A pilot study was carried out to evaluate the design and performance of an innovative green roof system installed on a two-storey ?green construction site office? for Gammon Skanska in Hong Kong. To allow the components and materials to be re-usable, the green roof system was modular by design and was constructed on the ground before putting and fixing onto the roof. To enable comparison of thermal performance, a typical site office adjacent to the green site office was selected as the baseline.The “Green Roof for Construction Site Office” received the Considerate Contractor Award Eco-business Award -WasteWise Scheme Skanka’s Internal Environmental Prize: “Skanska’s ambition to environmentally improve all steps in the production process includes an attention to our own site offices. Using established conservation methods, Gammon Skanska showed an innovative and systematic approach to implementing green design at a site office. The results include a reduction in energy consumption by 72 kW/sq.m per year, and the reuse of old site offices and structural steel. Office parts have also been designed for easy reuse of components and water tanks are used to harvest rainwater to reduce the use of freshwater.”

The green roof design is adopted in the Tsing Yi Green Site Office to enhance thermal insulation. It is found that the temperature inside the ceiling void of the CED office nearby has a high temperature up to 52oC at 2:00 under sunshine on a day with 33oC, while for the green site office, the temperature of the ceiling void is around 33oC under sunshine at 3:00p.m. Thus, it can be concluded that the green roof has made significant effect to the thermal loading of the site office building. Moreover, when using green roof, its shallow soil reduces rainwater run off to less than 50% of normal amounts, reducing the pressure on the ground level drainage system. The rainwater retained in the soil supports the plants, reduces the temperature, and through evaporation, it can humidifies the air and reduces the dust levels. The green roof also acts a buffer to attenuate the stormwater and allows some rainwater to be harvested and recycled for other uses. For example, drainage gutters were installed at the edge of the roof of the green site office to collect rainwater, some of which was being used for watering of the green roof. To make the watering more convenient to do, a water sprinkler and pipework was installed on the green roof and the system can be controlled remotely on the ground level.In terms of biodiversity, the green roof has attracted some inserts and birds which are not common in a normal roof system. During the operation of the green site office, the weeds problem did affect the growth of vegetation and the appearance of the green roof. In order to avoid weeds growing too fast and covering the whole roof, the original plant species (sedum) on one half of the green roof has been changed to keep weeds under control. But for the purpose of biodiversity study, we have allowed the other half of the roof area to have weeds.Construction Cost: With the information provided by the site staff, the material of the green roof cost around HK$160,000. With the addition of the labour cost of around HK$40,000, it could be concluded that the approximate cost of the green roof of the site office is around HK$20,000.Maintenance Cost: The green roof had been installed with automatic sprinkler system. Very little supervision is required to provide to maintain the green roof. Only very minimal reed removing work has to be done to the roof and the maintenance work would only involve a person around a few hours each time.Operational Cost: The reduced power consumption of the Green Site Office is seen from charts available on the website (see below). Comparisons of the energy consumption of the Green Site Office with the other two types of site offices can be seen very clearly from the shape of the lines. It is very obvious that the power consumption of the Green Site Office is much lower than the other two kinds, especially during hot summer days around May and June.