Balancing the benefits: Green Roof Innovation Testing (GRIT)

Hydrological Processes, 2018

Sustainable strategies such as green roofs have been implemented as stormwater management tools to mitigate disturbance of the hydrologic cycle resulting from urbanization. Green roofs, also referred to as vegetated roofs, can improve the urban landscape by reducing heat island effects, providing ecosystem services, and facilitating the retention and treatment of stormwater. Green roofs have received particular attention because they do not require acquisition and development of land and represent an application of biomimicry in design and construction. In this paper, we evaluate the effects of precipitation, evapotranspiration (ET), antecedent dry period (ADP), and seasonal variation on the run‐off quantity and distribution of an extensive, sedum covered, green roof on a commercial building in Syracuse, NY, USA. The green roof greatly facilitated retention of precipitation events without significant changes over the 4‐year study. The green roof retained on average 95.9 ± 3.6% (6.5 ...

Energy and Buildings, 2017

An increasing number of studies now demonstrate the benefits of rooftop greenery in reducing building energy consumption and improving the urban microclimate. However, the extent to which such thermal benefits are influenced by the components and design of rooftop greening systems has not received adequate attention. We report in this paper, results of a study to evaluate the effects of growing substrate and water retention layer on the thermal performance of a modular green roof system. The evaluation focused on surface temperatures of the green roof and plant responses in a field experiment with two main treatments: substrate types (normal garden top soil compared to soilless lightweight growing substrate) and the presence or absence of a water retention layer incorporated into the green roof system. Surface temperatures at three positions in the vertical profile of the green roof system, and plant responses (evapotranspiration and stomatal conductance) were compared under different levels of water availability in the growing substrate, which were imposed by sequentially subjecting the green roof to three phases of irrigation over a two-month period: irrigation to achieve well-watered condition, irrigation withheld to induce drought conditions, and irrigation restored to pre-drought conditions. Results show that surface temperatures tend to be higher with topsoil as the growing substrate. The water retention layer also reduced surface temperature at the mulch layer throughout the day. However, the water retention layer may lead to slight increase in concrete surface temperature, possibly due to heat stored in the water retention layer. The study suggests that incorporating a water retention layer can be beneficial to green roofs in sustaining soil moisture, evapotranspiration rates and plant health. We discuss other implications of the results in relation to irrigation needs and optimising the benefits of green roofs for cooling.

Water

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 m...

Sustainability

Green roofs are consistently being used to reduce some of the negative environmental impacts of cities. The increasing interest in extensive green roofs requires refined studies on their design and operation, and on the effects of their relevant parameters on green roof thermal performance. The effects of two design parameters, substrate thickness (ST) and conductivity of dry soil (CDS), and four operating parameters, leaf area index (LAI), leaf reflectivity (LR), stomatal resistance (SR), and moisture content (MC), were investigated using the green roof computer model developed by Sailor in 2008. The computer simulations showed that among the operating parameters, LAI has the largest effects on thermal performance while CDS is a more influential design parameter than ST. Experimental investigations of non-vegetated and sparsely vegetated green roofs in Melbourne were principally used to understand the effect of the substrate and enable better understanding of dominant heat transfer mechanisms involved. Investigated green roofs had three substrate thicknesses (100, 150 and 200 mm), and their performance was compared to a bare conventional roof. In contrast to the computer simulations, the experimental results for summer and winter showed the importance of MC and ST in reducing the substrate temperature and heat flux through the green roof.

Buildings, 2019

Urban environments are characterized by dense development and paved ground with reduced evapotranspiration rates. These areas store sensible and latent heat, providing the base for typical urban heat island effects. Green roof installations are one possible strategy to reintroduce evaporative surfaces into cities. If green roofs are irrigated, they can contribute to urban water management and evapotranspiration can be enhanced. As part of two research projects, lysimeter measurements were used to determine the real evapotranspiration rates on the research roof of the University of Applied Sciences in Neubrandenburg, Germany. In this paper, we address the results from 2017, a humid and cool summer, and 2018, a century summer with the highest temperatures and dryness over a long period of time, measured in Northeast Germany. The lysimeter measurements varied between the normal green roof layer (variation of extensive green roof constructions) and a special construction with an extra r...

Water and Environment Journal, 2018

Green roofs as solutions that can offer varying levels of stormwater management are the topic of current interest. In order to use this form of reconstructing retention capacity, it is important to understand the influence of meteorological conditions on the functioning of living roofs. The study presents the results of research, using of the ANOVA variance analysis method, on retention performance (i.e. volumetric control) and detention (temporal delay runoff) carried out in the years 2009-2014. The results indicate that the green roof can effectively retain rainfall and delay the initiation of runoff from the rainfall events included in the analysis. Understanding the hydrological performance of green roofs in different local meteorological conditions is key to the successful implementation and development of sustainable practices to control runoff in urban areas.

Greening Rooftops for Sustainable Communities, 2005

This study aims to provide technical data on the performance of green roofs in the City of Toronto, and to illustrate their benefits in an urban context. Two extensive green roof systems were installed on a community centre in Toronto. Both systems contained the same components that differed in materials and designs. The green roofs contained lightweight growing medium, 75 -100 mm in depth, that supported a variety of vegetation. The green roofs, and a reference roof, were instrumented to provide thermal performance and energy efficiency data, as well as runoff measurements. Although the vegetation was not well established in the first year of monitoring, nevertheless, the extensive green roofs reduced the building's energy demand by lowering the heat flow through the roof, especially in the summer. The green roofs were shown to be effective in delaying and reducing stormwater runoff and the retention efficiency depended upon the characteristics of the rain event (intensity and amount) and the wetting history of the growing medium. Preliminary observations and membrane temperatures recorded also suggest that green roofs could likely improve membrane durability by reducing heat aging, thermal stresses, ultra-violet radiation and physical damages.

Hydrologic performance of green roofs may differ in various climatic regions due to the specific precipitation climatology, building practices and green roof materials.

The Green roof innovation Testing (GriT) lab at the University of Toronto is a multiyear research project comparatively analyzing 33 extensive green roof modules with variables of composition and maintenance. Each module is continuously monitored through an array of nine thermal and hydrological sensors.

Ecological Engineering, 2016

Green roofs have been built and studied mainly in humid climates and provide many benefits, from thermal insulation to biodiversity. However, little is known about their performance in arid and semiarid regions where irrigation requirements can affect their sustainability. In order to examine and evaluate the performance of green roofs in a semiarid climate, 114-m 2 green roof modules were built and monitored for a year. The experimental setup consisted on nine different roof designs or modules, one of them with two additional replicates (11 modules in total). The study was performed in Santiago, Chile (33 • 26 S, 70 • 39 W, 570 MASL), a region with a typical semiarid climate. Three substrate depths (5-cm, 10-cm and 20-cm) and four commercial drainage systems were evaluated. The modules were implemented with a sprinkler irrigation system and instrumented to record air temperature, precipitation, and substrate water content and temperature at 5-min intervals. The results showed that substrate depth controls the amplitude in substrate temperature. The 10-cm and 20-cm depth modules showed a significant damping effect on substrate temperature. In addition, the 10-cm and 20-cm depth modules showed maximum daily temperatures up to 13 • C below the air temperature. In contrast, the 5-cm depth modules increased the amplitude in substrate temperature, reaching in average 13.8 • C more than the daily maximum air temperature in spring and 2.6 • C less than the daily minimum air temperature in summer. The 10-cm and 20-cm depth modules provided a stable and suitable substrate water content during the entire study period, while the 5-cm depth green roofs were more affected by the atmospheric demand. Although their limitations were partially overcome by increasing irrigation rates or adding a retention fabric to improve the substrate water holding capacity, the 5-cm depth green roofs experienced daily thermal amplitudes beyond the recommended value for plant development. Consequently, extensive (or thin) green roofs (less than 10 cm of growing medium) are unlikely to be recommended in arid or semiarid climates.