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