Green roofs: Performance

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Recently laid Sedum mats on a ground level podium roof.
Detail showing the early root development through the mat to the granularGravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices. planting medium.

Green roofA thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens. performance has not been reported to reduce over time. Controlled studies have instead indicated that maturing green roofs may have improved water retention properties [1].

The key hydrologic benefit which green roofs have over other forms of LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. is the proportion of the water returned to the atmosphere through evapotranspirationThe quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/dayThe combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration..

  • In Southern Ontario rainwater retention of extensive green roofs without irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47 is between 60% and 70%[2][3][4].
  • Including winter periods with snow accumulation and thaw, the annual retention of extensive green roofs is around 50% [3][4].
  • Using a compostDecayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil. based planting medium improves retention by around 10% i.e. 60 % for compostDecayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil. compared to 50% for granularGravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices..
  • Daily irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47 can reduce the annual retention by 20% compared to a roof without irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47. i.e. 40% for irrigated compared to 60% without irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47[4]. However, recirculating rainwater from a cisternTank used to store rainwater (typically roof runoff) for later use. was estimated to double the annual retention in Florida[5]. The research team modeled 87% retention for a green roofA thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens. coupled with a cisternTank used to store rainwater (typically roof runoff) for later use., compared to 43% for the green roofA thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens. alone.

An appropriate NRCS curve numbers for green roofs without irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47 in Southern Ontario is 90 [6][4].

Water quality

Many green roofs receive only rainwater, which is relatively clean when it lands. As such green roofs can contribute contamination, most notably in nutrient leaching during early establishment. Reported values of total phosphorus in green roofA thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens. runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. vary from less than 0.1 ppm to over 10 ppm. But, in dense urban centres, green roofs are increasingly being used to receive irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47 from harvested rainwater. Current Ontario Building Code permits the use of rooftop runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. to be reused in this manner, so long as it is 'free of solids'. A 'closed loop' system can be created by coupling a rainwater harvestingThe practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets. system to a green roofA thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.. by catching and reusing runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface., the only water leaving the system is through evapotranspirationThe quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/dayThe combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.. This prevents any runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. from leaving the site and so prevents any nutrient loadingThe total mass of a pollutant entering a waterbody over a defined time period.The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration. to the environmentRefers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer..

Total phosphorous concentrations reported in green roofA thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens. runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface.(abstracted from Hill 2017)

  1. Simon De-Ville, Manoj Menon, Xiaodong Jia, George Reed, Virginia Stovin, The impact of green roof ageing on substrate characteristics and hydrological performance, In Journal of Hydrology, Volume 547, 2017, Pages 332-344, ISSN 0022-1694, https://doi.org/10.1016/j.jhydrol.2017.02.006.
  2. http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/03/NRC_EastviewGRrept.pdf
  3. 3.0 3.1 T. Van Seters, L. Rocha, D. Smith, G. MacMillan; Evaluation of Green Roofs for Runoff Retention, Runoff Quality, and Leachability, Vol. 44 (1): 33 - 47 (2009). Innovative Approaches to Stormwater Management in Canada
  4. 4.0 4.1 4.2 4.3 Hill J, Drake J, Sleep B, Margolis L. Influences of Four Extensive Green Roof Design Variables on Stormwater Hydrology. J Hydrol Eng. 2017;22(8):04017019. doi:10.1061/(ASCE)HE.1943-5584.0001534
  5. Hardin, M.; Wanielista, M.; Chopra, M. A Mass Balance Model for Designing Green Roof Systems that Incorporate a Cistern for Re-Use. Water 2012, 4, 914-931. http://www.mdpi.com/2073-4441/4/4/914
  6. Curve Number and Runoff Coefficients for Extensive Living Roofs Elizabeth Fassman-Beck, Ph.D., A.M.ASCE; William Hunt, Ph.D., P.E., M.ASCE; Robert Berghage, Ph.D.; Donald Carpenter, Ph.D., P.E., M.ASCE; Timothy Kurtz, P.E., M.ASCE; Virginia Stovin, Ph.D.; and Bridget Wadzuk, Ph.D., A.M.ASCE