Changes

*drain covers (most common),  

*drain covers (most common),  

*small check dams or weirs across the roof deck, or

*small check dams or weirs across the roof deck, or

*modular tray systems.<ref>Rooftop Detention (Blue Roofs), Massachusetts Clean Water Toolkit http://prj.geosyntec.com/npsmanual/rooftopdetentionblueroofs.aspx (accessed 9 October 2018)</ref>  

*modular tray systems.<ref>Massachusetts Clean Water Toolkit, Rooftop Detention (Blue Roofs), Retrieved 9 October 2018 from http://prj.geosyntec.com/npsmanual/rooftopdetentionblueroofs.aspx</ref>  

The Ministry of Transportation do not currently permit blue roofs to form part of a SWM plan owing to concerns over long term maintenance; this affects relatively few projects<ref>Ontario Ministry of Transportation. (2016). Stormwater Management Requirements for Land Development Proposals. Retrieved March 7, 2018, from http://www.mto.gov.on.ca/english/publications/drainage/stormwater/section8.shtml#controls</ref>. The concerns arise from instances where building operators have found outlet control structures blocked with leaves or other debris. Without understanding the purpose of the device, these have been removed to prevent recurrence of the blockage. A study in New York City found that a modular system of trays outperformed an outlet flow control device in stormwater management<ref>Bloomberg, M., & Strickland, C. H. (2012). NYC Green Infrastructure Plan: 2012 Green Infrastructure Pilot Monitoring Report. New York. Retrieved from http://www.nyc.gov/html/dep/pdf/green_infrastructure/2012_green_infrastructure_pilot_monitoring_report.pdf</ref>. It might reasonably be expected that a modular system would present fewer opportunities for complete failure from a single action, either through clogging or breakage of an storage element.

The Ministry of Transportation do not currently permit blue roofs to form part of a SWM plan owing to concerns over long term maintenance; this affects relatively few projects<ref>Ontario Ministry of Transportation. (2016). Stormwater Management Requirements for Land Development Proposals. Retrieved March 7, 2018, from http://www.mto.gov.on.ca/english/publications/drainage/stormwater/section8.shtml#controls</ref>. The concerns arise from instances where building operators have found outlet control structures blocked with leaves or other debris. Without understanding the purpose of the device, these have been removed to prevent recurrence of the blockage. A study in New York City found that a modular system of trays outperformed an outlet flow control device in stormwater management<ref>Bloomberg, M., & Strickland, C. H. (2012). NYC Green Infrastructure Plan: 2012 Green Infrastructure Pilot Monitoring Report. New York. Retrieved from http://www.nyc.gov/html/dep/pdf/green_infrastructure/2012_green_infrastructure_pilot_monitoring_report.pdf</ref>. It might reasonably be expected that a modular system would present fewer opportunities for complete failure from a single action, either through clogging or breakage of an storage element.