Green roof infrastructure promises to become an increasingly important option for building owners and community planners. Green roofs can address many of the challenges facing urban residents. Life cycle costing indicates that green roofs cost the same or less than conventional roofing and they are an investment which provides a significant number of social, environmental and economic benefits that are both public and private in nature.
Green roofs are not a new phenomenon. They have been standard construction practice in many countries for hundreds, if not thousands, of years, mainly due to the excellent insulative qualities of the combined plant and soil layers (sod).In the cold climates of Iceland and Scandinavia sod roofs helped to retain a building 's heat, while in warm countries such as Tanzania, they keep buildings cool.
Until the mid-20th century, green roofs were viewed mainly as a vernacular building practice. However in the 1960 's, rising concerns about the degraded quality of the urban environment and the rapid decline of green space in urban areas, renewed interest in green roofs as a "green solution" was sparked in Germany and Switzerland .New technical research was carried out, ranging from studies on root-repelling agents, membranes, drainage, lightweight growing media, to plant suitability.
The Benefits
High water retention
• Run-off attenuation reduces sewer overflows
Improves environment
• Creates a beneficial monoclimate by cooling and humidifying the surrounding air.
• Absorbs greenhouse gases.
• Absorbs air pollution and dust.
• Reduces the heat island effect, which is the main cause of ozone production
Habitat for animals and plants
Building fabric
• Protects the roof from mechanical damage and ultra violet radiation – thus extending the roof life.
• Can provide extra insulation
(See article about the National Research Council Canada research)
• Reduces noise levels
Can use recycled materials
• Many of the materials used in green roof construction are manufactured from recycled building materials, plastics and rubber reducing landfill disposal.
Types of Green Roof
'Extensive'
Features:• Build-up height 50 – 150mm• Suitable for large areas • Lightweight • Easy to install • Simple to design • Self-sustaining plant community • No irrigation • No / low maintenance Use for:• Lightweight roof decks• Inaccessible roofs • Flat or sloping roofs • Reducing water run-off Planting:• Moss and sedum• Moss and sedum and herbs • Sedum and grass and herbs • Grasses and herbs |
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'Intensive'
Features:• Build-up height 150 – 1500mm• Wide range of plants and trees • Regular irrigation • Regular maintenance • Often indistinguishable from natural gardens • Usually a mixture of hard and soft landscaping • Good insulation properties Use for:• Natural gardens• Recreation and sport • Growing food Planting:• Lawn• Shrubs • Edible plants • Generalist perennials and grasses • Small deciduous trees and conifers |
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Construction
The basic build up of a green roof is three layered: drainage, filter and vegetation layer. Each layer needs to fulfil several functions to decrease the height and the weight of the overall build-up.
Vegetation
Type of planting depends on depth of the growing medium layer as well as other factors. (see above)
Growing medium
Grain size, water retention, air volume, weight and nutrient reserves. The soil needs to be stable, not prone to settlement, well aerated even with water saturation and free of weeds.
Drainage
This layer retains drains water off the roof, protects the root proof layer from being mechanically damaged, retains water for times of drought and provides the substrate with a balanced supply of water and air.
Insulation
(Warm roof rigid insulation)
Root membrane
This prevents roots from damaging the waterproofing. The membranes specification depends on the planned landscape and the slope of the roof.
Movies
• Design guidance for planting on sloped roofs
• An inspirational short film about the roof of Vancouver's Conference Centre
Publications
• 'Green Roofs' by Hassell and Coombes, CIBSE Knowledge Series: KS11, 2007
Further information
• 'Living roofs' - promotes green roofs and living roofs in the UK (www.livingroofs.org)
Disclaimer
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