Conventional Green Roof Research Articles

A Field Study to Investigate the Hydrological Characteristics of Newly Established Biochar-Amended Green Roofs

Green roofs (GRs) have been researched for decades, yet their implementation remains constrained due to several reasons, including their limited appeal to policymakers and the public. Biochar, a carbon-rich material, has been recently introduced as an amendment to GR substrate to enhance the performance of GRs through reduced runoff volume, improved runoff quality, and increased soil fertility. This paper aims to investigate the impact of biochar amendment on the hydrological performance of newly established GRs. Six 1 m × 1 m GR test beds were constructed, comprising of five biochar-amended GR test beds, and one conventional test bed (without any biochar in its substrate). The water retention capacity and runoff outflow delay of the six test beds were studied with the application of artificial rainfall using a nozzle-based simulator. Biochar was found to increase the water retention capacity and effectively delay runoff outflow in the biochar-amended GRs. After nine artificial rainfall events of 110.7 mm rainfall in total, 39.7 to 58.9 L of runoff was retained by the biochar-amended GRs as compared to 37.9 L of runoff retained by the conventional GR. Additionally, the test bed without biochar quickly started releasing runoff after 300 to 750 s, whereas test beds with fine biochar particles could delay runoff outflow by 700 to 1100 s. The performance of the non-biochar and biochar-amended test beds varies according to the values of biochar-related variables such as biochar particle sizes, amendment rates, and application methods. The observational data illustrated that the GR test bed with medium biochar particles applied to the bottom layer of the GR substrate was the optimal biochar-GR design. This selection was determined by the combined performance of high retention rates, long runoff outflow delays, and few other factors, such as lesser loss of biochar caused by wind and/or water.

Eco-friendly green roof from biodegradable substrate for stormwater quality improvement

Currently, there is a significant surge of interest in green roof technology for construction buildings due to its numerous environmental benefits, such as stormwater management, energy efficiency, and enhanced urban biodiversity. However, the issue of potential pollutant release from green roof substrates into runoff water, causing water pollution, needs to be addressed. To tackle this concern, a lab-scale green roof model was assessed, utilizing a biodegradable substrate made from banana peels and eggshell waste (organic fertilizer). Three models were tested: a conventional green roof (control), a green roof with chemical fertilizer, and a green roof with organic fertilizer. Various water quality parameters, including pH, total suspended solids (TSS), nitrogen, phosphorus, and potassium (NPK), and chemical oxygen demand (COD), were evaluated. The results demonstrated the effectiveness of organic fertilizer in reducing TSS and COD levels, where the eco-friendly green roof with biodegradable substrate exhibited an impressive performance, achieving a higher COD removal percentage (78%) compared to the green roof with chemical fertilizer (50%). The utilization of organic fertilizer led to an enhancement in the quality of stormwater runoff, resulting in NPK removal percentages ranging from 17% to 25%. Additionally, the organic fertilizer fostered healthier vegetation growth, leading to a greater number of leaves compared to the chemical fertilizer. These findings highlight the potential of eco-friendly green roofs as a sustainable and effective tool for stormwater management, provided suitable substrate materials are employed.

Substrate composition impacts long-term vegetation development on blue-green roofs: Insights from an experimental roof and greenhouse study

Green roofs provide ecosystem services and can promote biodiversity in urban areas. Blue-green roofs have an additional water storage compartment under the substrate to reduce roof water runoff, thereby also reducing drought stress which is beneficial for green roof vegetation. In order to study which blue-green roof design supports the highest plant diversity, we assessed the effect of different substrates and seed mixtures on vegetation development in a short-term greenhouse experiment and long-term blue-green roof experiment. A ten-week full-factorial greenhouse experiment was performed for six substrate composition and four seed mixture treatments. On an experimental blue-green roof, we annually surveyed plants from 2013 to 2021 in nine different treatments (five replicates each), that varied in substrate composition, substrate depth and seed mixture that was initially applied. Two treatments resembled conventional non-green roofs (100% gravel) and conventional extensive Sedum green roofs. The results of the greenhouse experiment showed that seed mixture is more important than substrate composition in shaping the initial species richness and species composition. However, on the experimental roof the substrate composition was an important determinant of species richness and species composition long-term. Plant species richness on the experimental roof was lowest in the gravel treatment (resembling conventional non-green roofs), and highest in treatments where locally collected soil was used, likely due to additional species that appeared from the seed bank present in the transplanted soil. Soil was never completely covered with vegetation on unfertilized substrates that contained 20% or less dense and organic materials. Plant species richness on conventional Sedum roof substrate was higher on the experimental blue-green roof compared to an adjacent non-blue roof, highlighting that blue-green roofs can promote biodiversity more than conventional green roofs. For future construction of blue-green roofs in our region, we recommend the addition of 30% locally collected soil to a 6 cm deep lightweight substrate to maximize long-term plant cover and plant species richness.

Impact of design variables on hydrologic and thermal performance of green, blue-green and blue roofs

Abstract Blue-green and blue roofs are increasingly promoted to adapt to climate change by providing multiple benefits. However, uncertainties about their design and how they differ from conventional green roofs hinder their implementation. This study investigates the potential of green, blue-green, and blue roofs to control urban stormwater and improve microclimate by monitoring their performance in Toronto, Ontario, Canada. Experimental setups were built and varied with the following design factors: substrate type and thickness, drainage layer thickness and orifice size. The results revealed that blue-green roofs with organic and FLL (blended according to the German Forschungsgesellschaft Landschaftsentiwicklung Landschaftsbau) substrates significantly improved detention compared to green roofs with similar substrates. The organic blue-green roof achieved maximum retention, but FLL blue-green roof did not have higher retention than FLL green roof. The blue roof with smaller orifices had comparable hydrologic performance to vegetated roofs but suffered from long water standing durations. Organic substrates followed by FLL substrates result in the highest air cooling in the noon, but blue roofs had the highest air cooling in the evening. In-substrate temperatures in blue-green roofs were lower than those in green roofs. Trade-offs between the benefits and drawbacks need to be considered in future designs.

Hydrologic and Thermal Performance of a Full-Scale Farmed Blue–Green Roof

Conventional green roofs have been widely accepted as a climate change adaptation strategy. However, little is known about the potential of blue–green roofs and rooftop farms to control urban stormwater and improve microclimates. This study evaluates a farmed blue–green roof’s hydrologic and thermal performance over an entire growing season in Toronto, Ontario, Canada. The runoff discharge from three plots planted with various crops was monitored. The substrate and air temperatures at two elevations of different cultivated and self-sowing plant species were collected and compared to a control roof. Results indicate that planting and harvesting activities impacted the hydrologic performance. Mean values for retention ranged from 85–88%, peak attenuation ranged from 82–85%, and peak delay ranged from 7.7 to 8 h. At the lower elevation, the mean air temperature difference above okra, tobacco, and beet was 2.5 °C, whereas, above squash, potato, and milkweed, it was 1.4 °C. Maximum and moderate air-cooling effects were observed in the afternoon and evening, but a warming effect was observed in the early morning. Farmed blue–green roof evaluated in this study provides a runoff control and microclimate improvement comparable to or better than conventional green roofs, in addition to other benefits such as improving food security.

Rethinking green roofs- natural and recycled materials improve their carbon footprint

Green roofs contribute to climate regulation and urban heat island mitigation, thereby counteracting heat stress. Ecosystem services of green roofs are discussed, but their production and demolition processes have not been fully addressed.In this study, a life cycle analysis (LCA) was conducted to assess the possibility of creating of a carbon-neutral green roof and to evaluate and compare the global warming potential (GWP) of two green roofs: 1) a conventional green roof (GR-c) with expanded clay, pumice, and compost in the substrate and a polypropylene drainage, and 2) an alternative green roof (GR-a) with recycled bricks and compost in the substrate and a cork drainage. LCA refers to a functional unit (FU) of a 218 m2 green roof (substrate depth of 9 cm; lifespan of 40 years). The results showed that the use of a brick substrate can reduce the GWP to 3139 kg of CO2 eq/FU (−50%) and the use of cork drainage to 441 kg of CO2 eq/FU (−69%). Apart from production, demolition was a key element required for further improvement, accounting for 32% (GR-c) and 55% (GR-a) of the GWP. Once produced, a green roof can take up 783 gCO2/(m2·a) because of plant uptake. To become CO2-neutral, a GR-c and GR-a would have to last 88 and 53 years, respectively. Furthermore, the GWP was influenced by green roof maintenance and plant CO2 uptake.We concluded that recycled bricks and cork are promising green roof materials.

Combining an Intensive Green Roof with Seismic Retrofitting of Typical Reinforced Concrete Buildings in Israel.

This study suggests an intensive green roof as part of a sustainable and hazard-resistant conceptual design for the retrofitting of old buildings in Israel. The roof is suggested to be built with waste-based materials. A five-story reinforced concrete residential building was retrofitted with: Case 1: concrete wall strengthening (CWS)-conventional concrete + conventional green roof; Case 2: CWS-waste-included concrete + waste-based green roof; Case 3: seismic isolation columns (SIC)-conventional concrete + conventional green roof; and Case 4: SIC-waste-included concrete + waste-based green roof. Palekastro, Nuweiba, Tabas, and Erzincan ground motions were used for a structural dynamic time-history analysis of the retrofitted buildings. Life cycle assessments of cases 1–4 were performed using ReCiPe 2016 midpoint and endpoint evaluations. A two-stage analysis of variance (ANOVA) was used to analyze the ReCiPe endpoint results. According to the seismic results, Case 3 and Case 4 were much more preferable to Case 1 and Case 2, whereas according to the environmental evaluations, Case 4 was the most preferable to the other cases.

Blue-green roofs with forecast-based operation to reduce the impact of weather extremes

Conventional green roofs have often been criticised for their limited water buffer capacity during extreme rainfall events and for their susceptibility to droughts when additional irrigation is unavailable. One solution to these challenges is to create an extra blue water retention layer underneath the green layer. Blue-green roofs allow more stormwater to be stored, and the reservoir can act as a water source for the green layer throughout capillary rises. An automated valve regulates the water level of the system. It can be opened to drain water when extreme precipitation is expected. Therefore, the water buffer capacity of the system during extreme rainfall events can be maximised by integrating precipitation forecasts as triggers for the operation of the valve. However, the added value of this forecast-based operation is yet unknown. Accordingly, in this study, we design and evaluate a hydrological blue-green roof model that utilises precipitation forecasts. We test its performance to capture (extreme) precipitation and to increase evapotranspiration and evaporative cooling under a variety of precipitation forecast-based decision rules. We show that blue-green roofs can capture between 70 % and 97 % of extreme precipitation (>20 mm/h) when set to anticipate ensemble precipitation forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF). This capture ratio is considerably higher than that of a conventional green roof without extra water retention (12 %) or that of a blue-green roof that does not use forecast information (i.e., valve always closed; 59 %). Moreover, blue-green roofs allow for high evapotranspiration rates relative to potential evapotranspiration on hot summer days (around 70 %), which is higher than from conventional green roofs (30 %). This serves to underscore the higher capacity of blue-green roofs to reduce heat stress. Using the city of Amsterdam as a case study, we show the high upscaling potential of the concept: on average, potentially suitable flat roofs cover 13.3 % of the total area of the catchments that are susceptible to pluvial flood risk. If the 90th percentile of the ECMWF forecast is used, an 84 % rainfall capture ratio can translate into capturing 11 % of rainfall in flood-prone urban catchments in Amsterdam.

Moving beyond habitat analogs: Optimizing green roofs for a balance of ecosystem services

Green roofs are increasingly popular as a result of their ability to provide local ecosystem services in the urban landscape. The number and intensity of the services provided is a function of the abiotic and biotic design elements chosen for a given green roof design. We manipulated experimental semi-intensive green roof plots in order to uncover which combinations of design choices created the system most capable of providing ecosystem services with the lowest number of trade-offs. We monitored performance of planted vegetation, weeds, soil carbon, and stormwater runoff in a complete factorial experiment involving three different green roof system types (conventional green roof, blue-green roof, and quasi-traditional green roof), two plant community types (restoration and horticulture), and two arbuscular mycorrhizal fungal treatments (inoculated or non-inoculated). We hypothesized that more natural designs (i.e., less engineered systems) had the potential to provide a greater amount of ecosystem services with fewer trade-offs. However, our results showed that natural design elements did not consistently perform better than engineered design choices, which does not support the idea that habitat templates should guide green roof design. We found system type to be most impactful on ecosystem services. In particular, the blue-green roof provided the most services with the lowest number of trade-offs and may represent the ideal intermediate worthy of further investigation.

Improving the Efficiency of Green Roofs Using Atmospheric Water Harvesting Systems (An Innovative Design)

Conventional green roofs, although having numerous advantages, could place water resources under pressure in dry periods due to irrigation requirements. Moreover, the thermal efficiency of green roofs could decrease without irrigation, and the plants could get damaged. Therefore, this study aims to improve the efficiency of conventional green roofs by proposing a new multipurpose green roof combined with fog and dew harvesting systems. The analysis determined that the average water use of green roofs in the summer (in humid regions) is about 3.7 L/m2/day, in the Mediterranean regions about 4.5 L/m2/day, and in arid regions about 2.7 L/m2/day. During the dry season, the average fog potential in humid regions is 1.2 to 15.6 L/m2/day, Mediterranean regions between 1.6 and 4.6 L/m2/day, and arid regions between 1.8 and 11.8 L/m2/day. The average dew potential during the dry season in humid regions is 0.1 to 0.3 L/m2/day, in the Mediterranean regions is 0.2 to 0.3 L/m2/day, and in the arid regions is 0.5 to 0.7 L/m2/day. The analysis of the suggested multipurpose green roof combined with fog/dew harvesting systems, in the summer, in three different climates, show that fog harvesting could provide the total water requirement of the green roofs, and that dew harvesting by PV (photo-voltaic) panels could provide 15 to 26% of the water requirements. Moreover, it could show a higher thermal impact on the building, higher efficiency in stormwater management, less dependence on the urban water network, and greater efficiency in decreasing urban air, water, and noise pollution. Finally, the novel green roof system could consume less water due to the shaded area by mesh and solar PVs and maximize the utilization of the roof area, as solar panels could be applied on the same green roof.

Comparison between cork-based and conventional green roof solutions

Green roofs are increasingly being used to improve the energy balance and stormwater management of buildings. This work examines the thermal behaviour, water drainage and vegetation growth of two green roof systems: a conventional system containing extruded polystyrene and polyethylene, and an alternative cork-based system. In the new system, an eco-friendly expanded cork layer is used to provide thermal resistance and water storage capacity in a more sustainable way, as cork is a natural material. This material can also be used as an uncoated finishing layer for architectural purposes.The main goal of this work was to see if the new system is reliable in real environmental conditions and to compare its behaviour to that of the conventional system for a full year. This work presents the results of a fully functioning prototype built in Portugal with a technical substrate 10 cm thick and planted with a variety of vegetation. The reference and proposed systems were installed side by side and monitored under spring, summer, autumn and winter conditions.The thermal insulation and the water drainage and storage capacity provided by the cork-based green roof were found to be similar to those of the reference solution, with the expanded cork layer showing a more marked temperature delay and a more effective management of rainfall events under dry conditions. Additionally, it was found that the growth and health of plants in the cork-based green roof were comparable to those of the reference solution, indicating the good performance of the cork system.

The heavy metal budget of an urban rooftop farm

Urban rooftop agriculture is a growing enterprise in the US with the goal of providing high quality, healthy, locally grown produce for city dwellers. However, air pollution abatement and the purification of stormwater are among the ecosystem services emphasized in studies of conventional green roofs. If rooftop farms actually capture pollutants, then accumulation of heavy metals in the soil could pose a problem over time. This study reports the heavy metal concentrations in soil, atmospheric deposition, and drainage output of 8 metals from the Brooklyn Grange Navy Yard Farm, rooftop vegetable farm in New York City, USA. Drainage of Pb and Mn were 6% and 14% of atmospheric bulk deposition, respectively, meaning that the Grange could be a net sink for Pb and Mn. Although there were small scale hotspots in the soil, farm-wide averages for heavy metal concentrations never exceeded guideline levels, and relatively low concentrations of Pb and Ba in the soil suggest that rooftop soils may be less vulnerable to contamination related to traffic and construction. In comparison to the growing seasons, we found relatively high concentrations of Pb and Cr in the soil during fallow periods when the soil was bare. To reduce the atmospheric deposition of heavy metals to soil, it is important to cover the soil with mulch, and discard the used mulch and unmarketable portion of vegetables, instead of recycling them via composting for soil amendments.

Evaporation from (Blue-)Green Roofs: Assessing the Benefits of a Storage and Capillary Irrigation System Based on Measurements and Modeling

Worldwide cities are facing increasing temperatures due to climate change and increasing urban density. Green roofs are promoted as a climate adaptation measure to lower air temperatures and improve comfort in urban areas, especially during intensive dry and warm spells. However, there is much debate on the effectiveness of this measure, because of a lack of fundamental knowledge about evaporation from different green roof systems. In this study, we investigate the water and energy balance of different roof types on a rooftop in Amsterdam, the Netherlands. Based on lysimeter measurements and modeling, we compared the water and energy balance of a conventional green roof with blue-green roofs equipped with a novel storage and capillary irrigation system. The roofs were covered either with Sedum or by grasses and herbs. Our measurements and modeling showed that conventional green roof systems (i.e., a Sedum cover and a few centimeters of substrate) have a low evaporation rate and due to a rapid decline in available moisture, a minor cooling effect. Roofs equipped with a storage and capillary irrigation system showed a remarkably large evaporation rate for Sedum species behaving as C3 plants during hot, dry periods. Covered with grasses and herbs, the evaporation rate was even larger. Precipitation storage and capillary irrigation strongly reduced the number of days with dry-out events. Implementing these systems therefore could lead to better cooling efficiencies in cities.

Helophyte mats (wetland roofs) with high evapotranspiration rates as a tool for decentralised rainwater management – process stability improved by simultaneous greywater treatment

Abstract Redensification of the housing stock is also creating challenges for the drainage of wastewater and rainwater in existing sewer systems, particularly in growing cities. One alternative here is the evaporation of rainwater, which reduces hydraulic loads on sewers. Rainwater evapotranspiration using helophyte mats on building roofs is a possible approach. Helophytes are able to transpire considerably more rainwater than extensively planted green roofs. Other than conventional green roofs helophyte mats in the form of wetland roofs require a permanent water supply on a daily basis. Greywater application can be an additional advantage in terms of nutrient supply of the wetland roof after being treated microbiologically within the plant carrier mat. The treatment of greywater using a helophyte-planted roof can help to meet the water and nutrient requirements of the helophytes even during rain-free periods. However, it must be ensured that the root mat treats the greywater to a sufficient extent. It was shown under practical conditions that a 0.1 m-thick helophyte mat is suitable for treating typical domestic greywater at loads of up to 15 L m−2 d−1.

Thermal Comfort Performances of a Green Roof Modular System Fixed on Gable Roofs

Urbanised areas are tend to heat up more than neighborhoods as consequences of global warming and urban heat island effect due to high built up areas. Moreover, buildings are the most energy consumptive assets in energy systems. Green roofing is a popular strategy for passive cooling of buildings in present green building industry and it has become the best solution to overcome problems related to thermal comfort. A conventional green roof made up with the combination of vegetation layer, growing media, drainage layer and root barrier layer add a huge weight up on the existing roofs. Hence, green roof modular system is more efficient in cost and performances basis with less weight as separated by prefabricated or non−prefabricated novel method. This study evaluated the effect of fixed green roof modular system to an existing building to ensure the thermal comfort performances of outdoor and indoor conditions of the building by innovating portable frame network and new growing media for green modules with low weight, low cost and easy maintenance. Developed module’s unit weight was 400 −450 g and it covered an area of 412.5 cm2. It was fabricated with fiber glass and a frame network was developed using canvas−rubber mixed materials with a unit weight of 622.5 g. Growth medium was prepared by using rubber and coir mixed material. Ipomea aquatica, Centella asiatica and Alternanthera sessilis were used as testing vegetation for the developed green roof modules. Leaf area (LA) and number of leaves were used as the parameters to select the best plant variety for the green roof. Subsequent thermal performance parameters of the green roof were analysed by comparing it with a normal roof of the same building. Leaf area of C. asiatica was significantly higher than I. aquatica and A. sessilis. Plant height, LA and number of leaves are the main factors that control the penetration of solar radiation; hence it influences the temperature. By considering statistical analysis and above facts, C. asiatica was selected as the best plant variety for the modules. A maximum indoor and outdoor temperature difference was reported after implementing the green roof modular system as 3 C. Control roof showed a maximum temperature difference of 2.3 C between indoor and outdoor. The maximum relative humidity difference between inside and the outside roof was recorded as 3.5% while control roof was given 3.3%.Therefore green roof modular system developed with fiberglass and frame network with coir-rubber mixed growing media for Centella asiatica can be suggested to enhance the thermal comfort of indoor and outdoor environments. Keywords: Green roofs, Green roof modules, Growing medium, Leaf area, Portable frame network

Lightweight Aggregate Made from Dredged Material in Green Roof Construction for Stormwater Management.

More than 1.15 million cubic meters (1.5 million cubic yards) of sediment require annual removal from harbors and ports along Ohio’s Lake Erie coast. Disposing of these materials into landfills depletes land resources, while open water placement of these materials deteriorates water quality. There are more than 14,000 acres of revitalizing brownfields in Cleveland, U.S., many containing up to 90% impervious surface, which does not allow “infiltration” based stormwater practices required by contemporary site-based stormwater regulation. This study investigates the potential of sintering the dredged material from the Harbor of Cleveland in Lake Erie to produce lightweight aggregate (LWA), and apply the LWA to green roof construction. Chemical and thermal analyses revealed the sintered material can serve for LWA production when preheated at 550 °C and sintered at a higher temperature. Through dewatering, drying, sieving, pellet making, preheating, and sintering with varying temperatures (900–1100 °C), LWAs with porous microstructures are produced with specific gravities ranging from 1.46 to 1.74, and water absorption capacities ranging from 11% to 23%. The water absorption capacity of the aggregate decreases as sintering temperature increases. The LWA was incorporated into the growing media of a green roof plot, which has higher water retention capacity than the conventional green roof system.