Above the congested roads, busy sidewalks and grey concrete, a sky-high urban farm is slowly taking root in the heart of downtown Toronto.

The roof of Ryerson University's George Vari Engineering and Computer Centre is one of a number of rooftop gardens in the city. On Monday, volunteers spent their time on the four-storey-high garden, tending to mustard greens, garlic patches and other plants.

The quarter-acre hidden gem is part of a citywide initiative to harvest unused real estate and go "green."

"If our roofs are covered in soil and plants then it both cools the city and also soaks up some of that rain water so that it can drain more slowly," Ryerson's urban agriculture coordinator Arlene Throness told CTV Toronto on Monday.

Rooftop garden

Rooftop garden

According to the Environmental Protection Agency, the surface temperature of a green roof can be slightly cooler than the outside air temperature. This helps to combat the "urban heat island effect," where a city is warmer than the surrounding rural areas due to its bustling population.

Rooftop gardens in Toronto are not a new phenomenon. In 2012, the city began applying a bylaw requiring green roofs on new industrial buildings.

Advocates of the idea say it's a smart solution to finding green space in a city where real estate can cost a fortune.

"(In) downtown, every square-footage is priceless so we've been able to take 30,000 square feet and turn it into a place where the community can come and engage in new programming," said Sarah Beldick, the general manager at Cooper Koo YMCA, home of another rooftop garden.

Rooftop garden

Despite the benefits of planting a green roof, however, digging one up is not an easy task. A garden's weight load must be taken into account when designing a rooftop green space, and there must be adequate drainage. All the planting materials also have to be lifted up to the roof.

"Imagine trying to do your gardening but you're trying to bring all the soil, all the plant material, everything up to the roof. Depending on how high your roof is, it can be a daunting task," said Brett Sverkas, a senior manager at the Earth Rangers Centre.

With a report from CTV Toronto's Janice Golding