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[[File:YorkU2.jpg|thumb|Detail showing the early root development through the mat to the granular planting medium.]]  
 
[[File:YorkU2.jpg|thumb|Detail showing the early root development through the mat to the granular planting medium.]]  
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Green roof performance has not been reported to reduce over time. Controlled studies have instead indicated that maturing green roofs may have improved water retention properties <ref>http://www.sciencedirect.com/science/article/pii/S0022169417300768</ref>.
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Controlled studies have indicated that maturing green roofs may have improved water retention properties <ref>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.</ref>.
    
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.   
 
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.   
*In Southern Ontario rainwater retention of extensive green roofs without irrigation is between 60% and 70%<ref>http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/03/NRC_EastviewGRrept.pdf</ref><ref>http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/03/GR_york_fullreport.pdf</ref><ref name=Hill>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</ref>.  
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*In Southern Ontario rainwater retention of extensive green roofs without irrigation is between 60% and 70%<ref>http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/03/NRC_EastviewGRrept.pdf</ref><ref name=VS>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</ref>
*Including winter periods with snow accumulation and thaw, the annual retention of extensive green roofs is around  50% <ref>http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/03/GR_york_fullreport.pdf</ref><ref name=Hill/>.
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<ref name=Hill>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</ref>.  
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*Including winter periods with snow accumulation and thaw, the annual retention of extensive green roofs is around  50% <ref name=VS/><ref name=Hill/>.
 
*Using a compost based planting medium improves retention by around 10% i.e. 60 % for compost compared to 50% for granular.
 
*Using a compost based planting medium improves retention by around 10% i.e. 60 % for compost compared to 50% for granular.
*Daily irrigation can reduce the annual retention by 20% compared to a roof without irrigation. i.e. 40% for irrigated compared to 60% without irrigation<ref name=Hill/>. However, recirculating rainwater from a cistern was estimated to double the annual retention in Florida<ref>http://www.mdpi.com/2073-4441/4/4/914</ref>. The research team modeled 87% retention for a green roof coupled with a cistern, compared to 43% for the green roof alone.
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*Daily irrigation can reduce the annual retention by 20% compared to a roof without irrigation. i.e. 40% for irrigated compared to 60% without irrigation<ref name=Hill/>. However, recirculating rainwater from a cistern was estimated to double the annual retention in Florida<ref>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</ref>. The research team modeled 87% retention for a green roof coupled with a cistern, compared to 43% for the green roof alone.
    
An appropriate NRCS curve numbers for green roofs without irrigation in Southern Ontario is 90 <ref>Curve Number and Runoff Coefficients for Extensive Living Roofs
 
An appropriate NRCS curve numbers for green roofs without irrigation in Southern Ontario is 90 <ref>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</ref><ref>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</ref>.
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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</ref><ref name=Hill/>.
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===Water quality===
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Many green roofs receive only rainwater, which is relatively clean when it lands. As such green roofs can contribute contamination, most notably in [[Water quality#Nutrient|nutrient]] leaching during early establishment. Reported values of total [[phosphorus]] in green roof runoff vary from less than 0.1 ppm to over 10 ppm. 
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But, 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 to be reused in this manner, so long as it is 'free of solids'.
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A 'closed loop' system can be created by coupling a rainwater harvesting system to a green roof. by catching and reusing runoff, the only water leaving the system is through evapotranspiration. This prevents any runoff from leaving the site and so prevents any nutrient loading to the environment.
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[[File:TP.PNG|thumb|Total phosphorous concentrations reported in green roof runoff(abstracted from Hill 2017)]]
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