When writing my post about Toronto mandating green roofs for new construction over 2000 sq m., I had remembered that Tokyo had a similar law, but hadn't provided any source material.
Since then, I decided to find out what localities or countries required green roofs by law. While I don't wish to discuss the merits or demerits of mandating green roofs by the force of law, I am happy to discuss the benefits from the green roofs themselves. Water Barrels and Storage
As mentioned above, Toronto's City Council passed a law in May of 2009 requiring roofs over 2,000 sq m. to have green roofs that cover between 20-50% of the roof's surface. For Toronto, storm water runoff and pollution control are significant issues, as well as reducing heating costs. However, it turns out the Tokyo is the leader in legislating green roofs.
The Fujitsu Research Institute reports that the law, Ordinance on Environmental Preservation, originally passed in December of 2000, implemented in April 2001, and revised in June of 2008, is focused on reducing petroleum in general, though it also spells out other green activities, including green roofs.
Greenroofs.com highlights that the Ordinance requires "...new buildings greater than 1,000 square meters (10,000 sf) or over one-quarter acre to green at least 20% of its useable roof space." For Tokyo specifically, a key goal in implementing this ordinance was to reduce the urban heat island effect. Mountain House Freeze-Dried Food
Not to be outdone by the Japanese, Switzerland has a federal law requiring green roofs. The law, however, is administered at the Canton (state) level, with each state taking a slightly different interpretation.
Again, greenroofs.com is a wealth of information, with this quotation:
"The cities of Basel, Zürich, and Luzern, for example now require that every new flat roof be planted per building code. As of the end of 2005, approximately 20% of the flat roofs are green roofs within Basel in the city of Basel in a year there will be around 80. This equals about 80 city projects totaling 480,000 m2, with the outlying countryside projects also around 80, estimated at 500,000 m2 (Dr. Stephan Brenneisen, 2005)."
As we have seen, legislating green roofs does create great benefits for the localities in which the laws are past. Buildings use less energy as the green roofs reduce HVAC costs, and they also reduce the load on storm water management systems. Additionally, major cities reduce the urban heat island effect. Clearly, these are tremendous benefits, though I don't personally believe they should be mandated by law.