What is a blue roof and how can it help your business?

Capturing rainwater on your flat roof can reduce flooding risks, energy use and your water bill

8 minutes read

You might have heard of green roofs—growing plants on the roof as a way to insulate the building, reduce the heat island effect and soak up carbon emissions.

Now there’s a new twist on this idea: blue roofs. It’s a pioneering concept of storing rainwater on your roof. The goal is to control runoff in order to reduce flooding risks from backed-up stormwater pipes, a major source of business insurance claims.

Blue roofs are more affordable than green roofs and offer a host of sustainability benefits. These include reduced air conditioning and heating costs because of the insulating effects of the stored water. Businesses can also cut their water bill by using the rainwater for toilets, irrigation, cooling towers and cleaning needs.

The number-one insurance claim in Canada is for flooding, and it's doubled in the last 10 years.

“Blue roofs are more proof that sustainability innovation is everywhere,” says Bruce Taylor, president of Enviro-Stewards, an environmental consulting firm. He said his own business was able to cut its water, air conditioning and heating bills significantly after he worked with his landlord to install a blue roof during a roof renovation. The company was able to reduce its emissions by 15% per employee thanks to its demonstration “affordable smart blue roof,” as Taylor calls it.

Blue roofs are still a novel idea, with only a few examples so far worldwide. But Taylor says there’s growing interest among businesses and insurance companies.

Taylor’s company is now installing a smart blue roof for Credit Valley Conservation, a conservation authority in Mississauga, Ont. An insurance industry association is covering the costs as a pilot project because of the potential for reducing flood damage risks.

“The number-one insurance claim in Canada is for flooding, and it's doubled in the last 10 years,” Taylor says.

When you get a 100-year storm more frequently due to climate change, the stormwater system wasn’t designed for that and it overflows and creates a lot of damage.

What is a blue roof?

A blue roof is a roof adapted to store rainwater. Valves are installed on stormwater drains to control flow during a rainstorm. The idea is to prevent stormpipes from backing up, which can cause flooding damage to the building and send untreated water into rivers and lakes.

“We’re getting more intense and frequent storms due to climate change,” Taylor says. “When you get a 100-year storm more frequently due to climate change, the system wasn’t designed for that and it overflows and creates a lot of damage.”

A small dam-like structure is erected on the roof to contain the rainwater. Water depth typically doesn’t exceed 10 or 15 centimetres. “It would be hard to get more than a few inches because it’s constantly evaporating,” Taylor says.

Your roof is literally a swimming pool liner anyway, so all you’re doing with a blue roof is storing water there.

Aerial view of Enviro-Steward’s blue roof under construction. Source: Enviro-Stewards

Sensors are installed to detect leaks, measure the water level, gather data on energy savings and gauge weather conditions (hence the term “smart blue roof”). The data can be monitored remotely, and the water can be drained in advance if a storm is expected.

“We put a weather station on the roof,” Taylor says. “If my weather data says there’s an inch of rain coming, I can drop an inch of water down the storm drain before it starts to rain. Then I can take 100 percent of that storm when it comes.”

Pipes and valves can also be installed to allow building occupants to use the water for business needs, such as for toilets, irrigation, cooling towers and cleaning machines and equipment.

A blue roof can only be installed on a flat roof, not a peaked one. Buildings built to code generally have enough load bearing capacity and are adequately sealed against water infiltration to permit a blue roof. An installer typically checks the building’s existing roof structure and membrane before proceeding with the installation.

Your building isn’t as hot with a blue roof, so your air conditioning isn’t working as hard.

The water containment area can cover all or part of a roof, depending on other structures or equipment present on top of the building. For example, if mechanical equipment is located on the roof, the containment area can be built around it. Water is typically drained off before the winter.

If a roof is peaked or otherwise inappropriate for a blue roof, there is an alternative: setting up a rainwater harvesting tank. This can be used to divert rainwater from your stormpipes and provide water for business uses. For example, rainwater tanks on the city of Guelph’s bus terminal are used to wash buses in the building. Taylor notes that rainwater is actually better than city water for washing buses as the (naturally distilled) rainwater reduces the need for spot remover.

Is a blue roof safe?

A blue roof is at least as safe as a conventional roof, Taylor says. Leak sensors are installed to detect any water infiltration. “We know right away if it leaks, which isn’t necessarily the case for a roof without a sensor,” he says.

Canadian building codes require roofs to be sealed against water infiltration and strong enough to bear typical snow loads.

A structural engineer inspects the roof to ensure it is up to code and can support a blue roof. “If your building was built to code, it can hold this,” Taylor says. “Your roof is literally a swimming pool liner anyway, so all you’re doing with a blue roof is storing water there.”

In addition, a building with a blue roof is less prone to stormpipes overflowing and causing indoor flooding damage.

What are the benefits of a blue roof?

A blue roof’s key advantage is controlling stormwater pipe runoff in order to reduce the risk of flood damage, which can be costly and disruptive to a business and lead to higher insurance premiums. Leak sensors installed with a blue roof alert you to water infiltration, further reducing water damage risks.

Other “co-benefits,” as Taylor calls them, include:

  • Reduced energy bills

    A blue roof acts as an insulation barrier that moderates temperatures inside the building. That can reduce your air conditioning and heating use and increase the service life of those systems.

    Taylor gives an example to show the impacts. One warm spring day when the outside temperature was 30 C, he measured the temperature of the black tar-and-gravel roof covering part of a building. It was a sizzling 60 C. On a renovated section of the same roof that had a white roofing membrane, the temperature was lower but still fairly hot—40 C. But in a bucket of water on the same roof, the temperature was 20 C. That was cooler than the temperature inside the building, which was 25 C.

    “Your building isn’t as hot with a blue roof, so your air conditioning isn’t working as hard,” Taylor says.

  • Reduced water bills

    Water stored on a blue roof can be used for business purposes, which can reduce your water bill. Taylor said his building’s blue roof provides water to toilets and a “living wall”—a wall of plants—in his business, so he doesn’t need to use any municipal water. And the plants are healthier than comparable living walls that use city water.

    You may also be eligible for a credit on any municipal stormwater charges and benefits under water conservation incentive programs.
  • Prolonged waterproof membrane life
    Less heat may prolong the life of your roof’s waterproof membrane.
  • Sustainability benefits
    Having a blue roof helps your business hit sustainability goals for certification, reporting and social entrepreneurship purposes. Your sustainability efforts can also create customer, employee, stakeholder and community goodwill.

If you were replacing the roof anyway, the extra cost of a blue roof is likely to be paid off by the savings.

Communities can benefit too through reduced infrastructure costs.

“The conventional approach to increasing storm drain capacity is to retrofit the stormwater infrastructure,” Taylor says. “You have to dig up the street, put in bigger pipes, take somebody's property, build a pond on it, dredge it every few years. It’s a lot less expensive to reduce the runoff at the source.”

How can I install a blue roof?

A blue roof installer typically starts with a feasibility study to determine if a building can support a blue roof or needs any roof renovation. The study also outlines a design for the blue roof, costs and quantifies potential savings.

Next, the installer does a detailed design, obtains permits and builds the roof. They typically remain involved to help monitor sensors, operate valves and maintain the system.

How much does a blue roof cost?

The cost can vary greatly depending on the contractor and type of blue roof. A blue roof with a basic water containment area, pipes, sensors and valves costs about $1 to $2 per square foot, Taylor says. Renovation to the roof to bring it up to code is extra.

“If you were replacing the roof anyway, the extra cost of a blue roof is likely to be paid off by the savings from a lower water bill and reduced heating and air conditioning,” he says.

What is the difference between a green roof and a blue roof?

A green roof is a roof that has been converted to a green space. It typically includes a root barrier, drainage and irrigation systems, a growing medium and plants. It generally costs $10 to $20 per square foot to build—about 10 times more than a blue roof.

A blue roof stores rainwater on top of a building. It generally consists of a small barrier to contain the rainwater, sensors, pipes and valves.