Large Scale Energy Savings
A study conducted by the City of Chicago showed that each Fahrenheit degree decrease in temperature yields a 1.2 percent drop in the energy used for cooling processes. Using this calculation, if all buildings in Chicago were covered with green roofs the annual savings would reach $100,000,000. This would also significantly reduce the urban heat island effect and the creation of ozone resulting from the combination of chemicals produced by automobiles and coal firing. Air quality would also be drastically improved, which would in turn lead to lower rates of pollution-related disease and illness. Storm water runoff would also be reduced significantly, as rain would percolate through permeable material rather than rushing over impermeable surfaces.

Small Scale Energy Savings
Some government agencies provide funding for projects proven to be energy efficient above minimum building code standards. Energy costs are reduced by up to 25 percent for a typical two-story building where a large majority of the square footage is roofing.

Aesthetics
Installation of a greenroof not only beautifies a house or other building, but also enhances quality of life for residents and workers. This is especially true for extensive, garden-like green roof systems, which in turn may raise property values and increase rents.

Source: CMHC/OAA Continuing Education for Architects
(http://www.cmhc-schl.gc.ca/en/imquaf/himu/himu_002.c)