An “extensive” green roof is one with relatively thin soils, most typically 3-4″, and thus planted with a limited mixture of plant materials, mostly those that are extremely drought tolerant, such as sedums. An “intensive” green roof is one with deeper soil depths, and thus able to support a great variety of plants. An “extensive” green has many environmental benefits, including insulation of the structure below and absorbtion of storm water. But it lacks the many other benefits of creating a more complex ecosystem, with a variety of trees, shrubs and plants, able to replace the habitat and life that was extinguished when the building obliterated all life that had existed in its footprint, sequester greater amounts of carbon, support a more pleasing year-round variety of ornamental horticulture and, of particular interest to me, support the growing of plants for food.
When I purchased the apartment, the roof had been finished as a typical “extensive” green roof, with drought tolerant ground covers and a few clumbs of the ubiquitous and pedestrian Bayberry (Myrica pensylvanica).
Luckily, as a new building built to LEED “Platinum” standards, the roof deck was 14″ of steel reinforced concrete, with support columns at key points under the deck. This allowed a variable increase in soil depth, with berms rising from the existing 3-4″ to as much as 24″ of depth in certain places. The largest trees were placed over columns.