Ballasted roofing systems are fairly common and consist of a membrane that is usually loose laid on the roof deck and held in place by the weight of a natural stone or precast concrete pavers. The weight of the stone or pavers employs the force of gravity to hold the roof membrane in place and counter the uplift forces of wind.
While not technically an “ENERGY STAR” roof, recent research by Oak Ridge National Laboratory (ORNL) indicates that ballasted roof systems can save energy. To learn more about this, please visit: http://www.ornl.gov/sci/roofs+walls/staff/papers/21.pdf
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. Green roofs absorb heat and act as insulators for buildings, reducing energy needed to provide cooling and heating. For more information on green roofs, please visit: http://www.epa.gov/heatisland/mitigation/greenroofs.htm