Effective green building means wringing multiple benefits from building elements and products. Green roofs – with their ability to affect stormwater run-off, in-house energy use and even city pollution levels, and their range of relevance from single dwelling houses to suburbs and city blocks – bring a lot of multi-tasking capacity to the planning table.

A green roof describes, in essence, a layer of planting medium and vegetation covering an impervious roof surface. As a retrofit, a green roof can involve lightweight modular plant-holding components installed over an existing sealed roof. A non-trafficable green roof with 100–300 millimetres of growing medium and little to no irrigation is known as an extensive or semi-extensive green roof. In a new building, a green roof can range from a lightweight modular system to a deeper engineered soil bed on a membrane, right through to the intricately designed 150-millimetre-deep hills and dales of Renzo Piano’s California Academy of Sciences building (above) in San Francisco. A trafficable landscape, with a media depth of 200 millimetres or more and an in-built irrigation system, is known as an intensive green roof. Some of the benefits a green roof brings to a building and to the greater townscape are:

• Water Depending on the depth of soil and vegetation chosen, a green roof can retain up to 90 per cent of stormwater run-off, making it an effective water bank particularly useful in cities with combined sewerage/stormwater systems. Combined with a smaller-scale rainwater-harvesting tank, a green roof makes it a relatively simple task to harvest 100 per cent of a roof’s run-off. In addition, a green roof can filter roof water and can even be engineered to filter the greywater for reuse.
• Urban heat mitigation A 2007 thermal-imaging exercise in Chicago revealed the heat of two adjacent buildings: one black-roofed structure measured 40 degrees Celsius, while the City Hall’s green roof registered 21 degrees Celsius. Chicago and, more recently, the city of Toronto are actively championing the green roof as a way to reduce the urban heat-island effect and the associated issues of high pollution and increased energy use for cooling buildings.
• Carbon sinks In addition to the reduction of pollution through reducing city heat, the types of plants used on a green roof can increase the reduction of CO2 in the air. C4 plants are generally hardy, water efficient and soak up a greater amount of CO2 than trees, which can lose up to 40 per cent of the CO2 fixed in photosynthesis through photorespiration. Australian native C4 plants include black speargrass (Heteropogon contortus) and kangaroo grass (Themeda triandra).
• Energy saving and noise reduction The thermal mass combination of soil and captured water is an effective way to keep a building cool and quiet. Green-roof manufacturers claim a noise reduction as high as 40 decibels; another incentive for the commercial adoption of green-roof systems.

  In addition to the heating and cooling savings inside the building envelope, mounting solar panels over a shade-tolerant green roof can increase the efficiency of the photovoltaic cell (PV) panels by up to 30 per cent.

• Green can be beautiful From habitat re-creation and assisting biodiversity, to the practical benefits of ‘sky farming’ and the emotional satisfaction of a beautiful landscape, a green roof delivers valuable real estate to a building. Best of all, designing and creating a green roof can be as simple or complex as you make it.