Singapore has been actively greening its urban landscape for many years. Sky gardens, living walls and world-class gardens are being constructed everywhere across the island, rapidly transforming the city state from a “Garden City” to a “City in the Garden.”

The Housing & Development Board (HDB) is Singapore’s public housing authority, with mandates to plan and develop housing estates to house an entire nation. Today, more than 80% of Singapore’s population is living in HDB’s 1 million plus flats in 23 towns across the island.

HDB builds homes and transforms towns to create a quality living environment for all Singaporeans. It aims to provide residents with green and sustainable living environment, and bring energy-efficient, water and waste management features into its estates.

HDB incorporates sky gardens, which include turf, shrubs, trees and recreational features, in the design of newer housing estates. It also retrofits older housing estates with lightweight greenery systems that contain small, hardy, low-profile plants on the roofs. This sustainable and environmentally-friendly approach proved to be a successful greening solution for inaccessible roofs, and a complement to the sky gardens that are normally found on the podium level.

Research by Singapore’s National Parks has shown that green roofs not only lower the building’s surface temperature by as much as 18 degrees C, they also reduce the glare reflected onto neighbouring blocks by about 15%.

Punggol was selected by HDB to be developed into its first eco-town in Singapore. Its Waterway and Treelodge@Punggol are two successful iconic developments alongside with multiple sustainable initiatives and urban solutions in this green community.

In this project, HDB aimed to retrofit a “second” roof on top of three residential blocks at the Punggol housing estate with green roofs to add greenery, lower the interior temperature, reduce reflective glare and create a pleasant view for the neighbouring blocks.

Like many high rise residential towers, rooftop equipment and piping are scattered across the rooftops on the HDB blocks, in this case they were covered by a “second” roof made of large concrete tiles in shingled fashion. The Town Council wanted to cover the tiles with greenery to improve visual appeal and reduce interior temperature.

The green roofs are located on top of the 23rd floor of HDB BLK 667B spanning across three residential blocks. The only access to the rooftop was up one staircase from the elevator, and there were many obstacles and various rooftop equipment. Therefore, it was important to select a system that was light and clean to transport and easy and quick to install, with minimal maintenance in the long term.

The XF309 Light buildup was selected because it is lightweight and flexible to transport to the rooftop and install over the large concrete shingles. The system consists of an XF309 pre-vegetated mat installed on top of an XF159 water retention fleece and an XF411 root barrier. It weighs only 33 kg/m2 (6.6 psf) when fully saturated, which is one of the lightest weight systems on the market and is perfect for retrofits.

In addition, the XF309 is a mineral wool based carrier and the system is completely soil-less. This complements the Public Utility Board’s Active Beautiful Clean Waters (ABC) Programme, a strategic initiative to improve the quality of water and life in Singapore.

The pre-vegetated mats contain small low-profile tropical succulents such as Sedum sarmentosum, Portulaca gradiflora, Portulaca pilosa, Portulaca oleracea and Plectranthus prostratus that are pre-grown in local nurseries to a minimum vegetation coverage of 80% before delivering to the rooftop for an instant green effect with seasonal visual interests.

Shade cloth was used to cover the entire green roof initially to allow the plants to root firmly into the water retention fleece underneath. The shade cloth was removed after two weeks of recovery and establishment.

All the components come in roll format – they are easily rolled out and the mats follow the contour of the tiles nicely due to their flexible nature. No loose growing medium was used to ensure a clean and quick installation. In addition, this mat system is easily retractable and replaceable to allow access to the piping underneath should inspection or repair be needed in the future.