Green roofs are not roofs that are painted or tiled green, but rather, roofs that are either completely or partially covered with vegetation. The surface of the roof is covered with a waterproofing membrane, then layered with specifically designed growing substrates and then the plants are literally planted in the new ‘ground’.

Not only are these green roofs aesthetically pleasing, there are many environmental aspects that yield great benefits for urban communities. Green roofs have actually been around for centuries in places such as Iceland and Sweden, but the modern trend began in Germany in the 1960’s.

Europe is currently the leader in green roof technologies and North America is becoming more active in the green roof movement. It is believed that over 10% of the rooftops in Germany have been planted.

In the 1930’s, a series of rooftop gardens were planted on the roofs of Rockefeller Center in Manhattan that are still thriving today.

Many companies and organisations have been created to support and construct green roofs. Universities and research centres all over the world are working to develop the technologies to further the advancement of green roofs.

The International Green Roof Association sets guidelines and standards for worldwide promotion of the green roof idea, and more and more cities are beginning to see the benefits of green roofs and are offering incentives to companies, individuals and builders.

There are numerous benefits to green roof including their ability to reduce the energy needed to cool the building in the summer and decrease the heat lost in the winter. A green roof can retain up to 75% of the rainwater and will slowly release it back into the atmosphere rather than allowing the water to pass to the storm water drains.

Pollutants such as heavy metals and nitrogen can also be filtered out. Carbon dioxide and other air pollutants can be filtered through plants processes like photosynthesis. The life span of a green roof can last two to three times longer than that of a traditional rooftop.

Anyone that lives, or has lived in a big city, is familiar with the urban island heat effect.

This when traditional building materials soak up the suns radiation and re-emit it as heat. This process can increase a city’s temperature by 4C! It has been estimated that if all the buildings in a given major city were ‘greened’, the temperature could be reduced by as much as 7C.

In February of 2007, the HK Architectural Services Department did a study of green roof applications in Hong Kong. They found that there are three types of roof gardens in the region: sky gardens, podium gardens, and green roofs on existing low maintenance buildings. Podium gardens, in particular, are very common, and they provide much needed leisure and functional open space.

There are several reasons why we don’t see more green roofs in Hong Kong besides the lack of government incentivesand basic public knowledge. Many of Hong Kong’s older buildings are very tall and narrow, and they offer little space.

Some buildings are unable to be retrofitted to withstand the weight load of the soil and vegetation. Newer buildings and residential areas are providing better space for green roofs.

Hong Kong also has unique climatic constraints such as high winds, high summer rainfall and low winter rainfall that make it harder to have green roofs.

Despite difficulties, even Hong Kong is recognising the green roof trend. This spring, HSBC bank initiated a programme with its new Green Card, in conjunction with the University of Hong Kong, pledging HK$5 million to create green spaces and rooftop gardens in selected schools in Hong Kong.

We must believe that our individual efforts collectively will have a positive impact towards the greening of our world.

Supplied by Jenny Selevan

_{(Issue July/August 2008)}