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===Water Balance===

===Water Balance===

Green roofs are primarily a practice used to partially achieve water balance and erosion control objectives through interception and [[Evapotranspiration| evapotranspiration]] of precipitation.  The key hydrologic benefit which green roofs have over other forms of LID is the proportion of the water returned to the atmosphere through [[Evapotranspiration| evapotranspiration]].

Green roofs are primarily a practice used to partially achieve water balance and erosion control objectives through interception and [[Evapotranspiration| evapotranspiration]] of precipitation that falls on the roof.  The key hydrologic benefit which green roofs have over other forms of LID is the proportion of the water returned to the atmosphere through [[Evapotranspiration| evapotranspiration]]. Considerable research has been conducted in recent years to define the runoff reduction or water retention capacity of extensive green roofs in climates similar to those that occur in Ontario and Canada.

* Controlled studies have indicated that maturing green roofs may have improved water retention properties <ref>De-Ville, S., Menon, M., Jia, X., Reed, G., Stovin, V. 2017. The impact of green roof ageing on substrate characteristics and hydrological performance. Journal of Hydrology. Volume 547. pp 332-344. https://doi.org/10.1016/j.jhydrol.2017.02.006.</ref>.  

* Controlled studies have indicated that maturing green roofs may have improved water retention properties <ref>De-Ville, S., Menon, M., Jia, X., Reed, G., Stovin, V. 2017. The impact of green roof ageing on substrate characteristics and hydrological performance. Journal of Hydrology. Volume 547. pp 332-344. https://doi.org/10.1016/j.jhydrol.2017.02.006.</ref>.  

===Water Quality===

===Water Quality===

Many green roofs receive only rainwater, which is relatively free of contaminants like nutrients and heavy metals, but can contribute contaminants to roof runoff, most notably through leaching of dissolved [[Nutrients]] and [[Heavy metals]] from the growing medium substrate during early establishment.<ref>Vijayaraghavan, K., Harkishore Kumar Reddy, D., Yun, Y. 2018. Improving the quality of runoff from green roofs through synergistic biosorption and phytoremediation techniques: A review. Sustainable Cities and Society. 46 (2019) 101381. https://www.sciencedirect.com/science/article/abs/pii/S2210670718319383?via%3Dihub</ref>  Concentrations of Total [[Phosphorus]] in green roof runoff have been observed to vary from 0.4 mg/L to over 10 mg/L.<ref>Hill J., Drake J., Sleep B., Margolis L. 2017. Influences of Four Extensive Green Roof Design Variables on Stormwater Hydrology. J Hydrol Eng. 2017;22(8):04017019. https://ascelibrary.org/doi/abs/10.1061/%28ASCE%29HE.1943-5584.0001534</ref>. Such concentrations are well above the Ontario Provincial Water Quality Objective of 0.03 mg/L for Total Phosphorus (OMOEE, 1994<ref>Ontario Ministry of Environment and Energy (OMOEE), 1994. Policies, Guidelines and Provincial Water Quality Objectives of the Ministry of Environment and Energy. Queen’s Printer for Ontario. Toronto, ON.</ref>) to control excessive plant growth in rivers and streams. To improve the quality of runoff from green roofs, Vijayaraghavan et al. (2018) recommend considering the sorption capacity of the growing medium substrate, the phytoremediation potential of plants, and incorporation of sorbent [[Additives]] to growing medium substrates.<ref>Vijayaraghavan, K., Harkishore Kumar Reddy, D., Yun, Y. 2018. Improving the quality of runoff from green roofs through synergistic biosorption and phytoremediation techniques: A review. Sustainable Cities and Society. 46 (2019) 101381. https://www.sciencedirect.com/science/article/abs/pii/S2210670718319383?via%3Dihub</ref>

Many green roofs receive only rainwater, which is relatively free of contaminants like nutrients and heavy metals, but can contribute contaminants to roof runoff, most notably through leaching of dissolved [[Nutrients]] and [[Heavy metals]] from the growing medium substrate during early establishment.<ref>Hathaway, A.M., Hunt, W.F., Jennings, G.D. 2008. A field study of green roof hydrologic and water quality performance. Transactions of the ASABE. Vol. 51 (1) pp. 1-8. https://elibrary.asabe.org/abstract.asp?aid=24225</ref> <ref>Vijayaraghavan, K., Harkishore Kumar Reddy, D., Yun, Y. 2018. Improving the quality of runoff from green roofs through synergistic biosorption and phytoremediation techniques: A review. Sustainable Cities and Society. 46 (2019) 101381. https://www.sciencedirect.com/science/article/abs/pii/S2210670718319383?via%3Dihub</ref>  Concentrations of Total [[Phosphorus]] in green roof runoff have been observed to vary from 0.4 mg/L to over 10 mg/L.<ref>Hill J., Drake J., Sleep B., Margolis L. 2017. Influences of Four Extensive Green Roof Design Variables on Stormwater Hydrology. J Hydrol Eng. 2017;22(8):04017019. https://ascelibrary.org/doi/abs/10.1061/%28ASCE%29HE.1943-5584.0001534</ref>. Such concentrations are well above the Ontario Provincial Water Quality Objective of 0.03 mg/L for Total Phosphorus (OMOEE, 1994<ref>Ontario Ministry of Environment and Energy (OMOEE), 1994. Policies, Guidelines and Provincial Water Quality Objectives of the Ministry of Environment and Energy. Queen’s Printer for Ontario. Toronto, ON.</ref>) to control excessive plant growth in rivers and streams. To improve the quality of runoff from green roofs, Vijayaraghavan et al. (2018) recommend considering the sorption capacity of the growing medium substrate, the phytoremediation potential of plants, and incorporation of sorbent [[Additives]] to growing medium substrates.<ref>Vijayaraghavan, K., Harkishore Kumar Reddy, D., Yun, Y. 2018. Improving the quality of runoff from green roofs through synergistic biosorption and phytoremediation techniques: A review. Sustainable Cities and Society. 46 (2019) 101381. https://www.sciencedirect.com/science/article/abs/pii/S2210670718319383?via%3Dihub</ref>

In dense urban centres, green roofs are increasingly being used to receive irrigation from harvested rainwater. Current Ontario Building Code permits the use of rooftop runoff for landscape irrigation purposes, so long as it is 'free of solids'.

In dense urban centres, green roofs are increasingly being used to receive irrigation from harvested rainwater. Current Ontario Building Code permits the use of rooftop runoff for landscape irrigation purposes, so long as it is 'free of solids'.

*[http://www.vitaroofs.com/ VitaRoofs]

*[http://www.vitaroofs.com/ VitaRoofs]

*[https://www.zinco.ca/ Zinco]

*[https://www.zinco.ca/ Zinco]

==Green roofs gallery==

==Green roofs gallery==