Nearly 25% of every major urban center in the world is comprised of rooftops. That's a lot of heat and storm water runoff. Green roofs are more expensive than standard roofing, but they provide a wide range of benefits, to both the general public and the building owner, among them:

Air quality - through photosynthesis, green roofs help filter the particulate matter and pollutants from the air and generate oxygen.

Heat reduction and lower energy costs - through evapotranspiration, green roofs help combat "urban heat island effect." Plants and soil also heat less quickly and thoroughly than roofing materials and do not reflect heat back into the air, which results in less radiated heat and a lower ambient temperature. According to research, a drop in air temperature of just a single degree centigrade reduces peak load energy consumption by as much as 4% and helps reduce smog by as much as 10%. Green roofs also provide substantial insulation to the buildings they shield, having a cooling effect in summer and retaining heat in winter, lowering energy bills for the building owner in question.

Storm water - green roofs retain 50% or more of the annual rainfall they experience, which reduces storm runoff and results in less flooding and aids in erosion control and maintaining water quality in the area. This helps to maintain safe drinking water and protects aquatic plants and wildlife.

Noise reduction - plant material cuts noise, as opposed to the hard, flat surfaces traditionally found on most rooftops. They help quiet the environment and reduce the amount of sound penetrating the buildings they top.

Longer lasting roofs - traditional roofs have a general lifespan of 20 years, as opposed to the 40-60 year lifespan of a properly installed green roof.

In addition to those stark improvements, other benefits can be realized, including the therapeutic effects of increased green space, community gardens, biodiversity, and aiding the efficiency of solar panels.