Extensive greenroof on Maryland Department
of the Environment’s
office building.
Source: Katrin Scholz-Barth Consulting
(click on thumbnail
for enlarged view)

There are two primary types of green roofs, intensive and extensive. The main distinction between the two is soil depth. Intensive green roof soils are deeper than 6 inches, allowing for the introduction of larger plants. Intensive green roofs are often used as roof gardens. Extensive green roofs have a thin layer of soil and support succulent plants. The primary function of extensive green roofs is to provide environmental benefits including stormwater management. 

Monitoring of green roofs has been conducted almost exclusively on extensive roofs. This monitoring has focused largely on peak flow and volume reductions. These results are presented below. Data is not as widely available for pollutant and nutrient concentrations. Nutrient export from the soil media is possible during the initial plant establishment period. Research is ongoing to evaluate soil compositions that will minimize nutrient export.