Blue roofs

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Conceptual diagram of flow control drain cover type blue roof/ rooftop detentionThe temporary storage of stormwater to control discharge rates, and allow for sedimentation.

Blue roofs can be a very economical and low maintenance way to manage unwanted rainwater on a flat roof. For rooftop retention on moderately sloped roofs, a green roof is going to be a better choice. On steeply sloped roofs, the best plan would be rainwater harvesting, or diverting roof leaders to a landscape integrated LIDA 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..


Blue roofs, or rooftop detentionThe temporary storage of stormwater to control discharge rates, and allow for sedimentation. is a popular stormwater management option for many developments. The appeal is the relatively low cost and simplicity of modelling and forecasting performance.

Blue roofs are ideal for:

  • Sites without significant space at ground level for infiltrationPenetration of water through the ground surface.,
  • Zero-lot line projects with outdoor amenity requirements

The fundamental components of a blue roof are:

  • A flat roof
  • Flow control device(s)

Types of blue roof

Water can be slowed in running from a flat roof (up to 2%) through the use of:

  • drain covers (most common),
  • small check damsStructures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales. or weirs across the roof deck, or
  • modular tray systems.

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[1]. 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[2]. 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.

Blue roofs in the treatment trainAlso called the water cycle, this is the process of water evaporating condensing, falling to the ground as precipitation and returning to the ocean as run-off.A combination of lot level, conveyance, and end-of-pipe stormwater management practices.

Blue roofs can be used in combination with green roofs by employing a scaffold system to keep the vegetation rooting layer out of the ponded water. Blue roofs could also be employed in combination with a rainwater harvesting system which had daily usage demand. This could offset some part of the cisternTank used to store rainwater (typically roof runoff) for later use. capacity.

Resources to review



In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
Lists are ordered alphabetically; link updates are welcomed using the form below.

Flow control devices


These products are designed to suspend 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. (or other) systems above a layer of free water for the detentionThe temporary storage of stormwater to control discharge rates, and allow for sedimentation. period.

  1. Ontario Ministry of Transportation. (2016). Stormwater Management Requirements for Land Development Proposals. Retrieved March 7, 2018, from
  2. Bloomberg, M., & Strickland, C. H. (2012). NYC Green Infrastructure Plan: 2012 Green Infrastructure Pilot Monitoring Report. New York. Retrieved from
  3. Richard Hammond (2017). Evaluating Green and Blue Roof Opportunities in Canadian Cities. UWSpace.