Also known as “living roofs”, green roofs serve several purposes such as, absorbing rainwater, providing insulation, creating a habitat for wildlife, and helping to lower urban air temperatures and combat the heat island effect.
Green roofs can be categorized as extensive, intensive, or bio-diverse, depending on the depth of planting medium and the amount of maintenance they need.
By contrast, extensive roofs are designed to be virtually self-sustaining and require lower levels of maintenance. Extensive roofs are usually only accessed for maintenance and therefore they can be established on a thin layer of soil. Even a thin layer of rockwool laid directly onto a watertight roof can support most mosses and Sedum species.
Another important consideration is between pitched green roofs and flat green roofs. Pitched sod roofs tend to be of a simpler design than flat green roofs because the pitch of the roof reduces allows for easier water run off and therefore reducing the risk of water penetrating through the roof.
Traditional roof gardens, which require a reasonable depth of soil to grow large plants or conventional lawns, are considered intensive. They involve higher levels of maintenance, irrigation and feeding. Intensive roofs are “park-like” with easy access and may include an extensive range of planting such as kitchen herbs to shrubs and small trees.
These are another form of extensive green roof (i.e. low maintenance) that make use of recycled materials (e.g. crushed brick, crushed concrete etc.) in the growing medium. This type of green roof is intended to create a natural wasteland and can be seeded initially with selected species to create some growth. However, the basic design is to attract fauna and flora of all kinds. As the growing medium is not high nutritional value, larger plants are not likely to survive and will die off creating further suitable habitat for invertebrates which in turn will attract other fauna.