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 the ENERGY STAR program is not structured to apply its label to complete roofing systems, the EPA recognizes that ballasted EPDM 1 roofing systems are a very effective means of significantly lowering the roof top surface temperature similar to reflective roofing products. In studies conducted by Oak Ridge National Laboratories (PDF, 2.6 MB), roof top temperatures of ballasted EPDM roofs were 30-40% lower than black membranes and are therefore a viable energy saving option for building owners in cooling dominated climates trying to lower their air conditioning costs.
In addition to providing a cooling benefit, ballasted EPDM roofing systems also save energy by eliminating the 3-8% loss in R-value from the thermal bridging of mechanical fasteners as the natural weight of the stone or paver ballast holds the roofing system in place. Ballasted EPDM systems are known for their sustainability due to their proven long life cycle, low environmental impact and the recyclable nature of the major components. Ballasted roofs have great aesthetic appeal; eliminate the need for periodic cleaning to retain reflectivity and have well documented resistance to hail damage.
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: https://www.epa.gov/heat-islands/using-green-roofs-reduce-heat-islands
1EPDM is an extremely durable synthetic rubber roofing membrane (ethylene propylene diene terpolymer) widely used in low-slope buildings in the United States.