While there is no standard classification for roof top greening covers they can be generally classified as being extensive or intensive green roofs.¹ Extensive green roofs are characterized by their low weight, low capital cost and low maintenance.  Intensive green roofs are characterized by their increased weight and capital cost, and their intensive planting and higher maintenance requirements.  Extensive and intensive green roofs are either accessible or inaccessible but are always constructed to provide accessibility at least for roof maintenance activities.   

Green roof top covers can vary from small-scale and simple designs that utilize a single plant species, to very large and extensive covers of landscaped gardens with numerous species.  It is the desired function of the roof space that usually drives the green roof design, resulting in designs requiring different soil depths to accommodate various plants, shrubs and trees.  The additional structural support necessary to accommodate higher roof loads must also be taken into consideration. 

Plant species are selected that have properties such as shallow root systems, good regenerative qualities, resistance to direct solar radiation, drought, frost and wind.  Vegetative cover can consist of a thin layer of moss and lichens to an assortment of native grasses, shrubs or even intricately landscaped gardens with multiple species and a soil substrate of 6 inches or more.  Green roof top covers can be found atop anything from low density residential dwellings in rural settings, to commercial, institutional and even large-scale industrial sites of at least 10 acres.

Green roofs can also be classified according to their functionality, such as:

·        Single layer systems with free drainage.

·        Multi-layer system with a freely-drained basal drainage layer.

·    Multi-layer system that incorporates restricted drainage into its design, thus creating additional roof top storage.

The waterproofing component of a green roof is by far the most important factor for the long-term success for the system.  A typical green roof is a composite system of several layers of protective materials to achieve waterproofing and to convey water away from the roof deck.  While the actual design specifications of a green roof will vary somewhat depending upon the manufacturer, a generalized, generic design consists of the following components (http://www.roofmeadow.com/). ²   

Green roof cross-section
(Source: American Wick Drain Corp.)

·        A waterproof membrane installed atop the existing roof, followed by a root barrier, a layer of insulation (optional),

·        A drainage layer, applied over the entire roof area, must be present to carry away excess water. On very shallow extensive green roofs the drainage layer may be combined with the filter layer. 

·        A filter fabric for fine soils, the engineered growing medium or soil substrate (minimum of 2.5 to 3-inches to support a diverse and healthy plant community) and finally the actual plants  

·        (Additional items can include a "wind blanket," such as a jute or coco liner-type mesh, to help stabilize and establish the roots of the new plants, as well as measures to prevent shearing and erosion on roofs of 20º or more. Measures on steep roofs include the installation of additional support with cross battens. A raised grid structure is installed to secure the growing substrate. A shallow layer of gravel or pebbles are placed from 18" to three feet within the outside perimeter of the roof to provide for additional drainage, fire control and roof access.        

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References

¹ Bass, B., 1999: Modeling the Impact of Green Roofs on Toronto’s Urban Heat Island. Environment Canada, Green Roofs for Healthy Cities.

² Roofscapes, Inc., Green Technology For the Urban Environment.,  (http://www.roofmeadow.com/), C. miller