Greening roofs for sustainable development of compact tropical cities

Abstract

As an exceptionally compact city, Hong Kong attempts to adopt the sustainable development mode. Urban greening offers a feasible way to attain environmental sustainability. With seven million population concentrated largely in 200 km2 of land, the urban public open space provision at merely 3 m2/person is usually low. High-rise buildings with barren flat rooftops and facades are seldom employed to introduce nature into the vertical dimension. Numerous hardly serve any useful purpose. Besides providing outdoor amenity and recreational spaces, green roofs can bring significant cooling and amelioration ofth e urban heat island effect especially for the tropical city. The scientific aspects of establishment and the environmental benefits need to be ascertained in the context of the humid-tropical regime. An extensive green roof was retrofitted on a four-storey building at the University of Hong Kong in 2006, denoting the first research green roof in the city. It covers 240 m2 of four plots planted respectively with turfgrass (Zoysia tenuifolia, Korean Velvet Grass), groundcover vine (Arachis pintoi, Perennial Peanut) and shrub (Duranta repens, Golden Dewdrops), plus a barren control plot. Environmental monitoring equipments with data loggers were installed at the plots. The objectives o fth e study were to evaluate: (1) growth performance o fth e three vegetation growth forms at the rooftop habitat; (2) evaporative cooling of air temperature; (3) surface temperature reduction; (4) thermal insulation effect; and (5) heat flux to the indoor space below the roof. Significant reduction in surface and below green roof temperatures and energy conservation were recorded, with about 15°C drop at summer midday for turfgrass and groundcover, and more so for shrub. Top floor indoor temperature was reduced by about 2°C under the vegetated plots. The research offered valuable findings and practical experience to establish and maintain extensive green roofs using different vegetation types.