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 infiltrationThe slow movement of water into or through a soil or drainage system.Penetration 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)


Things to consider when planning a rooftop detentionThe temporary storage of stormwater to control discharge rates, and allow for sedimentation. system[1]
Advantages Limitations
  • Do not require ground level space, so particularly well suited where ground space is limited or sites with large flat roofs,
  • May not require additional sewer connections,
  • Easy to retrofit if structural support and waterproofing are adequate,
  • Relatively low cost
  • MUST be regularly inspected and maintained,
  • Limited application on sloped roofs or sites with small building footprints,
  • Do not provide any co-benefits such as habitat creation or air quality improvements associated with vegetated practices,
  • Mechanical systems and other rooftop architectural features must not be adversely impacted by ponded water.


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.[2]

Because all roofs in Canada must be designed to support peak snow loads, this provides capacity for supporting an equivalent weight of ponded water. The Ontario building code permits designers to assume that snow and water loads are not cumulative, up to a maximum depth of 150 mm (water equivalent), if precautions are taken to prevent flooding by providing a sufficient number of roof drains with overflows. It is therefore uncommon for roof water detentionThe temporary storage of stormwater to control discharge rates, and allow for sedimentation. systems to be designed to permit more than 150 mm of water to accumulate. This also means that these systems can be retrofitted onto existing roof structures that have been designed to support conventional snow loads.[3]

The Ministry of Transportation do not currently permit blue roofs to form part of a SWMStormwater Management plan owing to concerns over long term maintenance; this affects relatively few projects[4]. 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[5]. 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.

For 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.

  2. Massachusetts Clean Water Toolkit, Rooftop Detention (Blue Roofs), Retrieved 9 October 2018 from
  3. Richard Hammond (2017). Evaluating Green and Blue Roof Opportunities in Canadian Cities. UWSpace.
  4. Ontario Ministry of Transportation. (2016). Stormwater Management Requirements for Land Development Proposals. Retrieved March 7, 2018, from
  5. Bloomberg, M., & Strickland, C. H. (2012). NYC Green Infrastructure Plan: 2012 Green Infrastructure Pilot Monitoring Report. New York. Retrieved from