"A green roof, or rooftop garden, is a vegetative layer grown on a rooftop. Green roofs provide shade and remove heat from the air through evapotranspiration, reducing temperatures of the roof surface and the surrounding air. On hot summer days, the surface temperature of a green roof can be cooler than the air temperature, whereas the surface of a conventional rooftop can be up to 90°F (50°C) warmer." - EPA
Green roofs significantly reduce heating and cooling costs by better insulating the building. They also capture rainfall and reduce runoff. Designs can be as elaborate as a rooftop park with full trees and habitat for wildlife, breaking up the hardscape of urban living.
Green roofs require a substantial amount of maintenance and can be costly to implement correctly. Because they are installed on flat-roofed buildings, many existing structures could not be retrofitted with a green roof.
Example 1: EPA
Description: EPA webpage for Green Roof information. Page goes through the benefits and costs and has links for references.
Example 2: Urban Design Tools for Low Impact Development
Description: An overview of how green roofs work and links to other references
Example 3: Minnesota
Description: A step by step guide to the process to designing and installing a green roof.