A few weeks ago I attended a seminar on vegetated roofs offered by the RI Green Building Council and learned that a lot more goes into these systems then you might at first think. Vegetated roofs have several components beyond just the roof deck, soil, and vegetation. A properly designed system also includes a waterproofing layer, root protection barriers, insulation, drainage components, and geotextile and other erosion control materials. Some also include irrigation. Plantings are selected not solely on appearance but with consideration to rainfall amounts and anticipated wind loads.
The finished product helps mitigate stormwater runoff but can also reduce urban heat island effect. Pictured below is the vegetated roof installed on the CBLS building at the University of Rhode Island.