Green roofs and walls YourHome

Green roofs and walls YourHome

From the turf roofs of Viking dwellings in Scandinavia to the ‘hanging gardens’ of ancient Babylon, green roofs have a history reaching back thousands of years. Modern green roofs and walls are building elements designed to support living vegetation in order to improve a building’s performance. Also known as ‘living’ roofs and walls, they are emerging as important additions to the palette of construction techniques for creating healthy, ecologically responsible buildings.

Photo: Paul Downton

A green roof can be an oasis in the city landscape.

A green roof is a roof surface, flat or pitched, that is planted partially or completely with vegetation and a growing medium over a waterproof membrane. They may be ‘extensive’ and have a thin growing medium (up to 200mm deep) with ‘groundcover’ vegetation, or ‘intensive’ and have soil over 200mm deep supporting vegetation up to the size of trees.

Green walls are external or internal vertical building elements that support a cover of vegetation rooted either in stacked pots or growing mats.

Green roofs are an accepted part of modern building in Europe where some city and national governments have mandated their use. The Austrian city of Linz, for example, requires green roofs on all new residential and commercial buildings with rooftops larger than 100m 2. and German green roof building has been encouraged by the Federal Nature Protection Act, the Building Code and state-level nature protection statutes. Australian examples are less common but in 2007 a national organisation was formed to promote green roofs, and Brisbane City Council included green roofs in its proposed action plan for dealing with climate change.

The growing interest in green roof and wall construction has been encouraged by the increasing availability of technologies that make their construction easier and more economical.

Earth-sheltered houses have green roofs by design, and anyone who has grown climbers across a vertical trellis has had some experience in creating green walls. The growing interest in green roof and wall construction has been encouraged by the increasing availability of technologies that make their construction easier and more economical.

Green roofs and walls have become common features in illustrations of modern architectural and urban design proposals but their implementation remains limited by perceptions of high costs and questions over their utility. A changing climate and increasing demands for high levels of environmental performance are likely to see these arguments continue to shift in their favour.

Photo: Paul Downton

Lettuce and herbs growing on an experimental green wall in Adelaide.

Green roofs can be particularly effective in denser, more urban environments, where they can compensate for the loss of productive landscape at ground level. Examples range from herbs growing on a 2m 2 bicycle shed roof in Sheffield, England, to vegetables growing on a 558m 2 organic rooftop farm in Brooklyn, New York. ‘Green wall’ techniques can be used on homes in suburban settings as part of aesthetic enhancement, to improve the overall climate responsiveness of individual dwellings, and even to treat wastewater.

The benefits of green roofs include:

  • longer roof lifespan
  • improved sound insulation
  • reduced heating and cooling requirements
  • reduced and slowed stormwater runoff
  • capture of gaseous and particulate pollutants
  • alleviation of urban heat island effects
  • increased biodiversity.

There is also the potential for green roofs to provide carbon sequestration.

Many of these benefits also apply to green walls.

Green roofs and walls YourHome

Green roofs are sometimes referred to as the ‘fifth façade’. Each of the two kinds of green roof — intensive and extensive — is appropriate for different purposes.

The intensive roof is typically much heavier, supports more substantial vegetation and is more expensive than ‘extensive’ roofs, which are often light enough to be retrofitted to existing buildings without the need to upgrade their structural capabilities.

There are no rigid categories for green roofs; the following descriptions are based on common use.

Photo: Paul Downton

Extensive curved green roof with native grasses.

Types of green roofs and walls

Extensive green roofs

Extensive green roofs have a shallow profile. They provide much of the environmental performance benefits of deeper, intensive green roofs but cannot support general foot traffic. Their shallower profile means less substrate volume to store water and support root growth (although there is still enough to reduce and slow stormwater runoff), and this limits the variety of plant options to those that tolerate environmental stress, especially drought and desiccating winds.

Source: Paul Downton

Cross-section of typical extensive green roof.

Extensive green roof characteristics

  • Shallow growing medium — typically less than 200mm
  • Roof structure similar to conventional roof coverings
  • Weight 60–200kg/m 2
  • Vegetation generally limited to low, shallow-rooting and groundcover plants that are tolerant of drought, wind exposure and temperature fluctuations
  • Not suitable for general access
  • Relatively economical
  • Some thermal and acoustic insulation benefits
  • Relatively easy to retrofit on existing roofs
  • Low maintenance

Extensive green roofs, often situated in urban areas, require low maintenance vegetation tolerant of heat, cold, drought and wind. Although there is limited experience in Australia of such roofs, it is likely that many native plants from coastal and arid inland regions are suited to use in such harsh and demanding environments.

Intensive green roofs

Intensive green roof profiles can range from 200mm to over 1m deep. This increases the volume of growing media available for root development and water-holding capacity and greatly extends the variety of plants that can be grown. The additional weight demands a stronger physical roof structure than extensive green roofs but allows for foot traffic. Intensive green roof gardens can be as richly planted and landscaped as ground level gardens. They require the same level of maintenance as conventional gardens.

Photo: Paul Downton

Intensive inner-city green roof on Adelaide apartment buildings.

  • Deep growing medium — 200mm or greater
  • Requires stronger roof structure
  • Weight 180–500kg/m2 or more
  • Wide range of plantings possible, from groundcover to trees
  • Suitable for access and use as roof garden — wide scope for design and multiple uses
  • Relatively expensive due to structural requirements
  • Substantial thermal and acoustic insulation benefits
  • Difficult to retrofit on existing buildings
  • Regular maintenance required

In between these types there are semi-extensive (extensive with areas of deeper soil) and semi-intensive roofs (intensive with areas of shallower soil).

Green walls

Green walls are like vertical gardens and may be inside or outside a building. In their more elaborate form, green walls are ‘living walls’ and may incorporate water elements including ponds and fish. Green walls may also be incorporated into the cooling strategy of a house, as a kind of evaporative air conditioner, and they may even be designed as part of a water treatment system. Like green roofs, green walls at their simplest can be made on a low-tech DIY basis or be quite sophisticated and expensive. An increasing number of proprietary green wall systems incorporate irrigation systems.

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