The District gives permits for three types of roofs:
1) Extensive Design, which is usually approved quickly, according to DC Greenworks. The roof has a shallow bed of growing medium (three to four inches deep), consisting mostly of expanded shale and slate and only 30 percent organic material. A higher percentage would make the roof heavier and potentially lead to rot.
2) Semi-Intensive, which contain deeper growing mediums that support plants with deeper roots.
3) Intensive systems, , which contain vegetables, bushes and even trees can be grown
The benefit of the former is that you can easily include or exclude different components, such as irrigation, to customize to a user’s need. A modular system, on the other hand, allows one to easily take up different sections to look for leaks or to replace dead plants. Also, modules usually come pre-planted, and thus the new roof has complete plant coverage sooner. A modular system is heavier and more expensive.
Resources on green roof design and function can be found on the District Department of the Environment, DC Greenworks and Green Roofs for Healthy Cities. (Wash Post, 7/23/2012)