel Berndtsson, 2010). Extensive vegetated roofs may be installed on sloped surfaces, with the slope angle as high as 45° (Mentens et al., 2006). The role played by extensive green roofs in urban environment is more functional than recreational.
In other words, installation of an extensive green roof may be finalized to mitigate urban stormwater related problems, improve roof runoff quality, provide thermal insulation, reduce noise, etc. (Hopkins and Goodwin, 2011). An example of extensive green roof installed in Melbourne, Australia is shown in Figure 3-1(a).
Intensive roofs are characterized by a much deeper soil layer supporting a more diverse plant community including ground covers, small trees and shrubs; requirement of regular maintenance such as weeding, fertilization and irrigation; high structural load; and more costly installation and maintenance (Czemiel Berndtsson et al., 2009, MacIvor and Lundholm, 2011).
They are often designed as roof gardens made accessible for human use (Hopkins and Goodwin, 2011). Intensive green roofs are typically installed on roofs with a slope of less than 10°
(Mentens et al., 2006). Figure 3-1(c) shows an intensive green roof in Sydney, Australia.
A third type of green roofs is recognized, namely simple-intensive (also called semi-intensive).
This category is a combination of extensive and intensive green roofs (FLL, 2008, Hopkins and Goodwin, 2011, Bianchini and Hewage, 2012), however the extensive type must represent 25% or less of the total green roof’s area (Bianchini and Hewage, 2012). It has greater biodiversity potential than an extensive green roof (Hopkins and Goodwin, 2011). It also requires frequent maintenance as intensive ones (Czemiel Berndtsson, 2010). A semi-intensive green roof constructed in Henderson, New Zealand is presented in Figure 3-1(b).
Being less costly, relatively lightweight and low maintenance, extensive green roofs have a wider application than the other two categories (Carson et al., 2013). As a result, quantifying the environmental benefits of extensive green roofs is often the focus of most research studies since the proof of such benefits can increase the application of extensive green roofs on existing buildings (MacIvor and Lundholm, 2011).
(a) (b) (c)
Figure 3-1 (a) An extensive green roof: THE VENNY located at Holland Park, Kensington Road, Melbourne, Victoria; (b) A semi-intensive green roof: WAITAKERE CENTRAL CIVIC CENTRE located at Henderson, New Zealand; (c) An intensive green roof: M CENTRAL located on Harris Street, Ultimo, NSW (Hopkins and Goodwin, 2011)
While there exist three different categories of green roofs, they share a similar structure (Hathaway et al., 2008). Materials, detailed design specifics, and installation methods for green
roofs may vary from site to site (Carson et al., 2013), partly due to the lack of internationally established guidelines for their design. A typical green roof (shown in Figure 3-2) consists of (in the construction sequence) a root barrier, a drainage layer, a filter fabric, a water retention mat, an engineered substrate layer, and a vegetation layer (Hathaway et al., 2008). Detailed information of each layer, such as the requirements, functions and materials, is described below.
The root barrier layer. This layer has a two-fold purpose: 1) provides a waterproofing membrane to the roof structure below it; 2) prevents plant roots from penetrating the roof structure (Lazzarin et al., 2005). Usually, it is a thin layer of low-density polyethylene or polypropylene (Bianchini and Hewage, 2012).
The drainage layer. All green roofs need good drainage since excessive water can either potentially cause root rot and prevent healthy plant growth, or encourage root growth which can block the drains and damage the root barrier and the roof structure. For extensive green roofs, the layer can be made up of a drainage board, or a drainage sheet. A drainage board has little tanks, which have holes at the bottom of each tank. Therefore, water is accumulated in the tanks and excessive water is drained through the holes (Lazzarin et al., 2005). A drainage sheet is a geotextile fabric with nylon coils attached on the underside (VanWoert et al., 2005).
Those should be made from light and thin materials, like polyethylene and polypropylene. For intensive green roofs, the layer can be made up of aggregates (Bianchini and Hewage, 2012, IZREAL, 2014).
The filter sheet. This layer permits the rainwater to pass, but stops the soil particles of the upper layer from being drained with rainwater and blocking the drainage layer (Lazzarin et al., 2005). Materials such as polymeric fibres (polyester, polyamide, polypropylene and acrylic fibres) or polyolefins are used to manufacture this layer (Bianchini and Hewage, 2012).
The water retention layer. The layer is used to retain some rainwater and nutrients for plant use during dry periods. It is a sheet made of mineral wool or polymeric fibres (VanWoert et al., 2005, Bianchini and Hewage, 2012). The selection of material and thickness of this layer is limited by the capacity of the building to bear the weight of water absorbed (Bianchini and Hewage, 2012).
The growing media layer. The substrate layer contributes to thermal performance and water runoff mitigation. It also supplies nutrients and water that plants need for their biological functions. In addition, it provides room for roots to settle and strengthen, and to withstand strong wind and other extreme weather conditions (Bianchini and Hewage, 2012). The substrate design of an extensive green roof requires a constant trade-off among water holding capacity, weight, hydraulic conductivity, oxygen diffusion, and supplying the plants with the needed nutrients and water (Hilten et al., 2008). The long-term stability of a substrate requires the substrate to resist decomposition and erosion caused by rainfall, wind and frost. In many cases, the proceeding requirements of a substrate have been achieved by limiting the amount of organic and fine materials in a substrate mix, as suggested by the German guidelines (FLL, 2008, Emilsson, 2008). As a result, green roof substrates usually have low nutrient exchange capacity because these processes occur on the surfaces of organic matter and fine particles (Brady and WEIL, 2008). The low nutrient exchange capacity of the substrates may have an impact on the quality of stormwater runoff and the long-term stability of the vegetation systems as the nutrients are constantly leaving the systems. However, most of the nutrient leaching occurs shortly after installation or after fertilization applications (Emilsson et al., 2007). Nagase and Dunnett (2011) advised that 10% organic matter in a substrate is optimal to sustain a stable growth of an extensive green roof regardless of water availability, whereas a substrate containing 50% organic matter encourages excessive growth in a well-watered environment.
The ideal range of porosity for a typical green roof substrate is between 0.25 and 0.75 (She and Pang, 2010). The substrate should have a low volumetric mass which may be 800–900 kg/m3 (Lazzarin et al., 2005). To maintain a good balance between weight and performance, the soil mix generally contains a larger proportion of porous minerals and a smaller proportion of organic materials (Bianchini and Hewage, 2012).
The vegetation layer. Plants are selected specifically for the intended substrate depth, substrate composites, and climate conditions.
Figure 3-2 A schematic representation of a typical green roof system showing its layered structure (Hopkins and Goodwin, 2011)