Evert Lindquist of Canada's National Observer writes:

Green rooftops are sprouting up across Canada, sowing the seeds of climate benefits that range from heat moderation in concrete jungles to vegetables served fresh from the roof. In major cities, green roofs also catch and clean stormwater and absorb carbon dioxide, a key contributor to global heating.

The concept of greenery on rooftops dates as far back as 600 BC, when Nebuchadnezzar reportedly built the 20-metre-high Hanging Gardens of Babylon for his wife Amytis. Today, while Toronto is no Seventh Wonder of the World, it does have more than 1,000 green roofs.

A 10,000-square-metre green roof can turn a building from a “heat sink” into a “life-generating force,” explains green roof artist Terence McGlade. “There are so many side benefits to it besides emitting wonderful, clean air into the atmosphere from plants’ oxygenation.”

In Vancouver, a 1,850-square-metre green roof over the public library’s central branch grows drought-resistant bunchgrass and kinnikinnick. Sandra Korpan, associate director of facilities management, said this inverted roof has an elevated protective membrane letting stormwater flow under the soil into large spouts that guide it down the building into the sewer system.

Green roof soil also binds to pollutants, eating them and purifying water so it’s cleaner when it eventually drains into a lake or ocean, added Jennifer Portsmouth, business development manager for Para Space Landscaping in Burnaby, B.C.

Some people don’t realize “the green roof is actually part of the building’s cooling system,” she said, noting it can spare 20 to 50 per cent of the air conditioning in summer.

Green roofs stay substantially cooler than those covered in black asphalt. It’s crucial when rooftop temperatures of 50 C or 60 C can dwarf street-level temperatures of 25 C, according to Kerry Ross, a Calgary-based green roof professional.

They can also retain two-thirds of rainfall, sweating water through plants and filtering it down into ground-level storm systems.

“It’s not about stopping water,” Portsmouth said. “It’s about slowing the water down.”

Build costs vary based on size and greenery, but green roofs can be adapted to climate conditions anywhere in Canada with the right structure, plants and soil.

Read more: Green roofs reap climate benefits. How do you build one?