A Green roof, also known as a rooftop garden or rain garden, is a roof that is partially or completely covered with plants and provided with an irrigation system.

Such greenery reduces the ‘urban heat island’ effect which makes a built-up area significantly warmer than its surroundings.

Green roofs also:

  • Conserve energy use in the building by keeping temperatures down so less air-conditioning is needed;
  • Improve air quality;
  • Reduce noise pollution;
  • Enhance a building’s aesthetics;
  • Store rainwater and reduce the amount and speed at which water flows to the ground.

Rooftop rain gardens are relatively inexpensive and can hold large amounts of water.

The rooftop gardens, traditionally installed to beautify the skyline and reduce the heat around a building, can help absorb rainwater and reduce the speed of water flow.

For a residential project with an extensive green roof, installation generally does not exceed 1 per cent of total construction cost. Such gardens can store anything from about six to 34 litres per sq m.

This comprises soil-like material, membranes and storage trays. The water is then eventually absorbed by the plants as they grow. The soil-like material also helps to regulate the flow of water; therefore, the speed of any excess water that flows down to ground level is reduced. The system is also light and can typically be installed in an average building.

Roof gardens can help alleviate flooding as ‘the sudden flow of water is minimised’. Depending on the size of the project and whether there is easy rooftop access, installing a green roof of about 400 sq m could take a month. Growing the plants takes two to three months before that.

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