Captions: Left: Ken TTDI, Malaysia: An exemplary low energy commercial development that uses the Green Plot Ratio to evaluate the amount of the existing greenery on the site, which is then replenished on the façades in order to reduce ambient temperatures and mitigate solar heat gain.
Middle: The Boutiq, Singapore: An urban residential scheme that celebrates recreational sky gardens providing social spaces that are more condusive to promoting interaction and relaxation.
Right: Acqua Residences, Philippines: a high density waterfront residential scheme that seeks to employ horizontal, vertical, and diagonal greenery to create social spaces that help to reduce the ambient temperatures.
ASIAN economic growth has seen cities such as Tokyo, Hong Kong and Singapore ﬂourish and urban densities increase. Singapore’s spatial constraint of being an island, coupled with a predicted population growth from ﬁve million people in 2011 to six million by 2020 (1), has seen a continued urban densiﬁcation and a consequent increase in urban temperatures, as vegetated areas are surrendered to urban development (Wong et al, 2006).
Studies have also shown that the high rise, high thermal mass building typology and anthropogenic sources of heat (such as air-conditioning plants and artiﬁcial hard surfaces) further magniﬁes the impact on the urban climate, causing the urban heat island eﬀect (UHI). This can be deﬁned as the diﬀerence in temperature between the rural and urban areas (Arnﬁeld, 1999; Wong, 2010,
The negative impacts of replacing the natural landscape with human structures (thus exacerbating UHI) include increased health risks through higher ambient temperatures, aggravated atmospheric pollution, increased emissions of ozone precursors and increased energy consumption for cooling of ﬁve percent for every one degree of ambient temperature rise (Wong, 2010).
Greening the urban habitat plays a major role in reducing the eﬀects of UHI in addition to enhancing space for the health and well-being of civil society, reducing building running costs and enhancing asset value. This includes planting and landscape that can enhance the physiological and psychological health and well-being within the high density built environment (Ulrich, 1981: 523–556; 1986: 29–44).
Various types of rooftop greenery, including shrubs, turf and clay soil of diﬀerent thicknesses, can contribute to economic savings of between 1 and 15 percent in annual energy consumption and 17 to 79 percent in space cooling load (the rate at which heat must be removed from the space to maintain a constant space air temperature) of commercial properties in Singapore (Wong, 2003). Green roof structures and their water absorbent and evapotranspiratory properties can also assist in the reduction of ambient temperatures (Tan et al, 2009).
With a rapidly densifying urban habitat to cater for a growing population, Singapore is faced with having to reduce urban temperatures and provide habitable alternative social spaces that can foster community. This is especially true as the existing vegetated open areas are, unless otherwise planned for retention and enhancement, potentially being depleted through urbanisation.
The challenge, therefore, lies in looking at alternative means to green the urban habitat that explore, in a more quantiﬁable way, diagonal and vertical planes in addition to the horizontal plane of the ground, podium or rooftop. This leads us to consider the green plot ratio.
Singapore’s cultural imperative of creating a garden city has seen the deployment of far-reaching legislative guidelines that promote the replenishment of greenery as well as the incorporation of sky courts and landscaped terraces in a bid to restore balance in an increasingly high-density environment (Tan, 2010). This has led to the formulation of more quantiﬁable measures of planting to ensure that the adverse climatic eﬀects of high-density development can be mitigated through more balanced architectural and integrated landscape design.
The Singapore Government’s commitment to researching the eﬀects of rooftop gardens, vertical and horizontal planting can be seen in what has been deﬁned as the green plot ratio (GnPR). Planted surfaces are an eﬀective way of counteracting the heat island eﬀect and the absorption of heat in the building fabric and its subsequent re-radiation. The GnPR addresses this issue by assigning values to particular plants based on the surface area of greenery. This is achieved by adapting the leaf area index, a biological parameter that is used to monitor the ecological health of natural ecosystems and to mathematically model and predict metabolic processes. As such, it can be used to quantify planning metrics in biological terms (Ong, 2003).
For instance, a hypothetical site that has 12 trees (and therefore a particular GnPR value) may be developed and see the consequent removal of the said number. By assigning values to diﬀerent types of planting, the ability to replenish the same “green value” of the 12 trees by alternative means (for instance, turf and shrubs across the vertical or diagonal surfaces) will ensure a balance of leaf area is retained on the site, and its correlating social, economic and environmental beneﬁts.
Turf, palms, shrubs and trees are the major groups that have been assigned GnPR values based on the leaf area index. Turf has the lowest GnPR at 2.0, as the leaf area index of a blade of grass is less than that of the other categories. Despite palm trees being larger structures, their leaf area index is still less than a shrub and has a value of 2.5. Given their greater density of leaf coverage, shrubs have a value of 3.5, whilst trees have the highest leaf area index at 4.0. The ability to quantiﬁably ascertain the eﬀectiveness of planting on the building through the creation of a green metric goes some way to integrating architecture with landscape as opposed to being considered in isolation.