Cities could stop raw sewage from being dumped into local waterways during heavy rains if they installed more rooftop gardens on their buildings, says a Canadian environmental group.

A report released Tuesday by Ecojustice Canada, formerly known as the Sierra Legal Defence Fund, addresses a problem faced by 89 Ontario municipalities such as Ottawa that have combined sanitary and storm sewers, carrying both human waste and rainwater.

During storms, the sewers become overloaded and are designed to overflow into waterways such as the Ottawa River to prevent them from backing up into people's homes. Sewage overflows and spills due to that design have been blamed for bacterial contamination at Ottawa beaches on the river.

Researcher Liat Podolsky, co-author of the Ecojustice study titled Green Cities, Great Lakes: The Green Infrastructure Report, said rooftop gardens or "green roofs" absorb rainwater, preventing it from flowing into the sewer system and overloading the system's capacity.

"We do think that it could have a huge impact in mitigating some of the impacts of excess stormwater," she said.

Ron Standish, director of wastewater and treatment for the City of London, Ont., said green roofs are a great idea in some situations.

"Obviously it works best on flat roofs, and certainly as opportunities for new development go forward, that's one solution," he said.

But he added that converting an existing roof into a green roof is more complicated and expensive, raising questions about who should pay for it.

Paving, landscaping solutions

Other infrastructure that the report recommends to absorb water and prevent it from flowing into storm sewers include:

• Downspout disconnections, which direct water flowing off roofs onto lawns or into rainbarrels instead of storm sewers.
• Permeable pavements such as cobble stones, paving blocks, or special porous asphalt and concrete that allow rainwater to trickle through to an underlying stone reservoir. The reservoir temporarily stores water until it can be absorbed into soil underneath.
• Biotention areas, landscaped depression planted with vegetation next to parking lots and sidewalks to absorb stormwater.

The report recommends:

• Incorporating such infrastructure into development planning and standards such as building codes.
• Establishing incentives and funding to encourage their installation, such as a program in Toronto that offers grants for property owners who want to plant vegetation on their roofs and a program in St. Catharines, Ont., that supplies subsidized rain barrels.