Green roofs are technological solutions for increasing greening within the built urban environment while offering numerous environmental and aesthetic advantages (Dunnett and Kingsbury, 2010; Getter and Rowe, 2006; Spronken-Smith and Oke, 1998; Takebayashi and Moriyama, 2009). However, several of these advantages, such as storm water management and amelioration of the urban heat island effect, are expected to occur only if green roofs are implemented in adequately large urban scales (Akbari et al., 2001; Getter and Rowe, 2006). In addition, the positive impact of urban greening is expected to have significant ramifications when it participates in a green network, especially when it is interconnected with neighboring suburban green bodies such as forests, mountains, and hillslopes. The creation of an elevated urban greenway system through green roof construction and networking demands broad applications over extended surfaces of existing city buildings.
Thus, in an effort to improve the environment of contemporary cities, it is mandatory to construct green roof systems on existing building rooftops. Unfortunately, in most cases existing buildings are aged with minimal load-bearing capacity. In such cases, extensive green roof systems are the only option since their weight (90–150 kg·m−2) is tolerated by most existing building frameworks. Extensive green roofs are characterized by minimal substrate depth (25–150 mm) and are planted either with succulent or herbaceous plants that demand minimal maintenance. An alternative solution, especially for semiarid and arid climates, is the use of adaptive green roofs (Kotsiris et al., 2013; Ntoulas et al., 2013a) that use a minimal substrate depth in conjunction with drought-tolerant plants and prudent irrigation applications.
Since, it is acknowledged that the heaviest part of an extensive green roof system is the growing substrate (Scrivens, 1990), selection of appropriate lightweight materials is of utmost interest. However, apart from the weight factor, substrates must provide sustainable plant growth under the harsh and extreme conditions of a green roof that involve increased solar radiation, wind drifts, and occasionally significant shading from adjacent buildings. Thus, green roof substrates must fulfill several criteria, such as maintaining adequate moisture for plant growth, facilitating the quick removal of excess water, providing support and anchoring for the plants, providing nutrients, and possessing a pH and an EC appropriate for plant growth. In addition, green roof substrates must be able to withstand wind erosion, as wind drifts are a significant factor on building roofs (Beattie and Berghage, 2004). The materials that have customarily constituted green roof substrate mixtures are mainly of inorganic nature (Beattie and Berghage, 2004; Thuring et al., 2010) such as calcined clay, Zeo (Nektarios et al., 2011a, 2015), perlite (Kotsiris et al., 2012, 2013), sand, heat-expanded shale (Rowe et al., 2006), heat-expanded slate (Olszewski et al., 2010), Pum (Ntoulas et al., 2012, 2013b), and lava (Nektarios et al., 2003; Tsiotsiopoulou et al., 2003).
Organic substances, such as peat and composts, have also been used, but at a smaller percentage to prevent substrate subsidence due to decomposition (Williams et al., 2010). Given the effort to minimize the negative environmental impact of the horticultural use of peat (Kotsiris et al., 2012) regarding both peatland degradation and total carbon dioxide (CO2) emissions, composts can offer an alternative choice as green roof organic constituents (Nektarios et al., 2011a; Ntoulas et al., 2012, 2013a, 2013b).
Beattie and Berghage (2004) indicated that an extensive green roof growth substrate must consist primarily of inorganic materials, while large quantities of composted or other organic substances should be avoided. Rowe et al. (2006) evaluated the use of heat-expanded shale in the establishment, growth, and survival of several stonecrops (Sedum sp.) and native wild plants. The researchers evaluated 60%, 70%, 80%, 90%, and 100% of heat-expanded shale participation in the substrate mix in conjunction with varying proportions of sand, peat, and compost. They concluded that the substrates with higher shale contents resulted in slightly reduced plant growth and lower visual quality characteristics, irrespective of the plant species studied, whereas moderately high quantities of heat-expanded shale (80%) did not have any negative effects on plant growth and reduced the load weight of the structure. Thuring et al. (2010) evaluated the effects of expanded clay and shale amended with spent mushroom compost on the growth and dry weight of five succulent and herbaceous plants. Despite some individual plant responses, they found that plants grew better in expanded clay compared with expanded shale because of its enhanced moisture- and nutrient-holding capacity. This was especially profound during the drought-stress periods.
In addition to the physical and chemical characteristics of a green roof substrate, an environmental parameter needs to be taken into consideration. More specifically, an issue that has only superficially been investigated concerns the fact that green roof substrates must be environmentally friendly either by absorbing potential pollutants or by having minimal pollutant leaching capacity themselves. To date, only sparse leaching studies have investigated the leaching potential of green roof systems mainly after the application of fertilizers and pesticides. Nikologianni et al. (2009) reported that metalaxyl-m leaching was enhanced in intensive type substrates without organic fraction or amended with reduced organic portion. Vijayaraghavan et al. (2012) reported that green roof systems were prone to NO3−-N and phosphate leaching. Czemiel Berndtsson et al. (2008) reported that ammonium (NH4+-N) and NO3−-N, phosphorus (P), zinc (Zn), and copper (Cu) exhibited increased concentrations in the leachate of the first runoff (first flush effect) compared with latter runoff events.
Apart from the technical aspects of green roof substrate formulation, financial issues are also of major importance. Under the current economic restraints, it is obligatory to reduce construction costs if green roofs are to be widely applied. Substrates are considered as one of the most expensive parts of a green roof system reaching a proportion of 40% to 50% of the total construction cost depending on the applied depth in each case. Thus, it is necessary to investigate the potential of using locally available materials that will be suitable as green roof substrate constituents. The materials should be inexpensive, should have the capacity to conform to existing guidelines, and should possess characteristics that will improve the capacities of substrates currently available in the market.
Locally available inorganic materials include Pum, crushed bricks and tiles, TC, and Zeos. Pumice is a porous volcanic rock, which is chemically inert. It is created by volcanic action, and the porosity of the material results from the voids created by the entrapped vapors during the cooling process of lava. Because of its light weight, absorbency, and structural resistance, Pum has been used in numerous agricultural applications including green roof systems (Kotsiris et al., 2012, 2013; Nektarios et al., 2011a, 2015; Ntoulas et al., 2012, 2013a, 2013b).
Crushed tiles and bricks are deformed or defective residues from the masonry industry that have been crushed and sieved. Their basic constituent is kneaded clay soil that has been fire hardened or air-dried. They have been promoted in the green roof industry as recyclable materials that are eco-friendly, having the advantage of adding a terra-cotta color to the green roofs.
Clinoptilolite Zeos are hydrated aluminosilicates and characterized by high surface areas and high cation exchange capacities with a 5:1 silica to alumina ratio. Clinoptilolite Zeo has the capacity to absorb and retain NH4+-N and potassium (K) cations in its microtunneling structure, which are then provided to the plants in a slow-release mode of action (Huang and Petrovic, 1994).
Thermally treated clay is marketed as providing increased water-holding and cation exchange capacity. The exact nature of the clay as well as the treatment procedure fall under proprietary restrictions.
The aim of the present study was to evaluate different locally available materials that are capable of participating in an extensive green roof growth substrate according to existing guidelines either as stand-alone products or in various mixes. In addition, it was of major interest to evaluate the environmental behavior of the final mixes, such as their nitrate leaching potential at first flush.
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