Green Roof Substrate Research Articles

Effect of rice husk addition on the physicochemical and hydrological properties on green roof substrates under subtropical climate conditions

Green roof substrates provide water, nutrients, and physical support to vegetation development, as well as being a key element in the hydrological performance of the system. In this regard, the use of aggregate components that improve physicochemical properties of the substrates optimises the ecosystem services provided by green roofs. This study demonstrates the effects of using rice husk as an aggregate material in engineered substrates for extensive green roofs. Sixteen different substrates compositions were tested with different proportions of natural (NRH) and carbonized (CRH) rice husk incorporated into a base mixture with fixed proportions of vermicompost, vermiculite, and construction waste. The study consists of two phases: i) laboratory analyses to assess the substrates' physicochemical properties, and ii) a pilot-scale field experiment to evaluate hydrological performance under real subtropical climate conditions and vegetation development using a monoculture planting of Sedum rupestre. The findings showed that CRH has the potential to improve some physicochemical properties, such as water holding capacity, bulk density, and porosity. Additionally, CRH substrates presented a slightly increased average retention rate (up to 7%) comparing to local topsoil. The average stormwater retention rate for all engineered substrates was 77.73%. Multiple linear regression models were satisfactorily fitted to observed data from the open field experiment allowing an estimation of potential substrates’ retention rates. Moreover, vegetation development was positively affected by adding CRH comparing to the NRH. Carbonized rice husk presents as a useful material for improving green roof substrates properties and could be a sustainable alternative for local agriculture waste management.

Influência do substrato dos telhados verdes na redução do escoamento superficial quando submetido às condições climáticas da Região Metropolitana do Recife

O processo de urbanização acarreta a diminuição de áreas permeáveis, gerando uma redução na taxa de infiltração e consequente aumento do escoamento das águas pluviais, intensificando a frequência e a magnitude dos alagamentos. Contudo, o uso de telhados verdes vem se mostrando uma prática eficiente na redução e retardo desse escoamento. Nesse sentido, buscou-se identificar a classe textural ótima para o substrato de um telhado verde submetido às condições climáticas da Região Metropolitana do Recife (RMR). Para isso, realizou-se a calibração e validação do modelo Hydrus-1D com uso do volume drenado por um protótipo de telhado verde. Em seguida foram feitas simulações do fluxo de água no solo com diferentes classes texturais. Após a validação foi verificada uma boa eficiência do modelo na representação dos valores observados. Por fim, as classes texturais que apresentaram as maiores capacidades de retenção média foram a franca arenosa, franca, franco siltosa e silte. Com isso, para região estudada, o substrato do telhado verde deve ser composto por uma dessas categorias de solo. Na falta das mesmas, devem ser empregadas a areia, areia franca, franco argilo siltosa ou franco argilosa.

Pretreatments of wheat straw for possibility use in maintenance-free compressed green roof substrates

Green roofs are a key measure used to alleviate the urban heat island effect and reduce stagnant water from urban runoff. However, the expensive substrate material and high construction cost restrict the large-scale application of green roofs. Wheat straw (WS) contains abundant nutrients needed for plant growth; however, burning this straw waste causes severe environmental pollution. In this study, six pretreatment methods, including mixed, soaked, hydrothermal, ultrasonic, microwave, and steam explosion, and three liquid media, including deionized water, acetic acid, and wood vinegar, were used to treat WS to improve its properties and decrease its environmental pollution effects. The results showed that, compared to other treatments, physical methods (microwave, ultrasonic) used in conjunction with wood vinegar could effectively break down the particle structure of WS and increase porosity. Most of the crystallinities were reduced by 1.45–6.66% compared to WS when using these methods, and the absolute values of the surface zeta potential were reduced by 4.224.67 mV. Additionally, the contents of macromolecular lignocellulose, which is difficult to decompose, were reduced by 3.6613.75%. Compared with the use of deionized water or acetic acid, wood vinegar reduced the compressive force of the WS substrates and the energy consumption of compression. Hydrophilic groups such as hydroxyl were introduced into the WS to increase the water absorption rate. Overall, the microwave-assisted wood vinegar pretreatment was an effective method for improving the WS properties for use as a potential alternative substrate material in green roof construction.

Plant growth of Atriplex portulacoides affected by irrigation amount and substrate type in an extensive green roof system

Extensive green roofs in coastal areas planted with halophytes can contribute to the species preservation of local salt marshes that are being destroyed and threatened by urban development and enhance the local landscape character of coastal areas. Additional, benefits of halophytes include drought tolerance and the ability to withstand saline conditions in coastal areas. Herein the plant growth of A. portulacoides a native Mediterranean halophyte established on an extensive green roof system that received the effects of two irrigation water amounts (100% ET0 and 75% ET0) defined by substrate hydraulic properties and three substrate types are presented. The duration of the experiment was 285 days. The substrates contained soil (S), pumice (Pum), pure sphagnum peat (P) and/or grape marc compost (C) in different volume ratios (15S:70Pum:15P, 15S:70Pum:15C and 15S:70Pum:7.5C:7.5P). The height, shoot canopy diameter, growth index, and ground cover percentage of plants were measured at regular intervals, and at the end of the experiment, both shoot and root dry weights were also measured. It was found that irrespective of substrate type, plant growth of A. portulacoides was healthy and similar in size for both irrigation treatments therefore with an aim to conserve water, the use of the 75% ET0 irrigation treatment is proposed. The use of grape marc compost in the substrate (15S:70Pum:15C) is also proposed as it improved the substrate's nutrient content increasing most of the measured plant growth parameters of A. portulacoides. The use of grape marc compost as an alternative organic material to peat in green roof substrates would also contribute to the preservation of wetlands.

Green Roof Enhancement on Buildings of the University of Applied Sciences in Neubrandenburg (Germany) in Times of Climate Change

The reduction in evaporative surfaces in cities is one driver for longer and hotter summers. Greening building surfaces can help to mitigate the loss of vegetated cover. Typical extensive green roof structures, such as sedum-based solutions, survive in dry periods, but how can green roofs be made to be more effective for the longer hot and dry periods to come? The research findings are based on continuous vegetation analytics of typical extensive green roofs over the past 20 years. -Survival of longer dry periods by fully adapted plants species with a focus on the fittest and best adapted species. -Additional technical and treatment solutions to support greater water storage in the media in dry periods and to support greater plant biomass/high biodiversity on the roofs by optimizing growing media with fertilizer to achieve higher evapotranspiration (short: ET) values. The main findings of this research: -The climate benefits of green roofs are associated with the quantity of phytomass. Selecting the right growing media is critical. -Typical extensive green roof substrates have poor nutrition levels. Fertilizer can significantly boost the ecological effects on CO2 fixation. -If the goal of the green roof is a highly biodiverse green roof, micro-structures are the right solution.

Performance of Two Seashore Paspalum (Paspalum vaginatum Sw.) Varieties Growing in Shallow Green Roof Substrate Depths and Irrigated with Seawater

The continuing decline in global drinking water reserves necessitates finding alternative water sources for turfgrass irrigation, especially in southern semi-arid Mediterranean countries. The aim of the present study was to evaluate the potential of using seawater for irrigating two varieties of seashore paspalum (Paspalum vaginatum Sw.), “Marina” and “Platinum ΤΕ”, growing in shallow green roof substrates, and to determine their recuperative capacity after the termination of the salt stress period. The greenhouse study comprised of 48 lysimeters equipped with extensive green roof layering. Treatments included: (i) two substrate depths (7.5 cm or 15 cm) and (ii) three seawater irrigation regimes (7 mm, 15 mm, or 45 mm every two days). Measurements included the determination of green turf cover (GTC) as well as the leaching fraction (LF) and leachate electrical conductivity (ECL) draining from the lysimeters. It was found that during the 46-d salt stress period, none of the seawater irrigation regimes managed to maintain acceptable GTC levels for both seashore paspalum varieties. Increasing the green roof substrate depth from 7.5 cm to 15 cm resulted in GTC improvement. During the recovery period, the use of potable water as irrigation source improved GTC levels. After 40 d the recovery was complete since GTC exceeded 90% in all treatments for both varieties. Regression curves correlating GTC response to ECL can be used to estimate the leaching requirements of turfgrasses grown in shallow green roof systems when irrigated with saline water.

Effects of shading and composition on green roof media temperature and moisture

Three of the primary functions of green roofs in urban areas are to delay rainwater runoff, moderate building temperatures, and ameliorate the urban heat island (UHI) effect. A major impediment to the survival of plants on an unirrigated extensive green roof (EGR) is the harsh rooftop environment, including high temperatures and limited water during dry periods. Factors that influence EGR thermal and hydrologic performance include the albedo (reflectivity) of the roof and the composition of the green roof substrate (growing media). In this study we used white, reflective shading structures and three different media formulations to evaluate EGR thermal and hydrologic performance in the Pacific Northwest, USA. Shading significantly reduced daytime mean and maximum EGR media temperatures and significantly increased nighttime mean and minimum temperatures, which may provide energy benefits to buildings. Mean media moisture was greater in shaded trays than in exposed (unshaded) trays but differences were not statistically significant. Warmer nighttime media temperatures and lack of dew formation in shaded trays may have partially compensated for greater daytime evaporation from exposed trays. Media composition did not significantly influence media temperature or moisture. Results of this study suggest that adding shade structures to green roofs will combine thermal, hydrologic, and ecological benefits, and help achieve temperature and light regimes that allow for greater plant diversity on EGRs.

Effects of substrate depth and native plants on green roof thermal performance in South-East Australia

Three experimental green roofs in Melbourne with depth of 100, 150 and 300 mm have been assessed to quantify their thermal performance. To evaluate the benefit of substrate depth, temperature was recorded every 50 mm along a vertical profile. Green roofs consisted of scoria substrate and a mix of three species of plants: Lomandra longifolia, Dianella admixta and Stypandra glauca. Statistical analyses applying the hierarchical partitioning technique showed that solar radiation is the main driver affecting the green roof surface temperature, air temperature has strong correlations with the variations of the temperatures recorded below the surface, while moisture content has the least influence. Temperature profiles of the green roof show that the first 50 mm do reduce the heat flowing through the green roof substrate regardless the total green roof substrate depth. Differences in thermal performance arise at deeper points, where thicker green roofs are able to delay the change of substrate temperatures. Similar effects were found for the heat fluxes measured at the interface between the green roof and building roof. These results confirmed that green roofs may be used as a sustainable passive technology to reduce building energy consumptions for South-East Australia climate.

Impact of the Hydrogel Amendment and the Dry Period Duration on the Green Roof Retention Capacity

Abstract Climate changes as well as the urbanisation and economic development influence the characteristics of the stormwater runoff in the cities. The sealing of drainage basin surface leads to an increase of the runoff intensity, thereby decreasing the rainwater infiltration. This situation can lead to the risk of flooding in urban areas. Therefore, especially in great cities there is a need for application of such solutions that will support the operation of the sewage systems. The examples of such solutions are, among others, the green roofs. The paper presents the results of investigation of the water retention capacity of 4 green roof models containing following growing media: (1) the typical green roof substrate without any amendments, (2) the substrate with addition of about 1 % by weight of hydrogel (the cross-linked potassium polyacrylate), (3) the substrate containing about 0.25 % by weight of hydrogel, (4) the substrate with addition of expanded clay and perlite. The models were not vegetated in order to investigate only the water retention capacity of drainage elements and substrates. The water retention capacity of green roof models was investigated in the laboratory conditions with use of artificial precipitations simulated after diverse antecedent dry weather periods (ADWP) amounting to: 1, 2, 5, 7, and 12 days. The intensities of artificial precipitations were relatively high and ranged from 1.14 to 1.27 mm/min, whereas their durations ranged from 7.75 to 12.56 min. These values of intensities and durations corresponded to the design rainfall intensities calculated using Blaszczyk’s equation for annual rain depth equal to 600 mm and the return periods ranged from 5 to 15 years. The obtained results indicate that the water retention capacity of green roof models, expressed as the volumes (or depths) of rainwater retained within their structures, increases with an increase of ADWP. Results indicate that the relation between ADWP and the amount of water retained in the layers of green roofs in the case of relatively short antecedent dry weather periods provided for the analysis (from 1 to 7 days) may be approximately linear. The results of the one-way ANOVA indicate that in the case of all models there is a statistically significant difference between the values of retention depth for specified ADWP (p < 0.001). During more than half of simulated precipitations, especially in the case of longer ADWPs lasting 5, 7, and 12 days the best water retention capacity had Model 3, with substrate containing about 0.25 % by weight of hydrogel. On the other hand, the results show that the weakest retention capacity had Model 2 (with substrate containing 1 % by weight of hydrogel). In the case of longer ADWPs (lasting 7 and 12 days) relatively weak water retention capacity had Model 4 (with substrate containing the addition of expanded clay and perlite). It can be concluded that too large amount of hydrogel added to the substrate can have an unfavourable impact on the water retention capacity of green roofs.

Smart capacitive moisture sensor calibration in mineral wool and green roof soil substrate

The environmental benefits of green roofs have been widely recognized. In recent years, increasing attention has been paid to moisture management in the green roof systems. The moisture in the green roof has an influence on its thermal and hydrological performances. An accurate measurement of water content in green roof substrate is important for irrigation monitoring, optimal irrigation management, and plant growth. Knowing the performance and characteristics of the sensor for the chosen substrate layer in a green roof system is essential. This paper presents laboratory calibration of the capacitive moisture sensor in two types of the green roof substrate layer. The volumetric water content of several mineral wool and soil samples, with the water content from low until saturated, were measured using the gravimetric method and related to frequency obtained by the sensor. The results have shown that the capacitive moisture sensor has a good response to water content variation.

Effects of pretreatments on physical and chemical characteristics of wheat straw used as a maintenance-free compressed green roof substrate material

Green roofs can effectively reduce urban runoff and the urban heat island effect. To solve the problems of traditional green roof substrate, such as difficult construction and high maintenance costs, as well as the problem of environmental pollution caused by straw burning, this paper selects wheat straw as a raw material of substrate. Six modification methods (mixed, soaked, hydrothermal, ultrasonic, microwave, and steam explosion) and three liquid media (deionized water, acetic acid, and wood vinegar) were used to treat wheat straw to improve its physicochemical properties and water-absorption expansion performance after compression. The results showed that, in terms of chemical properties, the microwave treatment led to a lower nutrients content (TN = 3.97–5.00 mg·g−1; TP = 0.22–0.29 mg·g−1, and TK = 3.27–4.65 mg·g−1) but effectively reduced the electrical conductivity (= 0.26–0.67 mS cm−1) and ash content (= 7.38–7.51%) of the substrate compared with other groups, effectively reducing the salt toxicity to the plant and improving the utilization rate of organic matter. As a modification medium, wood vinegar could effectively reduce the pH (= 3.33–6.12) and ash content (= 7.38–11.12%) of wheat straw, which was conducive to maintaining the acidity of the substrate over a longer time period. Regarding physical properties, the microwave-assisted acid hydrothermal treatment was the most effective in improving porosity, especially aeration porosity (= 21.73%), which was beneficial to the respiration of plant roots. For the expansion performance of the substrate after compression, the wood vinegar group yielded the highest water-absorption expansion rate (= 151–224%), and the microwave group yielded the highest relaxation ratio (= 6.75–9.64), effectively helping the compressed substrate to restore its porosity and support plant root growth. Overall, the microwave-assisted wood vinegar treatment is a suitable modification method for wheat straw as a green roof substrate.

Whether the carbon emission from green roofs can be effectively mitigated by recycling waste building material as green roof substrate during five-year operation?

Green roof (GF) as an important role of urban ecosystem services is more and more focused on carbon sequestration for the mitigation of climate change, which there is still a gap of longer period of investigation on carbon sequestration on GF. This work aims to quantify the carbon sequestration on green roofs from 2012 to 2017 by measuring and calculating parameter on substrate organic carbon and plant organic carbon, when using waste building material substrate (WBMS) as GF substrate for the recycling of waste solid. Green roof group 2 (waste building material substrate (WBMS) as substrate) and green roof group 1 (local natural soil (LNS) as substrate), planting same three native plants (N. auriculata, L. spicata, and L. vicaryi), were both three substrate depth of 20cm, 25cm, and 30cm, respectively. Results show that both innovative WBMS and LNS were a great capability of carbon sequestration and carbon storage on green roofs. Carbon storage of green roof group 1 and green roof group 2 was 65.6kg Cm-2 and 72.6kg Cm-2, respectively. Annual mean carbon sequestration of the WBMS was 1.8 times higher than LNS. The overall average carbon sequestration (12.8kg Cm-2year-1) in green roof group 2 using WBMS was 1.1 times than corresponding in green roof group 1 (11.4kg Cm-2year-1 using LNS). WBMS substrate and L. vicaryi could be considered as the most adaptable green roof configuration, which can be a recommendation to promote the carbon sequestration and the function of green roof for the better urban ecosystem services. Future work may focus on the GF carbon model, water interface, long-term monitoring, environmental impact, water quality and quantity, synthesized effect on GF ecosystem, low impact development (LID), management and simulation, and combination on intelligent urban system, based on LCA.

Assessment of recycled or locally available materials as green roof substrates

Importance of green roofs are growing due to their several benefits in urban areas. To provide a sustainable green roof system and reduce the negative environmental effects of green roof construction, recycled or locally available materials plays a crucial role. A study was conducted to evaluate the potential of four recycled materials (crushed concrete, crushed bricks, sawdust, and municipal waste compost) and five locally available materials in Istanbul (lava rock, pumice, zeolite, perlite and sheep manure) as green roof substrates over one year period. Twelve-substrate mixture were prepared at the site by mixing six inorganic mixtures (crushed concrete, crushed bricks, lava rock, pumice, zeolite, perlite) with two organic amendments (municipal waste compost and sawdust-sheep manure mixture) with a ratio of 8:2 by volume. Specific measurements such as plant growth index, chlorophyll fluorescence, plant coverage ratio and survival of plant taxa were performed on five native plant species, Allium schoenoprasum, Helichrysum italicum, Sedum lydium, Stachys thirkei and Thymus vulgaris. At the end of the 58 week study period, pumice and perlite-based substrates amended with municipal compost outperformed remaining substrate mixtures in terms of plant growth, plant stress, chemical and physical properties. Moreover, performance of substrate mixtures consist of concrete, crushed bricks, lava rock, zeolite amended with sawdust and manure were adequate in some cases, which could be preferred as a green roof substrate by admitting suggestions offered in the study.

The importance of unsaturated hydraulic conductivity measurements for green roof detention modelling

Characterising the unsaturated hydraulic conductivity of a green roof substrate is essential for accurately modelling runoff detention in response to rainfall events. In this paper, the unsaturated hydraulic conductivities for four representative green roof substrates were determined in an infiltration column using steady state and transient techniques. The conventional Durner-Mualem Hydraulic Conductivity Function (HCF) model, for which parameters were calibrated based on the measured Soil Water Release Curve (SWRC) data, was shown to provide a poor fit to the experimental data. A new three-segment HCF was, therefore, proposed to fit measured unsaturated hydraulic conductivity data. Detention tests were carried out on 100 mm and 200 mm deep substrates using four simulated storm events. The runoff and moisture content data collected during the detention tests was used to validate the HCFs using the Richards Equation. The new three-segment HCF resulted in simulated runoff and moisture content profiles that closely matched the measured data (with mean Rt2 = 0.754 for modelled runoff), in contrast to predictions made using the conventional Durner-Mualem model (with mean Rt2 = 0.409 for modelled runoff). It was also demonstrated that further simplification of the HCF to a function defined by moisture content at just two points – the saturated hydraulic conductivity and at an unsaturated hydraulic conductivity of 0.1 cm/min – provides a model that is fit-for-purpose for green roof runoff estimation (with mean Rt2 = 0.629 for modelled runoff).

Novel application and reused materials for extensive green roof substrates and drainage layers in Japan – Plant growth and moisture uptake implementation –

This study investigates the potential utility of reused materials as an alternative material for commercial substrate and drainage layers. The performances of reused materials were assessed using turf with trays on a roof for two years in Chiba, Japan. Two types of materials, a commercial green roof substrate and cocopeat, were used as the substrate. For the drainage layers, five different materials were used: a commercial green roof drainage layer, bamboo stems and nodes, and polyethylene terephthalate (PET) bottle caps and bottoms. In addition, a sample without any drainage layer was used. The trays were prepared with the combination of these two substrates and six drainage applications. Overall, reused materials were observed to function well as commercial green roof materials. The turf in cocopeat was able to survive better than turf in commercial green roof substrate. In the drainage layers, some reused materials, such as the PET bottle caps and bamboo nodes, showed higher final plant coverage than commercial drainage layers in cocopeat. Our study has addressed the importance of realizing the original substrate and drainage layer using reused materials for developing sustainable and inexpensive green roofs.

Performance of six different soilless green roof substrates for the Mediterranean region

The objectives of this research were to assess the physical properties of six different growing substrate mixtures destined for roof gardens and determine the influence of these substrates on the morphology, physiology, growth and flower quality of pansy (Viola × wittrockiana), Madagascar periwinkle (Catharanthus roseus), and Pavia lily (Longiflorum×Asiatic lilies (Lilium) ‘Pavia’). Six green roof growing substrate mixtures (by volume) were used [T1: coarse tuff+fine tuff+cocopeat (5:1:4), T2:coarse tuff+medium tuff+fine tuff+cocopeat (5:5:2:8), T3: medium tuff+fine tuff+cocopeat (5:1:4), T4:medium tuff+fine tuff + peat moss (5:1:4), T5: perlite+medium tuff+fine tuff+cocopeat (5:5:2:8), and T6: lightweight expandable clay aggregates+fine tuff+cocopeat (5:1:4)]. The T6 (LECA-cocopeat) had the lowest weight at field capacity and good aeration and WHC to sustain optimal plant growth. Medium tuff-peat moss (T4) produced the highest pansy flowers number per plants. However, peat moss (T4) has been identified by environmentalists as an unsustainable media. LECA-cocopeat (T6) had a higher number of lily flowers and leaf area than T1-T4. The cost of the LECA substrate ($US 215 m−3, T6) is extremely higher than that of volcanic tuff ($US 36 m−3, T1-T4), and perlite ($US 100 m−3, T5). Overall, T6 can be an ideal option in terms of physical properties of growing substrate and flower quality but it might not be the best option for green roofs when cost is the primary concern. In addition, certain growing substrates could be used to satisfy specific growing requirements without sacrificing performance. For example, the combination of medium tuff and peat moss (T4) produced the highest pansy flower numbers per plant but required a longer period to flower (95 days). So, this growing substrate could be used where plentiful blooms are desired and the time to bloom is not a concern.

Green roof substrate physical properties differ between standard laboratory tests due to differences in compaction

Green roofs are expanding internationally due to the well documented benefits they provide for buildings and cities. This requires transferable knowledge of the technological aspects influencing green roof design, particularly substrate properties. However, this is made difficult due to differences in substrate testing methods referred to in green roof guidelines and standards. Therefore, we tested a green roof substrate using laboratory-based methods from European (FLL), North American (ASTM) and Australian (AS) green roof guidelines and standards to determine how these methods vary in characterising substrate physical properties (bulk density, water permeability and water holding capacity at field capacity (WHC)). Further, we compared the results from the laboratory-based methods with measures of bulk density and WHC in green roof platforms to determine whether standard methods accurately represent substrate properties in-situ. Results from the standard test methods varied due to differences in sample compaction. The standard test methods that employ Proctor hammer compaction (FLL and ASTM) had greater bulk density (at field capacity and dry) and lower water permeability than Australian standard methods that employ free-fall compaction. WHC did not differ among the standard methods. The Australian standard method better reflected bulk density at field capacity and WHC of the substrate under in-situ green roof conditions. For mineral based substrates, our results suggest that for the FLL and ASTM testing methods, a single Proctor hammer drop will produce a degree of sample compaction equivalent to the free-fall method (AS) and be more representative of bulk density in-situ. Subtle changes in testing procedures would allow for more direct comparison of substrate properties between standard methods and help enable the international transfer of knowledge for substrate design.

Phosphorus and Metals Leaching from Green Roof Substrates and Aggregates Used in Their Composition

Green roofs are constructions made of different layers, each serving a dedicated function. Substrates and materials used in their composition are essential from the point of view of rainwater retention and plant development, but they may have an adverse effect on runoff quality. Literature studies show that phosphorus and heavy metals are of main importance. The total roofs area covered with green increased in the last years in cities as they are efficient in retention of rainwater and delaying of the runoff, therefore, protecting the cities against floods. As green roofs filtrate a significant amount of rainwater, materials used in substrates composition should be carefully selected to protect urban receivers against pollution. The aim of this study was to assess phosphorus and heavy metals leaching from different green roof substrates and their components with the focus on green roof runoff quality. Both commercially made green roof substrates and often used compounds (construction aggregates) were tested in laboratory batch tests for P, Cu, Ni, Cd, and Zn content in extracts. Based on the results of this study, it could be emphasized that a large part of commonly used construction aggregates can be a source of phosphorus, some also can release elevated values of nickel. Therefore, the materials should be carefully tested before use in the green roof substrate composition, not only for their physical properties reflecting water retention capacity, but also for chemical composition.

Detention-based green roofs for stormwater management under extreme precipitation due to climate change

Abstract Rooftops cover a large percentage of land area in urban areas, which can potentially be used for stormwater purposes. Seeking adaptation strategies, there is an increasing interest in utilising green roofs for stormwater management. However, the impact of extreme rainfall on the hydrological performance of green roofs and their design implications remain challenging to quantify. In this study, a method was developed to assess the detention performance of a detention-based green roof (underlaid with 100 mm of expanded clay) for current and future climate conditions under extreme precipitation using an artificial rainfall generator. The green roof runoff was found to be more sensitive to the initial water content than the hyetograph shape. The green roof outperformed the black roof for performance indicators (time of concentration, centroid delay, T50 or peak attenuation). While the time of concentration for the reference black roof was within 5 minutes independently of rainfall intensity, for the green roof was extrapolated between 30 and 90 minutes with intensity from 0.8 to 2.5 mm/min. Adding a layer of expanded clay under the green roof substrate provided a significant improvement to the detention performance under extreme precipitation in current and future climate conditions.

Use of organic wastes to create lightweight green roof substrates with increased plant-available water

Substrate design is important for stormwater retention and plant survival on green roofs. Green roof substrates are mostly inorganic, providing long-term stability, while organic components (<20% v/v) reduce substrate weight and increase water retention, depending on rate and type. Selection of organic components depends on availability and organic waste materials can improve sustainability and reduce costs. We aimed to use organic waste materials to create lightweight substrates with good aeration and increased plant-available water (PAW) to support plant growth on green roofs. We evaluated the effects of five locally available organic waste components (coarse coir, fine coir, composted green waste, almond hull, and pistachio shell) on the physical and chemical properties of a scoria-based substrate when added at 0, 5, 10, 15, and 20% by volume. We also examined their phytotoxic effects on plant growth using lettuce (Lactuca sativa) as bioassay species. Overall, higher rates (15–20 % v/v) of coarse coir, fine coir and composted green waste all reduced substrate weight and improved PAW. Pistachio shells and almond hulls reduced saturated bulk density but adversely affected plant growth, likely due to phenolic compounds. For the other amendments, rates above 10–15 % addition also decreased root and shoot growth in lettuce. This indicates a potential trade-off between water availability and growth in these substrates. Therefore, if a green roof needs to be amended to reduce weight but have improved PAW, coarse coir, fine coir and composted green waste would be the best choices.