Roofs have been greened in more than one way for centuries. Some of the first roofs that were literally green were the famed hanging gardens of Babylon. One of the ancient wonders of the world, exotic plants grew on several tiers of roofs and amazed all who saw them. In more modern times, green roof systems fall into two main categories: (1) garden roofs, and (2) energy efficient roofs. Although roofs that grow grass and other vegetation are also energy efficient, they differ from roofs that have structures and materials designed to conserve and save energy.
As a concept, energy efficient roofs became more well known to the public during the 1970's. However, they didn't truly become a major aspect of construction practices until the late 1990's and twenty-first century. The creation of the Green Building Council in 1998 was probably the main driving force behind green construction and eco friendly roofs. This council set up the LEED (Leadership in Energy and Environmental Design) rating systems that have become a popular and effective way of assessing the energy saving capabilities of structures and construction materials. Since products highly rated by LEED save consumers money in addition to helping to protect the environment, they have become quite popular.