The Hydrological Performance of Lightweight Green Roofs Made From Recycled Waste Materials As the Drainage Layer

Green roofs can be used for promoting infiltration and provide temporary storage spaces. Hence, in urban stormwater structural design, the investigation of the hydrological performance investigation is often required. Thus, this paper presents the results of a hydrological investigation in term of peak flow reduction and green roof’s weight using 0, 2, and 6% slope for three specimens drainage layer in green roofs. Three types of recycled waste are selected for each test bed which is rubber crumbs, palm oil shell, and polyfoam. Another test bed without a drainage layer as a control. The result indicates that rubber crumbs can be used as a stormwater control and runoff reduction while ensuring a good drainage and aeration of the substrate and roofs. From the results obtained shows that rubber crumbs are suitable as a drainage layer and a proposed slope of 6% are suitable for lightweight green roofs.


Introduction
In Malaysia, sustainable issues have become critical issues.Also, even though the average rainfall in Malaysia per year is 3500 mm, problems in the stormwater such as flash flood, water shortage, and water pollution still occurs.As stated by Ayub et al. [1], the conventional method in stormwater management such as using the concrete drain in a new development area cannot fulfill the expectation in controlling the volume and quality of runoff water.So, a green roof is a solution for sustainable problems.For other developed countries, green roof applications are widely used.However, in Malaysia green roof can be considered the relatively new concept and the less of application since only a few buildings in Malaysia practice the green roof construction on buildings.
Due to some sustainable practices, a recycled waste material for a green roof is introduced for the drainage layer in the green roof.This drainage layer is important since it must able to retain water during raining and ensuring a good drainage system and the aeration of the top soil and plant roots.A roof slope is believed can affect the water retention capacity.Thus the paper presenting a green roof system are tested under three different slope which is 0, 2 and 6% slope.The weight of designing a green roofs system is necessary.Thus, the issues of the live load in wet and dry conditions are considered in determining the lightweight green roofs.
Objectives of this paper are to share the preliminary result found regarding the drainage layer used in green roofs using three different recycled waste materials for intensive green roofs in attenuation runoff and as lightweight green roofs in simulated rainfall.

Research background
As stated by Berndtsson [2], there are two types of green roofs which are intensive and extensive.The extensive green roof is the focus of this research due to the wide application and the weight which is lighter than the intensive green roof.
Green roof systems consist of consists of waterproofing layer, drainage layer (40 mm), a filter layer, substrate layer or a topsoil (50 mm) and growing medium [3].Extensive green roofs are different from intensive because it is using smaller plants, less in maintenance and are established with thin substrate layers.
Chow, Bakar and Deck [4] in their research mention that it have been proven that it green roofs can produce many benefits such as stormwater mitigation, biodiversity, and habitat, save energy, air quality, aesthetics and quality of life, increased roof membrane durability, noise reduction and providing new amenity spaces.
According to the survey done by Ismail et al. [5] in Malaysia there are some obstacles of the application of green roofs, and it was proven that several factors were identified.The factors are past failure, challenging and high cost, complicated and hard to maintain, limited local expertise, lack of scientific research, fear of unknown risk, higher cost of materials supply, no design standards and guidelines, and believe that green roof is susceptible to fire.
Research on using recycled materials utilized for a drainage layer in a green roof are done by some researcher due to some sustainable practices to be applied in a green roof's design.Perez et al. and Rincon et al. [6][7] in their research are testing the use of rubber crumbs as a drainage layer by focusing on thermal reduction and hydrological performance.This recycled waste material can contribute to reducing waste materials production thus lessen the use of raw materials.Shahid et al. [8] also in their research mention that palm oil clinker can be used as the drainage layer because it has an excellent ability to drain the excess water and no effect on plant development.Roof slope hence is necessary for green roofs.The green roof slope is one of the factors that can affect green roof water retention capacity and the runoff [2].
As stated by Chow et al. [4], a good knowledge of engineering is essential in designing a green roof since all the critical aspects of design must be included such as the plant selection, environmental at the region and the weight of the system itself.

Study area and methodology
The study carries out in hydraulics laboratory of Faculty of Engineering, Universiti Malaysia Sabah.Test beds are placed under the rainfall simulator as shown in Fig. 1.Table 1 shows three test bed represent different recycled waste and one test bed as a control bed.The weight of the green roofs for dry conditions are measured using floor scale on 7-day antecedent dry conditions of the green roofs while the weights for wet conditions are collected after rainfall simulations have ended.
The inflow from rainfall simulator are for 15 minutes continuously.The value for outflow was taken for one hour continuously after the simulator started with five minutes interval.Three types recycled waste materials are used as the drainage layer as shown in Fig. 2.

Peak runoff
Three types of drainage layer are used in the testing, and four simulated rainfall has been done.Fig. 3 show the peak flow comparison among drainage layer using recycled waste materials on 0, 2, and 6% slope.Rubber crumbs have the lowest peak flow in all experiments data with the different slope in the range 17.67 to 26.00 cm 3 /min.From the data recorded, the rubber crumbs have the highest flow reduction and able to attenuate peak flow.Second waste materials that have better hydrological performance in attenuation of peak flow is polyfoam.It gives the range of peak flow from 23.33 to 39.00 cm 3 /min.Palm oil shells show the highest peak flow compare to other waste.The maximum peak flow for this waste is 43.33 cm 3 /min whereas the lowest is 36.67 cm 3 /min.

Weight of green roofs
In this study, all wasted materials studied have different weights.However, the most effective drainage layer is which gives the lowest weight.Fig. 4 and 5 show the details result from this study.Polyfoam has the lighter weight with 3.09 kg/m 2 increased weight on 0% slope.After the slope are increased, the weight of sample increased for the wet weight (0% slope: 3.51 kg/m 2 , 2% slope: 1.53 kg/m 2 , 6% slope: 1.28 kg/m 2 ).Followed by rubber crumbs (0% slope: 3.51 kg/m 2 , 2% slope: 1.53 kg/m 2 , 6% slope: 1.28 kg/m 2 ).The heaviest waste materials is palm oil shells (0% slope: 28.75 kg/m 2 , 2% slope: 25.17 kg/m 2 , 6% slope: 22.95 kg/m 2 ).From the figure also can be seen that the weight of the sample decreases as the slope are increased by 2 and 6% in slope.As shown in the results are believed due to the higher slope used to help water discharge and retention in the green roofs are reduced.
According to Table 2, for both 2% and 6% slope of green roofs, polyfoam have the highest reduction in weight where more than half percent of reduction which are 56.44% for 2% slope and 63.37 for 6%.Then, followed by rubber crumbs (2% slope:15.91%,6% slope: 17.13%) and palm oil shells (2% slope:12.33%,6% slope: 20.07%), respectively.For all the recycled waste materials, 6% of slope have the higher reduction in weight.

Conclusions
Based on capabilities in peak flow attenuation and lighter green roofs, this study found the rubber crumbs suitable as green roof drainage layer beside this waste able to retain water when it rains, suitable for filtration while ensuring good drainage and aeration of the substrate and roots.The usage of rubber crumbs as waste materials in green roofs layer are suitable for stormwater management where it can reduce stormwater runoff, so there is less water directed into the storm drains.Second wastes material that is suggested for a green roof is polyfoam.The proposed slope for the green roofs is 6% since it ensures that large amounts of water are not retained on the roof, this implies a reduction in weight of the green roofs system.However, further details study for the green roofs with a combination of natural fibres with the same intensity of rainfall is needed.

Fig. 2 .
Fig.2.Drainage Layer used in the Green Roofs: A. Palm Oil Shell, B. Polyfoam, C. Rubber Crumbs.A B C

Table 1 .
Materials used for control specimen and drainage layer.

Table 2 .
Reduction in weight of green roofs for drainage layer