CVC Wants to Put a Pond on its Roof


Blue roofs capture and reuse rainwater, instead of letting it flow into municipal sewers. They can reduce the amount of pollution entering our waterways and help us better manage intense storms. They can make life easier and less costly for residents and municipal governments as we experience the effects of climate change. Dealing with stormwater is a growing problem for municipalities. Cities such as Mississauga and Brampton are no exception.

CVC Head Office
CVC Head Office

Why is stormwater runoff such a problem for cities?

Without urban development, the water cycle follows a certain pattern. Rainfall first hits plants and trees, where evapotranspiration sends it back into the air. When plants and trees can’t hold more water, rainfall reaches the ground, where it either evaporates or seeps into the soil. Heavy rainfall events saturate soils, causing water to collect and to flow downhill as runoff.

When vegetation and loose soils are replaced with houses, roads and other hard surfaces, we drastically alter this cycle. With less vegetation to absorb rainfall, less evapotranspiration takes place and more water reaches the ground. Hard, impervious surfaces that dominate urban areas (compacted soil, roads, roofs, and so on) keep water from filtering through the soil. Instead, rainwater collects and runs downhill, becoming stormwater runoff. The more impervious surface, the more runoff.

Hoban and Wong Infographic

Stormwater runoff: flood risk, water quality, and negative ecological effects

High volumes of runoff cause several problems. First, it raises flood risk. Most flooding events in Ontario are flash floods. When lots of rain falls quickly, it can overwhelm municipal sewer systems. When this happens, flooding may occur, as it did in Mississauga in July 2013 and March 2017.

Second, runoff is often polluted. As it flows over roofs, roads and parking lots stormwater collects harmful pollutants – gas, road salt, phosphorus, and heavy metals, to name a few. These pollutants end up in our lakes and rivers and cause problems for both people and ecosystems.

Third, it has negative ecological effects, resulting in urban stream syndrome. When too much water flows through a river in a too short a time, it changes the river’s structure. Urban rivers are wider, flatter and have less tree branches, logs, and riffles, and their banks are often armoured with rocks to prevent erosion. This leaves less habitat for aquatic wildlife. Also, stormwater runoff is often quite warm, which changes the type of ecosystems found in urban streams.

CVC’s Integrated Water Management Team

Our Integrated Water Management (IWM) team works with municipalities and partners across the Credit River Watershed on Low Impact Development (LID) stormwater management practices. LID helps to restore the natural water cycle by filtering, storing and returning stormwater to the ground as closely to its source as possible. To do this, we use bioretention, permeable paving, green roofs, soakaways, rainwater harvesting and other tools. We’ve helped with dozens of LID practices across the Credit River Watershed.

Blue Roofs

CVC and the Region of Peel recently received funding from the Federation of Canadian Municipalities to examine the possibility of building a “smart blue roof” at our head office. This grant is through the Federation’s Municipalities for Climate Innovation Program.

We’ll look at retrofitting our existing flat roof into a smart blue roof, with real-time controls, for holding stormwater over an extended period of time. A network of interconnected sensors and real-time controls will let us save more water for future use and keep pressure off municipal sewers during storms.

We hope to build a demonstration roof so that industrial, commercial and institutional buildings can adopt blue roof technology in the future. Industrial, commercial and institutional lands usually makeup 20-30 per cent of the total urban area, so there’s a great opportunity to reduce runoff.

Industrial park
Industrial park with hard impervious surfaces contribute excess stormwater runoff.

If you’re interested in managing stormwater on your property and live in the Credit River Watershed, we can help. Learn more about programs for landowners.

Comments (8)

    1. Bernadeta Szmudrowska (Credit Valley Conservation)

      The City of Mississauga, like many other municipalities in Ontario including the City of Guelph and the Town of Newmarket, has a stormwater charge program which uses the fees towards improving and maintaining municipal stormwater infrastructure. For example, in Mississauga, by implementing low impact development (LID) and green infrastructure practices such as blue roofs on multi- and non-residential properties, the city may provide a credit to the property owner (up to 50%) based on categories such as peak flow reduction, water quality treatment, runoff volume reduction and pollution prevention.

      As part of this feasibility study, we will be examining the return on investment to the landowner when implementing such a blue roof system. The intent of this study is to make blue roof systems as financially feasible as possible to encourage IC&I property uptake and implementation. For more information on overcoming constraints to installation of LID by private property owners, please see the following 2018 CVC discussion paper (

    1. Bernadeta Szmudrowska (Credit Valley Conservation)

      Liz, thank you for your interest in this project! We will be posting the final results of this feasibility study in early 2019 on our CVC website and – please stay tuned.

    1. Bernadeta Szmudrowska (Credit Valley Conservation)

      Great project considerations, Karen! As part of this feasibility study, we will be investigating mosquito prevention’s as well as if and how a blue roof system would impact insurance rates. During our preliminary findings, we have noted that solutions for controlling mosquitoes include simply draining ponded rooftop water within 24 hours during mosquito breeding periods (mosquito pupae develop into adult flying mosquitoes after 2-3 days in standing water) or adding turbulence ie. an air bubbler to the blue roof (mosquitoes prefer to breed in standing water rather than moving water). If designed, sealed and waterproofed accordingly and qualified contractors are hired during the construction phase, then there should not be any problems with the functioning of such a blue roof and like green roofs, could even improve the lifespan of the roof. All these concerns will be examined as part of this project. The finish report will be on CVC’s website and in early 2019.

  1. I believe the word evapotranspiration is being used incorrectly in your opening paragraph. Merriam-Webster dictionary: Evaportranspiration is the loss of water from the soil both by evaporation and by transpiration FROM THE PLANTS growing thereon. Also, the rain doesn’t get absorbed when it ‘hits the plants’, it is taken up by the plant roots from the soil.

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