Blue roofs

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Conceptual diagram of flow control drain cover type blue roof/ rooftop detention

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 LID.


Blue roofs, or rooftop detention 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 infiltration,
  • 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 detention 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 dams 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 detention 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 SWM 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 train[edit]

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 cistern capacity.

For Review[edit]


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[edit]


These products are designed to suspend green roof (or other) systems above a layer of free water for the detention 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