Green roofs are areas on top of a building covered in soil and grass, or more extensive planting. They have become increasingly popular as a feature of new buildings or an add-on to existing structures, but are they worth having and if so what are the advantages? Green roofs’ soil and plant layers sit on top of the conventional waterproofed roof surfaces of a building. Whilst green roofs come in many different forms, usually a distinction is made between intensive, extensive, and biodiverse or wildlife roof types.
An intensive green roof has a deep soil, usually over 20 cm in thickness, and thus can support a wide variety of plants including trees and shrubs. Naturally the structure of the building needs to be sufficient, or be reinforced to hold up the increased weight. With the variety of plants in this type of roof, they often require extensive maintenance, similar to a conventional garden.
In contrast, an extensive green roof is becoming a fashionable feature of many buildings nowadays. They are composed of lightweight layers of free-draining material that support low-growing, hardy, drought-tolerant vegetation. Generally the depth of growing medium is from a few centimetres up to a maximum of around 10-15cm. These roof types have great potential for wide application because, being lightweight, they require little or no additional structural support from the building. Furthermore, because the vegetation is adapted to the extreme roof top environment (high winds, hot sun, drought, and winter cold), extensive green roofs require little in the way of maintenance and resource inputs. Extensive green roofs can be designed into new buildings, or ‘retro-fitted’ onto existing buildings.
The biodiverse, or wildlife roof is an attempt to compensate for loss of habitats by reproducing that habitat on a roof, so as to encourage species, like bees and other insects.
The economic benefits to a property-owner include insulation (because the soil layer adds thermal mass), cooling (as water evaporates), the ability to significantly reduce rainwater run-off from roofs, and their value in promoting biodiversity and habitat in built-up areas. Green roofs are important elements of sustainable construction, and also have ancillary benefits, such as being a pleasant area for people to relax in, whether the building is a residential or working property. They also can add a significant design element to a building, making it more visible and a “talking point” feature for visitors. There have been various studies of the energy performance of green roofs, for example, a good example of energy saving is Paradise Park in the London Borough of Islington. Owing to the thermal mass of the green roof, no air conditioning has been installed. This has led to a reduction in energy consumption of 3,800kW/hrs and a saving of 1.6 CO2 tonnes. This positive improvement in energy performance should be reflected in a better Energy Performance Certificate rating after the green roof is installed.
In conclusion, green roofs are another way to add sustainability to both new and existing buildings, which has both economic and environmental benefits, as well as being a method of making the building more pleasant for its users.