Green Roofs are generally defined as a roof that has been purposely cultivated with a layer of vegetation and soil, or a growing medium.
Application of green roofs has increased significantly over recent years, due to their economic and environmental credentials, to the point at which it is now a legal requirement in some parts, particularly Germany, to provide green roofs on newly constructed flat roof systems.
Their use has been assisted due to changing attitudes, with increasing attention on ‘being green’, and the threat of Global warming, has highlighted their benefits, particularly in saving energy through reduced ventilation and drainage requirements, and their contribution to biodiversity.
Another trend, which is the greater demand for outside space has also accelerated their popularity, particularly for intensive systems in built up, urban areas, where space is at a premium.
They are generally found on flat roofs, although the extensive system may be installed on a traditional pitched roof. They have a number of benefits, these include:
- Aesthetics – Highly visible, provides an outward sign of sustainability, a positive and distinctive green image for a building.
- Protection and Insulation – Vegetated layer protects the roof from the extremes of the weather and reduces thermal variance, providing insulation from the summer heat and retains heat during the winter.
- Mitigation of Urban Heat Island Effect - A particular concern in built up urban locations.
- Reduce runoff - from roofs by at least 50%, contributing to urban drainage and flood alleviation schemes.
- Reduced Need for Air Conditioning systems – Particularly in summer, and thus reduce energy consumption and CO2 output.
- Prolonged expected roof life – Green Roof systems can double roof life, thus reducing requirement for maintenance and repair.
Green Roofs consist of a number of elements; however they generally consist of plant, substrate, drainage, protection and waterproofing layers.
There are currently two main varieties of Green Roof specialism, these are:
These Green Roof systems have recently seen the greatest interest amongst consumers, primarily due to their relatively inexpensive installation, and wide ranging suitability for a diverse portfolio of differing properties.
These systems can be designed into new buildings or retro fitting onto existing buildings. They are composed of lightweight free draining material, which supports tough, draught resistant mosses and sedums on a layer of soil, from a few millimetres up to 10cm. These roof systems have two main benefits.
Lightweight - Applicable on a large scale, suitable for the majority of existing roofs, with little structural work required to facilitate their use.
Tough Vegetation - Adapted to harsh roof top climatic conditions, these being high winds, hot sun, draught, and winter cold. They therefore require little maintenance and remedial work.
These are typically referred to as a traditional approach, whereby they perform in the same fashion befitting that of a typical garden, as their larger soil profile; typically 150mm (6 inches) allows planting of larger plants and small trees.
Their main benefit, distinguishing them from extensive green roofs is their ability to provide a useable outside space.
Intensive roofs facilitate creation of diverse ecological habitats, providing shelter and food for a range of insects and birds. Intensive Roofs also allow a vast array of differing plant types to be grown, representing a real horticultural opportunity. Green roofs may also incorporate a pond see Blue Roofs.
Their main disadvantage is their relatively higher cost associated with their installation, due to the need for structural consideration to take the larger imposed and dead loads from the planting material.
Ideally intensive roof systems should be designed into the building within the original construction, however if the intention is to retrofit to an existing building, a number of structural alterations will need to be undertaken. It is recommended that a Structural Engineer be appointed, to ensure the construction adequate deals with loading requirements. Essentially the addition of rafters will increase the load bearing capacity of the roof, with larger and deeper beams having greater benefit.
Green Roofs have the potential to play a significant role in reducing our CO2 output, due to the scale and number of existing flat roofs currently within the built environment today. In order to meet and fulfil this potential, greater awareness needs to be attained, in terms of technical knowledge of their installation and application, whilst educating potential clients to the diverse array of benefits which Green Roofs may offer.