Blue roofs could help reduce the flooding effects of big storms

CBC

Published: April 23rd, 2021

You've probably heard of a green roof, which sits on top of a building and is covered with vegetation. But what about a blue roof?

You might have guessed that it has something to do with water. Indeed, a blue roof collects stormwater through a pond system, temporarily stores it and gradually releases it afterward — offering a way to conserve water and prevent water damage.

During a storm, rainwater can overwhelm urban sewer systems and send contaminated, untreated water into lakes and rivers. A blue roof could help solve this problem.

"It's a new form of green infrastructure," said Rohan Hakimi, an engineer in integrated water management with Credit Valley Conservation in Ontario. Hakimi said the benefit of blue roofs is most evident in industrial commercial areas, which make up almost 30 per cent of commercial lands in cities.

"When it rains in these areas, because of all these hard surfaces, the water doesn't have anywhere to go, so you get a lot of runoff and risk of flooding."

Water damage has become the leading cause of personal property claims in Canada, said Bruce Taylor, president of Enviro-Stewards, a company that provides businesses with sustainable solutions to environmental challenges.

He said our current urban infrastructure was not designed for the extreme weather conditions we are increasingly experiencing as a result of climate change.

"With climate change, you won't get the same amount of precipitation but you get it in a shorter duration in bigger, shorter storms," Taylor said. "If you get water faster than you designed for, then it fills up and it starts backing up and you get flooding. And flooding is very expensive wherever that occurs."

A blue roof system stores rainwater and slowly releases it using flow-control devices or structures, from customized trays to existing building risers that cause water to dam up. Together, they act as a temporary sponge, collecting and then releasing the water over time.

The stored water also provides the building with a cooling effect through evaporation, as well as additional water for reuse.

Enviro-Stewards will be doing an impact assessment for two community organizations based in London, Ont., including a food bank, where the water stored in the roof will be used for irrigation in their greenhouses and gardens, said Taylor.

An "active" blue roof has a greater capacity than a "passive" one, allowing it to store water for a longer period of time and releasing it at a faster rate. Since the food bank doesn't have the pool liner required for an active roof, they implemented a slow-release roof drain as a passive blue roof.

Taylor said that blue roofs are more affordable than green roofs for the general population, although some jurisdictions have managed to successfully combine the two. For example, the RESILIO project in Amsterdam will install 10,000 square metres of blue-green roofs on social housing complexes (see photo above).

A blue-green roof incorporates the water storage layer below the green areas throughout the roof surface. When it rains, the water slowly supplies the necessary soil moisture for the plants and trees.

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