Green roofs are an accepted part of modern building in Europe where some city and even national governments have mandated their use (Linz, in Austria requires green roofs on all new residential and commercial buildings with rooftops larger than 100m2, German green roof building has been encouraged by the Federal Nature Protection Act, the Building Code and state-level nature protection statutes).
Green roofs are particularly effective in denser, more urban environments, where they can compensate for the loss of productive landscape at ground level.
The benefits of green roofs include:
- Longer roof lifespan.
- Improved sound insulation.
- Reduced heating and cooling requirements.
- Reduced storm water run-off.
- Pleasant appearance.
- Garden and urban food source.
- Reduce urban heat island effect.
- Cool area around solar panels to reduce temperature that impacts efficiency.
Extensive green roofs
- Shallow growing medium – 50 to 200mm.
- Roof structure similar to conventional roof coverings.
- Vegetation limited to shallow rooting plants.
- Relatively economical.
- Relatively easy to retrofit
- Trapping of gaseous and particulant pollutants.
- Alleviation of urban heat islands.
- Increased biodiversity
Intensive green roofs
- Deep growing medium – 200mm or greater.
- Requires stronger roof structure.
- Wide range of plantings possible.
- Relatively expensive.
- Difficult to retrofit.
On top of the structural components, there are typically seven layers to a green roof:
- Waterproofing membrane (either built-up roof, single-ply membrane or fluid-applied membrane. Modified bitumen or plastic sheeting most typical).
- Root barrier (polyethylene sheeting, copper or copper compounds in the membrane).
- Insulation (optional).
- Drainage layer (synthetic drainage mesh, granular aggregate).
- Filter fabric (geotextile).
- Growing medium – also known as planting medium or substrate (manufactured soil, crushed brick or other inorganic material).
- Vegetation (shallow-rooted on extensive roofs, deeper-rooted on intensive roofs).
Green walls are constructed with plants rooted in sheets of fibrous material which may be fixed to a wall or frame, or they may be
constructed more like vertical arrays of pots or planters. Some proprietary green wall systems come in the form of modular panels. Plants may be pre-grown in these panels or planted after the panels have been installed.
Materials include steel for supporting frameworks, HDPE plastic for plant containers, and geotextiles. In exterior applications, irrigation may be from the top via soaker hoses or similar. Interior applications may use drip trays.
Both green roofs need to allow for irrigation of vegetation without loss of soil and to provide reservoirs of water to carry plants through periods of low water availability
Things to watch out for
When installing a green roof it is important to consider:
- The climate zone.
- Micro climate and roof orientation.
- Local habitats and species.
- Plant selection.
- Integration with building functions generally.
The correct growing medium for the climate and plant selection is essential, particularly for extensive roofs. Plant selection for green roofs requires careful consideration as different conditions apply to vegetation on the roof compared with ground level and long term plant maintenance is essential.
Plants that have been able to survive the hot Australian sun are:
- Stypandra glauca (blue flowers)
- Dianella revoluta
- Isotoma axillaris (purple flowers)
Source: Downton, P. Green roofs and walls ; ABC iView, thealzblog.com