Green Roofing
Green roofs are essentially an extension of an existing flat roof system that allows plants,  trees, and shrubs be planted and grow in a light-weight growing medium. Generally green roofs systems are located on top of a human-made structure and can be located below, at or above grade. There are generally three types of green roof systems:

The first of the three is a complete systems where all different components of the green roof (including roof membrane) are an integral part of the whole system.

The second type of Green Roof system is the modular systems. these modular systems are positioned above the existing roofing system.

the finals type of green roofing system is the pre-cultivated vegetation blanket. The pre-cultivated vegetation blanket consist of growing medium and plants that are basically rolled onto the existing roofing system complete with drainage mats and root barriers as required.
Typical green roof assembly on conventional roof Photo Courtesy of Sopranature