CHAPTER 2: LITERATURE REVIEW
2.17. Layers on a typical green roof system
2.17.2 Drainage layers
The characteristics of green roofs make them capable of being water retaining structures.
As such, Bianchini and Hewage (2012), suggest that it is of utmost importance to provide empty spaces between layers to allow free movement of excess water from the green roof system. The provision of empty spaces reduces the risk of potential leakage onto a structure’s roof. Taking into account the density of water, any extra water on a structure’s roof adds significant weight that could possibly impact on the structural integrity of a structure; it is therefore essential to ensure a proper drainage system is in place in order to remain within the carrying capacity of a structure’s roof. Having an effective drainage system further protects the root barrier from taking on excess water. Excess water encourages root growth and could potentially damage the root barrier layer and the roof structure. Drainage layers vary based on the green roof system, prevailing weather conditions and the assembly of a structure’s roof. Research conducted by Bianchini and Hewage (2012), state that materials such as polyethylene and polypropylene, that are both light and thin, are favoured for use in extensive green roof systems; whereas polymer based materials are preferred in other green roof systems due to the ease of transportation and installation, high strength, durability and low cost of production that they offer. In practice, the polymer material is generally bonded to either one or both sides of a geotextile material in order to prevent the migration of particles from the growing media which can essentially block drainage paths. Bianchini and Hewage (2012), further suggest that depending on the type of green roof system utilised, the thickness of the drainage material can vary from 1 cm to 1.5 cm. Intensive green roofs are designed such that they are able to carry higher loads as compared to that of an extensive green roof system. As such, the drainage material on an intensive green roof system can be thicker and heavier.
The drainage layer itself is typically around 4 cm in thickness, as it comprises of the drainage material and natural filtration and drainage i.e. through the use of small pebbles or stones.
Green roof systems tend to absorb an amount of water that is sufficient enough to supply all the vegetation and still keep the soil layer moist. However, any amount of water over and above this limit could result in, as mentioned earlier, the depletion of oxygen and creation of anaerobic conditions in the green roof system. As such, this excess water must be diverted to drainage outlets (through- or subsurface) for release.
58 2.17.2.1 Stone aggregates
Stone aggregates such as crushed stone, gravel and river rock can be utilised for use in a drainage layer. To ensure consistent and reliable drainage, these materials need to be washed, cleaned and made free of fine particles. The washing and cleaning of stone aggregates are also carried out to prevent the fine material from entering and clogging the adjacent geotextile material. Furthermore, stone aggregate selected for use should stem from a parent material that is of sound strength and has good long-term properties.
(Scholz-Barth & Weiler, 2009) 2.17.2.2 Lightweight aggregates
Lightweight aggregates the likes of balled or expanded clay, expanded shale or slate, along with other ceramic based products, are favoured for use in industry as an alternative to stone aggregates. This comes as a result of the attribute of being lightweight that the aggregates possess. Furthermore, the consistency in size that these aggregates offer, allow them to be graded and selected for specific purposes i.e. from that of a desired rate of compaction, to a flow rate through the selected aggregate. However, lightweight aggregates prove to be much more expensive when compared to stone aggregates. On a positive note, the excess cost can be offset against the cost of implementing a similar stone aggregate drainage system, as this would involve increased costs for strengthening a structure to carry the stone aggregate.
2.17.2.3 Synthetic and composite drainage products
In cases where load and depth factors become restricting, thin profiled products such drainage mats or panels are favoured as compared to several millimetres of aggregate.
These drainage products further offer ease of installation, properties of being lightweight and the ability to perform functions such as water retention and aeration.
For green roof systems that have a shallow growing medium layer (extensive), drainage mats or panels may be adequate in providing drainage, provided that they fall within the minimum weight criterion that is needed to prevent uplift due to wind action (Scholz- Barth & Weiler, 2009). In addition, if drainage mats or panels are to be utilised they must be checked to ensure that they provide adequate retention capacity for a given system.
For green roof systems that are intensive by definition, drainage mats or panels should not be considered as being sufficient in providing adequate drainage but instead, should be seen as a component to supplement a drainage system. Depending on the composition
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of a green roof system, one can utilise drainage aggregates together with the drainage mats or panels, in order to achieve the desired drainage rate (Scholz-Barth & Weiler, 2009).
2.17.2.4 Drainage mats
Drainage mats can be utilised in both vertical and horizontal drainage applications. When used horizontally and in a thin profile, it proves to be most effective. Most drainage mats are manufactured with an attachment of some form of geotextile or filtration fabric. As such, the specification of each type varies and should be selected for use based on the green roof’s system requirements.
2.17.2.5 Drainage panels
Much like drainage mats, drainage panels can be used in either vertical or horizontal applications. However, the intended application of the panels will determine the fabrication method of choice i.e. reservoirs being depressed or raised. Drainage panels retain water in reservoirs called “cups” and disperse excess volumes of water through openings in the “cones”. When the reservoirs are depressed they serve to retain water until they can no longer do so causing the excess water to escape through the openings.
Conversely, when the reservoirs are raised essentially, they can no longer function as reservoirs and the water begins to disperse through the openings.
2.17.2.6 Drains
Drains form an essential part of any green roof system. The main function of drains is to direct any excess water from the surface and that of the subsurface into the main stormwater system (Scholz-Barth & Weiler, 2009).
In order to evaluate the suitability of drainage an assessment based on the following performance characteristics should be considered:
• Water storage capacity (L/m2)
• Filling volume (L/m2)
• Flow rate (L/s/m2)
• Weight (both dry and saturated) (Kg/m2)
• Compressive strength (kN/m2)
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