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Created page with "thumb|Recently laid ''Sedum'' mats on a ground level podium roof. File:YorkU2.jpg|thumb|Detail showing the early root development through the mat to the..."
[[File:YorkU1.jpg|thumb|Recently laid ''Sedum'' mats on a ground level podium roof.]]
[[File:YorkU2.jpg|thumb|Detail showing the early root development through the mat to the granular planting medium.]]
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>.
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>.
*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/>.
*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.
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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[[File:YorkU2.jpg|thumb|Detail showing the early root development through the mat to the granular planting medium.]]
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>.
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>.
*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/>.
*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.
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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