Green roofs are vegetated layers that sit on top of the conventional roof surfaces of a building. Usually a distinction is made between 'extensive' and 'intensive' which refers to the required level of structural load bearing capacity of the roof as well as to the degree of maintenance the green roofs will require.

Intensive green roofs are composed of relatively deep substrates and can therefore support a wide range of plant types. Intensive green roofs (what most people think of as roof gardens) have in the past been rather traditional in their design, simply reproducing what tends to be found on the ground, with lawns, flower beds and water features. However, more contemporary intensive green roofs can be visually and environmentally exciting, integrating water management systems that process waste water from the building as well as storing surplus rainwater in constructed wetlands. Because of their larger plant material and horticultural diversity, intensive green roofs can require higher degree of maintenance including the usual pruning, clipping, watering and weeding as well as irrigation and fertilisation depending on the plant choices.



Conversely the extensive green roofs are composed of lightweight layers of free-draining material that support low-growing, tough drought-resistant vegetation. These roof types have great potential for wide application because, being lightweight, they require little or no additional structural support from the building, and because the vegetation is adapted to the extreme roof top environment (high winds, hot sun, drought) they require less in the way of maintenance and resource inputs. Extensive green roofs can be designed into new buildings, or 'retro-fitted' onto existing buildings.


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