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Cheap and quick green roofs

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Rooftop rain gardens are relatively inexpensive and can hold large amounts of water. And if the authorities accept a recommendation by an expert panel on floods to mandate that all buildings have these green roofs, they can be quick to install, too.

The 12-man panel, tasked by the Government to look into solving Singapore’s flooding woes in the long run, has recommended that building owners be required by law to build green roofs. These 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.

Local contractors The Straits Times interviewed yesterday said these gardens would cost from $20,000 to $180,000, depending on their size.

Property developer City Developments, a leader in green buildings here, spends $150 to $400 per sq m for a green roof for a new building, and $105 to $150 per sq m to retrofit an existing one. For a residential project with an extensive green roof, installation generally does not exceed 1 per cent of total construction cost, it said.

Contractors say such gardens can store anything from about six to 34 litres per sq m. The size can range from 200 sq m for a commercial building, to 1,200 sq m for the entire roof of a multi-storey Housing Board carpark.

The National Parks Board (NParks) said the cost of such gardens is usually between $100 and $150 per sq m, which means a commercial green roof costs between $20,000 and $30,000 and can hold 6,800 litres. An HDB carpark roof costs $120,000 to $180,000 and can hold 40,800 litres of water.

Mr Andy Chew, director of local firm Elmich, which designs, builds and installs green roofs, said the idea of rooftop gardens to help alleviate flooding could work for Singapore as large amounts of rainwater can be stored in the garden’s water retention system.

This comprises soil-like material, membranes and storage trays. The water is then eventually absorbed by the plants as they grow. He added that 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.

Elmich, which has been in the business for 26 years and has installed gardens such as the one atop Orchard Central mall, offers systems that can store between six and 28 litres per sq m.

Another firm, Prince’s Landscape & Construction, offers a proprietary solution whose water reservoir feature can store up to 34 litres of water per sq m.

Its manager Eugine Spicer said its 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, said Mr Spicer.