What is a green roof, or "living roof"?

A green roof system is an extension of the existing roof, and not potted plants as many assume. The system consists of a special water proof and root repellant membrane, a drainage system, filter cloth, a lightweight growing medium and plants on top. Green roof systems may also be modular, with the drainage layers, filter cloth, growing media and plants already prepared in movable, interlocking grids or trays. These are good for areas where access could be an issue. In North America, the benefits of green roof technologies are poorly understood, despite the efforts of several industry leaders. In Europe however, these technologies have become very well established. Cities like Chicago have launched a Green Roof Initiative, serving as a model for other cities. Answer excerpted from Ask the Green Architect, Eric Corey Freed's column on greenerbuildings.com.  more