How green does your rooftop grow? HKU wants to find out ...

PUBLISHED : Monday, 12 July, 2010, 12:00am
UPDATED : Monday, 12 July, 2010, 12:00am

University research is identifying the fittest plant species to insulate the city's rooftops in the hope of promoting green roof creation in new development areas as well as retrofitting for old buildings.

The experiment, started two months ago by Professor Jim Chi-yung from the geography department of the University of Hong Kong, is a sequel to one he did in 2006, which found that a planted rooftop can lower the surface temperature by as much as 26 degrees Celsius.

The 2,000-square-metre rooftop of the university's main library has been turned into an oasis, with 50 plant species under testing and being nurtured with compost.

The experiment is to find the species that can best endure the intense infrared and ultra-violet light in sunlight, is most drought-resistant, and grows with the thinnest layer of soil.

'A thin soil layer is important because many old buildings in Hong Kong were designed with a small loading,' Jim said. One square metre of roof area should be able to support a weight of at least 350 kilograms to make greening possible, and many schools would fit the requirement.

Despite the merits of reducing heat and saving air-conditioning costs, demand for green roofs remains low and commercial nurseries in Hong Kong are not keen on growing plants for the purpose, selling only eight to 10 species.

The 50 test species are imported mostly from Guangzhou, including colourful flowering plants such as beach morning glory, rainbow pink and leopard lily.

It costs HK$700 to create one square metre of green roof, comprising a root barrier, drainage, filter and water storage layers and soil.

While green roofs are common in Germany, Japan, the mainland and Southeast Asia, Hong Kong sees little effort in the private sector to create them, while the government has done this for dozens of its buildings.

Jim said scores of new private housing developments around the city with tall towers resting on huge podiums would be suitable for greening. 'But the so-called podium gardens you see now often have more space paved than planted.'

He sees potential in promoting green roofs in new development areas in Kai Tak, west Kowloon and three planned new towns in the northeastern New Territories, as well as in old building retrofitting works. The new millennium campus of the university will see several buildings with roofs converted into greenery.

The research is also monitoring the ability of the plantations to absorb rainwater and filter heavy metal and organic pollutants.

Dramatic drop

A green roof can save large amounts of money in air-conditioning costs

The temperature reduction proved in a 2006 planted-rooftop experiment was: 26 degrees Celsius