Extensive Green Roofs Research Articles

Testing standard growth substrates for establishing native dry sandy grassland species on extensive green roofs in Northern Germany

Green roofs are known to mitigate the negative effects of urban consolidation by offering diverse ecosystem functions compared to non-vegetated roofs. However, the support for native biodiversity might be improved by using native plant species. In a mesocosm experiment, we studied the suitability of three commercial green-roof growth substrates for the establishment of 27 native plant species from dry sandy grasslands of northwestern Germany over the course of four years. The substrates were mineral-based, but differed in the layering of organic matter. Total establishment rates reached 44–59% in Year 4, indicating the general suitability of the substrates. During the first weeks after seeding, with light irrigation, the vascular plant cover was greater in the similar substrates Zincolit® Plus (Z) and Zincolit® Plus-Leicht (ZL) with their compost-based organic mulch layers than in the substrate Sedumteppich (ST) with its organic matter evenly admixed with the mineral aggregates. In Years 2 and 3, however, the vascular plant cover was greater in the ST substrate, likely due to the better availability of water and nutrients from the organic matter compared to the dry surface-mulch layer variants Z and ZL. After severe drought events, the decline in plant cover was more pronounced in the ST substrate, likely representing a trade-off between lush growth and a susceptibility to drought. An indicator-species analysis revealed differences in species composition between the ST and Z/ZL substrates. Annual plant species were indicators of the ST substrate. Perennials, such as Thymus pulegioides and Achillea millefolium, were typical of the Z and ZL substrates. In addition to the general suitability of the tested standard substrates for target species establishment, the study indicated that a combination of different layers of substrate components resulted in different vegetation patterns that may have a positive effect on green-roof biodiversity.

Life Cycle Analysis and Life Cycle Cost Analysis of green roofs in the Mediterranean climatic conditions

ABSTRACT The European policy as well as the national legislation framework set targets toward energy reduction and decarbonization in all sectors. Considering the building sector a number of regulations have been defined in order to ensure the energy as well as the environmental upgrade of the building stock. The paper’s main target is to evaluate from the environmental perspective the green roofs technology in Southern Europe. Therefore, a Life Cycle Assessment has been performed following the international standards of ISO 14040 series, supported by SimaPro Software. The aim is to define the proposed green roof system, analyze its environmental impact and evaluate the air pollutants levels related to the life cycle of its construction materials. A comprehensive review of similar roof applications is presented and results are compared with the ones regarding conventional roof typologies. The economic feasibility of the proposed roof types is assessed and evaluated based on a Life Cycle Cost Analysis. Thus, an integrated evaluation approach for the examined solutions is introduced, mainly focusing on Mediterranean climatic conditions. Moreover, a sensitivity analysis is conducted in order to define the impact of green roofs construction’s life span in terms of maintenance and renovation. As concerns the comparison of the suggested two roof types, and based on the sensitivity analysis the Traditional Horizontal Roof is proved to cause less environmental impact comparing to the Extensive Green Roof mainly because of the components use. Finally, the use of different insulation materials is examined and evaluated from an environmental point of view for the suggested typologies that are most suitable for the Mediterranean climatic conditions. More specific based on the sensitivity analysis’ results, and as far as global warming category, in both green technologies, levels have dropped by approximately 61% (from 2,13 to 1,52 kgCO2 and from 3,08 to 2,46 kgCO2, respectively) when recycled extruded polysterene is used in the construction stages for insulation.

An energy balance model of heterogeneous extensive green roofs

We study a two-layer energy balance model of an extensive green roof. The model represents the evolution of the temperature in both the vegetation and the substrate layers. We focus on the modeling and the numerical approximation of the energy balance on the vegetation cover and the substrate. One characteristic of the model is that the vegetation layer is heterogeneous, that is, some parameters describing the vegetation are time–space-dependent. An approximate solution of the model is obtained by means of a numerical scheme based on finite volume method. This method has made possible to solve the model without any linearization. This work also includes the validation of the model with experimental data. Different scenarios have been simulated to verify how changes in some parameters affect the energy balance. Chosen parameters are: LAI, vegetation height and soil humidity. As expected, looking at the numerical results of the simulations, we can conclude that if any of these parameters increases green roof temperature is reduced.

Experimental evaluation of the rainfall retention and inorganic pollutant mitigation effect by dual-layer and polyacrylamide-modified green roofs.

Research on substrate layer modification and filler configuration is an important direction for improving the retention and interception efficiency of green roofs. In this study, four green roof modules were established using peat soil, vermiculite, and zeolite as the main substrate layer. In addition, a polyacrylamide (PAM) modifier was added and mixed with these substrates. By simulating seven rainfall events, this study calculated and analyzed the outflow of each green roof as well as the average event mean concentration (EMC) and cumulative outflow quantity (COQ) of turbidity, ammonia nitrogen ([Formula: see text]), nitrate nitrogen ([Formula: see text]), total nitrogen (TN), and total phosphorus (TP). At the same time, the nitrogen mass balance of each green roof was analyzed. The experimental results showed that the interception capacity of the LP module was higher than the unmodified green roofs under heavy rain conditions. The modules also showed an improved capacity to inhibit the leaching of [Formula: see text]; therefore, TN was effectively suppressed. However, suppression of [Formula: see text] did not significantly improve. The outflow turbidity from the MP and LP modules was low and stable, and the TP concentration showed no apparent change. After the simulated rainfall experiment with the rainfall of 426.3 mm, the proportion of TN leaching out of the LP module was at its lowest (0.85%), and the residual proportion of TN reached 80.7%. Overall, the addition of PAM to the dual-substrate layer can better form the soil aggregate structure, to improve the retention and purification effect of the extensive green roofs.

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.

Evapotranspiration Measurement and Estimation of Crop Coefficient for Native Plant Species of Green Roof in the Tropics

Extensive green roof is one of the sustainable urban stormwater management alternatives to manage and mitigate the urban surface runoff. In order to implement green roofs more effectively, suitable plant species and substrate components for tropical climate must be identified. The aim of this study is to investigate the evapotranspiration (ET) behaviors in extensive green roofs based on different substrate types and local native plant species. Four green roof test beds containing pro-mixing pot and burn soils were each vegetated with Axonopus Compressus (grass) and Portulaca Grandiflora (sedum). A weather station with soil moisture sensors was installed to measure the weather and soil moisture data. The results showed that the mean ET rates for grass-pot soil, sedum-pot soil, grass-burn soil and sedum-burn soil were 1.32 ± 0.41 mm/day, 2.31 ± 0.72 mm/day, 1.47 ± 0.39 mm/day and 2.31 ± 0.43 mm/day, respectively. It is noted that environmental parameters such as ambient temperature, solar radiation and wind speed showed significantly positive relationship (p value < 0.01) with ET rates of green roofs except relative humidity. The crop coefficients (Ks) for the studied green roof plant species are estimated based on actual and reference evapotranspiration rates. The sedum planted in burn soil showed the highest crop coefficient (0.64), followed by sedum in pot soil (0.62), grass in burn soil (0.39) and grass in pot soils (0.37), respectively. The findings in this study also showed that substrate with better water retention capacity generally improved the Ks values.

Dynamic modelling of rainwater harvesting with green roofs in university buildings

University buildings may contribute significantly to the urban demand of potable water, particularly in small and medium communities with large universities. In this regard, alternative sources of water supply contribute to minimizing the pressure on natural resources. This paper investigates the applicability of rainwater harvesting (RWH) systems to partially compensate the daily non-potable water demand in university buildings. The traditional daily water balance model was improved to enhance the accuracy of the performance analysis and design optimization of the RWH system by accounting for consumption and runoff variability. The novel algorithm was tested using real daily water consumption data for two university buildings located in Portugal. In addition, different scenarios were evaluated combing RWH systems with extensive green roofs and different runoff distributions over time. The findings indicate that the use of real daily water consumption data instead of average values influences the final results, especially in buildings with fluctuating water consumption patterns over time up to 11%. Regarding the runoff coefficient, it was verified that variable values are recommended compared to the average values. Runoff variability associated with the installation of green roofs was found to decrease RWH system performance due to reduced stormwater runoff. However, the scenario with 50% of the catchment area covered by extensive green roofs showed promising results, decreasing the potential water saving less than 6% and increasing the volume of retained water by almost 15%.

Comparative environmental and economic evaluation of green roofs under Mediterranean climate conditions – Extensive green roofs a potentially preferable solution

Green roofs constitute a prominent urban green infrastructure solution and are considered to be able to enhance urban sustainability and resilience. Existing literature, however, lacks a holistic evaluation of their life cycle impacts. In this study, a comparative environmental and economic life cycle analysis between green and flat roofs under Mediterranean climate conditions was conducted, examining all green roof types (extensive, semi-intensive, intensive), two types of drainage layer (granular and synthetic) for each one, all green roof layer materials, life cycle stages (production of materials, use and maintenance, disposal) and their quantifiable costs and benefits, aiming to address this research gap. Dynamic whole-building energy demand simulations were also conducted in its context, which additionally investigated the effect of various factors on green roofs' energy saving potential. Thermally insulated green roofs provided a small improvement in the energy consumption for heating and cooling (up to 8.30% and 3.50% respectively) but significant reductions in total life cycle energy consumption (8–31%), CO2 emissions (24–32%) and waste production (15–60%) rendering them a cleaner environmental option compared to flat roofs. However, they also produced a very important increase in total life cycle water consumption (279–835%). The extensive green roof types were the ones to perform better in all these indexes. They were also the only type to constitute a better economic choice than a flat roof for the private owner in a low discount rate (0.25%) scenario, both with or without public benefits. As this study shows, the green roofs' potential is not limited only to the improvement of the buildings’ energy efficiency, as numerous studies in the field of energy efficiency highlight, but compared to flat roofs, expand to reduce the embodied energy, greenhouse gas emissions and material waste. By means of environmental footprint, they are superior to flat roofs when only energy and emissions are considered, while in regions where water availability is a key environmental factor, this superiority is in question, together with their economic feasibility in countries where this technology is not financially supported.

Crassulacean plant succession over eight years on an unirrigated green roof in Beijing

The selection of plants on green roofs should be tailored to the local climate. However, little is known about the long-term performance of plants in a Dwa climate (in the Köppen climate classification). This study follows the performance of 10 Crassulacean species (four species of Phedimus and six of Sedum) grown at four substrate depths (5, 10, 15, and 20 cm) over eight years on an unirrigated roof in Beijing, China. We found that the climate in Beijing, particularly the specific distribution of precipitation, had a substantial influence on plant growth and the community dynamics on extensive green roofs, and the dry and cold winters were the main factors that affected the plant survival on unirrigated green roofs. In such a climate, species with rootstocks or thickened fibrous roots performed better than species with entirely fibrous roots. Although deeper levels of substrate promoted better plant survival and performance, the species richness at all four depths continued to decrease. By the end of this research, only Phedimus aizoon and Phedimus floriferus ‘Weihenstephaner Gold’ still survived, and Phedimus aizoon was the only species that was alive at all of the four substrate depths. This research emphasized the importance of long-term research. The results of this study also revealed that the substrate moisture content was a more important factor than the substrate depth on the survival of plants over winter. To increase the rate of survival of plants on extensive green roofs over winter in Beijing, we advise the provision of supplementary irrigation before winter in years when the plants have experienced a dry autumn.

Stormwater retention and detention performance of green roofs with different substrates: Observational data and hydrological simulations

Green roofs are widely considered as a promising nature-based solution for urban stormwater management. In this study, the stormwater retention and detention performance of 6 green roof modules with different types and depth of substrates at Beijing, China was investigated through 3-year continuous monitoring. The Hydrus-1D was applied to further explore the stormwater management performance of green roofs under extreme storms. The average event-based stormwater retention and detention rates of the green roofs with 10 cm substrates ranged between 81% and 87%, and 83%–87%, respectively; and the average time delays in runoff generation and peak discharge ranged between 82 and 210 min, and 63–131 min, respectively. Green roofs with 15 cm depth of substrates offered higher stormwater retention and peak runoff attenuation rates than those with 10 cm substrates. However, due to the high frequency (55 out of total 92) of light rainfall events (<10 mm) and short antecedent dry weather periods (3.8 days in average), no significant difference was found on stormwater control performance of those green roofs. The Hydrus-1D simulations revealed that green roof stormwater retention rate decreases exponentially with return periods of extreme storms but increases with substrate depth. There exists a critical depth of substrates and further increases in substrate depth beyond this critical value could not bring much improvement in stormwater retention performance of green roofs. The application of extensive green roofs with 10–15 cm substrates provides promising stormwater retention and detention performance in highly urbanized area of Beijing.

Water use efficiency, growth and anatomic-physiological parameters of Mediterranean xerophytes as affected by substrate and irrigation on a green roof

The water use efficiency, growth and anatomic-physiological parameters of Mediterranean medicinal xerophytes on a green roof was investigated, under the effect of water stress and different types of substrate. Rooted cuttings of Convolvulus cneorum, Origanum dictamnus, Sideritis athoa, Atriplex halimus and Lomelosia cretica were planted on a roof in Athens, Greece. Two substrate types, 10 cm deep, were used, i.e. grape marc compost: perlite: soil: pumice (3:3:2:2, v/v) and a lighter one, i.e. grape marc compost: perlite: pumice (3:3:4, v/v). Two irrigation frequencies were applied during the dry period (summer), normal and sparse, when substrate moisture was 17-20% and 5-11%, respectively. Water use efficiency was increased by sparse irrigation in A. halimus and L. cretica and by soil substrate in C. cneorum. Sparse irrigation reduced dry weight in O. dictamnus, S. athoa and L. cretica and in A. halimus in combination with soilless substrate. It increased leaf thickness in all species and reduced Chlολ in all species, other than S. athoa, while in C. cneorum, O. dictamnus and L. cretica this was evident only in the presence of soil in the substrate. In summer, one d before irrigation, stomatal resistance (Rleaf) was increased and maximum quantum yield of PSII (ΦPSIIο) decreased indicating water limitation, while both parameters were restored to normal levels one d after irrigation indicating that both can serve as sensitive indicators of the onset of water stress. ΦPSIIο values, never fell to critical levels, excluding irreversible damage to the photosynthetic apparatus. Therefore, all five plant species are recommended for use on extensive green roofs in semi-arid areas.

Plant selection for green roofs and their impact on carbon sequestration and the building carbon footprint

One of the most critical determinants of a green roof's performance is the type of vegetation planted on its surface. This study examines the factors influencing the choice of plant for green roofing, such as sunlight requirements, water requirements, and cold tolerance, in order to identify the preferred green roof plants for use in cold and dry climates such as Mashhad, Iran, and to provide a roadmap to assist decision making in this regard. For this purpose, initially, fifty different plant species were evaluated from four perspectives: (i) applicability in extensive green roofs, (ii) photosynthesis rate, (iii) availability, (iv) low cost. Then, green roofs with selected vegetation were used in a field test, and the amount of carbon uptake of each of them was measured over one year. Finally, by modeling these green roofs in Design Builder software, the reduction of building energy consumption was evaluated to comprehensively investigate the overall impact of green roofing on the carbon footprint of the building. This study found that the best plants for the climate experienced in the field test are Sedum acre, Frankenia thymifolia, and Vinca major, which enjoy good tolerance and performance characteristics and offer the best energy demand and carbon emission. Green roofs with these three plants could reduce a typical building's annual energy consumption by 8.5%, 8.0%, and 7.1%, respectively. After implementing a green roof with Sedum acre, Frankenia thymifolia, and Vinca major atop a 4-story building and measuring these plants' dry weight monthly over one year, the annual CO2 absorption of these plants through photosynthesis was estimated to be 0.14, 2.07, and 0.61 kg/m2. In addition to absorbing carbon through photosynthesis, the green roofs with these plants also reduced the building's CO2 emissions by 28.16, 26.48, and 23.44 kg/m2 respectively, by reducing the energy demand.

Comparative-experimental study of different roof types in a tropical climate

Introduction: The implementation of green roofs in buildings has become increasingly common, because it is an ecological construction alternative that provides great benefits, which range from reducing air pollution to improving air conditioning inside of the buildings. Objective: This article aims to evaluate whether extensive green roofs are an energy-valuable option in a tropical climate compared to a thermoacoustic, clay tile, and concrete roof. Methodology: An experimental study was carried out in summer (December) in a tropical climate, where the internal temperature of two identical small buildings was compared except for the roof type. Both small buildings were built with a low construction standard according to buildings of families with low income in this region. Data of internal temperatures and other climatic conditions were analyzed to compare the influence of the roof type on the inner shelter temperature. Results: In these experiments, the average inner shelter temperature of green roofs was lower than the same temperature for thermoacoustic, clay tile and concrete roof and resulted in average temperature differences of 1.8 °C, 0.9 °C and 1.5 °C, respectively (depending on environmental conditions). Conclusions: It was possible to show that the green roof has a greater thermal inertia compared to the others. However, the temperature difference between the green roof and the other roof types was in the same range as in studies conducted in other climatic regions, therefore no special benefit of the green roof has been derived in a warm-tropical climate in this study. However, the low building standard can be of great interest for a final evaluation and the authors recommend to conduct further studies.

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.

Outdoor thermal performance of green roofs across multiple time scales: A case study in subtropical China

In recent years, roof greening technologies have been developed and implemented worldwide, as green roofs are an effective nature-based solution for alleviating outdoor heat and reducing building energy costs. While most observational green roof studies have investigated the cooling effect of model or test plots, few studies have conducted full-scale measurements; however, to guide green roof implementation, the thermal performance of full-scale roofs is crucial. This experimental study explored the ability of extensive green roofs to reduce outdoor temperatures on multiple time scales. The outdoor cooling effect of green roofs and the main factors that drive its performance were analyzed using long-term observational data collected from May 2016 to April 2019 in Nanjing, China. The results suggest that the cooling effect of green roofs exhibits significant diurnal, seasonal, annual, and vertical trends, and that the cooling performance across different time scales corresponds to weather and soil characteristics. The best cooling effect occurred at a height of 60 cm. The green roof displayed a temporary diurnal warming effect but had a good cooling effect on a seasonal time scale, with the largest amount of cooling occurring during the summer (average of 0.28 °C). The findings of this study can support the development, management, and maintenance of green roofs in subtropical areas.

The thermal performance of a green roof on a highly insulated building in a sub-arctic climate

Green roofs are complex systems, with a vegetation layer covering the outermost surface of the building shell. An effective design may confer environmental and energy benefits. Most field studies evaluating green roof performance have been conducted in warmer climates with few studies of full-scale green roofs in cold regions. No study has so far evaluated the energy performance of a green roof in a sub-arctic climate. This study demonstrates the heat flow and thermal effect of an extensive green roof versus a black bare roof area on a highly insulated building in the sub-arctic town of Kiruna, Sweden, for the period from November 2016 to February 2018. Measured temperature and heat flux values were consistently higher and more variable for the black roof than the green roof, except during the snow-covered winter months when the responses were similar. The cumulative heat flux showed that the net heat loss was greater through the black than the green roof, but the values remained low. Overall, the study confirms that the energy benefit of a green roof on a highly insulated building in a subarctic climate is low.

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.

The seasonal microclimate trends of a large scale extensive green roof

At nearly 27,500 m2, the Jacob K. Javits Convention Center (JJCC) located in New York City, hosts one of the largest extensive green roofs in the United States. This paper explores three years of fine scale microclimate data collected at the JJCC green roof and its potential ability to reduce the urban heat island intensity (UHII). Surface energy fluxes and microclimate parameters on four different surfaces are analyzed before and after installation of the southern section of the green roof, offering a unique before/after, test/control study. The results indicate that the temperatures of the air above the green roof, and its exterior surface are different (e.g. lower) than those measured above and on, respectively, the black roof that preceded it. Differences in the maximum daytime air and surface temperature between the black and green roof were 1.80 °C and 18.4 °C, respectively. Installation of the green roof increased evapotranspiration, modifying the roof’s surface energy balance, and reduced the median summer nighttime UHII (compared to the pedestrian level station) by 0.91 °C. Though microclimatic conditions on two sections of the green roofs vary somewhat, the research findings generally support the statement that green roofs are a moderately effective strategy for mitigating the UHI effect.

Sustainable Urban Greening and Cooling Strategies for Thermal Comfort at Pedestrian Level

The increase of the urban warming phenomenon all over the world is gaining increasing attention from scientists as well as planners and policymakers due to its adverse effects on energy consumption, health, wellbeing, and air pollution. The protection of urban areas from the outdoor warming phenomenon is one of the challenges that policy and governments have to tackle as soon as possible and in the best possible way. Among the urban heat island mitigation techniques, cool materials and urban greening are identified as the most effective solutions in reducing the urban warming phenomenon. The effects produced by the adoption of cool materials and urban forestation on the urban microclimate were investigated through a computational fluid-dynamic (CFD) model. The CFD model was calibrated and validated thanks to experimental surveys within the Catania University campus area. The urban microclimate thermal comfort analysis and assessment were carried out with the Klima–Michel Model (KMM) and Munich Energy Balance Model for Individuals (MEMI). In particular, three scenarios were performed: cool, low, and high levels of urban greening. The cool scenario, although it produces air temperature at around 1.00 °C, determines the worst condition of outdoor thermal comfort, especially at the pedestrian level. On the contrary, a high level of urban greening, obtained by the extensive green roofs together with an urban forestation, guarantees the wellbeing of pedestrians, showing more convenient values of PMV and PET.

Hydrologic Performance of an Extensive Green Roof under Intense Rain Events: Results from a Rain-Chamber Simulation

The water storage capacity of a green roof generates several benefits for the building conterminous environment. The hydrologic performance is conventionally expressed by the runoff coefficient, according to international standards and guidelines. The runoff coefficient is a dimensionless number and defines the water retention performance over a long period. At the scale of single rain events, characterized by varying intensity and duration, the reaction of the green roof is scarcely investigated. The purpose of this study is to highlight how an extensive green roof—having a supposed minimum water performance, compared to an intensive one—responds to real and repetitive rain events, simulated in a rain chamber with controlled rain and runoff data. The experiment provides, through cumulative curve graphs, the behavior of the green roof sample during four rainy days. The simulated rain events are based on a statistical study (summarized in the paper) of 25 years of rain data for a specific location in North Italy characterized by an average rain/year of 1100 mm. The results prove the active response of the substrate, although thin and mineral, and quick draining, in terms of water retention and detention during intense rain events. The study raises questions about how to better express the water performance of green roofs.