In a first for a North American city, Toronto recently passed a new law mandating "green" rooftops for all new developments. Any new construction with floor space of more than 2,000 square meters must devote between 20 and 60 percent of its roof to vegetation. The rule applies to residential, commercial, industrial and institutional structures.

As expected, developers are less than thrilled with the new mandatory rules — least of all that they come during an economic downturn. Some estimate that green roofs could add more than $177,000 to the cost of a project; not including the ongoing maintenance, replacement and repair costs. "I don't think anybody is warm and fuzzy about having a green roof bylaw impressed on them as a prescriptive method," one developer told Reuters.

Still, the benefits to the city in terms of energy savings and rainwater runoff management are seen as cost-effective in the long term. According to the organization Green Roofs for Healthy Cities, more than 3.1 million square feet of green roofs were installed in 2008, a growth rate of 35 percent over the previous year. Though Toronto may have the lead in proactive legislation, the city of Chicago retains the crown for having the most green roofs in North America at more than 600. Still, it should be interesting to see if our neighbors to the North can inspire some U.S. cities to follow their example.