The Danish capital of Copenhagen has introduced legislation requiring any new building whose roof slopes less than 30 degress to have a green roof.
A "green roof" in this context, is a roof covered with lawn or other vegetation. Parliament House in Canberra is Australia's most obvious example of a green roof.
Green roofs aren't just green in colour. They are good for the environment because they help insulate buildings (reducing energy use) and they absorb carbon dioxide. Another advantage in Australian conditions is they retain water and can therefore reduce flooding in severe storms, and allow more water to be diverted from runoff into water tanks.
They also offer havens for biodiversity, the potential to grow food within cities, and simply more green spaces for nature-deprived city folk.
And, because they protect the roof underneath from temperature extremes and UV rays, it's estimated a green roof will last twice as long as the same roof without the green on top.