03_Green Roofs for Singapore

8/4/2019 03_Green Roofs for Singapore

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Green Roofs for Singapore

It is not uncommon to see green roofs in Europe, and increasingly inNorth American cities, many city governments have also implementeddemonstration projects to raise the awareness and evaluate theenvironmental benefits of green roofs. The motivating factorsfor promoting green roofs are varied, ranging from amelioratingenvironmental problems within cities, to re-introducing biodiversity offlora and fauna, improving the aesthetics of cities, utilizing untapped spaceon roofs, and using green roofs to supplement traditional engineeringsolutions for the citys drainage system. The implementation of greenroofs is often not attributable to one motivating factor or the other, but

is usually the result of a complex interplay between economic, socialand even political considerations in these cities. Increasingly, green roofadvocates have also linked green roofs to biophilic design of buildingsor public spaces, green building practices, smart growth of cities, and thegeneral concept of sustainable cities.

These cases perhaps reflect the multiple contributions of urban greeneryin the environmental, ecological, social and economic realms of cities, andthe awareness that more often than not, there is insufficient greeneryin the urban environment. From a range of studies conducted, it seemsclear that with the growing interest in green roofs worldwide, effortsto green up roofs can only help correct the ecological imbalance andenvironmental problems brought about by excessive urbanization, andimprove the quality of life of urban dwellers.

For Singapore, the case to install more green roofs is strong. Land usein Singapore will increasingly be high-density and high-rise. There will

be an inevitable impact on the physical environment and ambience ofthe Garden City painstakingly nurtured over the last 40 years, if we failto allow greenery to keep pace with the increasing amount of concrete,glass and steel surfaces associated with increased land developments. Alogical solution to the competition for space is to integrate greenery ontobuilt structures, for instance on roofs, decks and even building facades.


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Green roof technology, given its lightweight feature and emphasis oncreating self-sustaining communities that do not require intensive-maintenance, is a useful technology that can be used on many existingand new roofs in Singapore. With widespread implementation, not onlywill there be significant greening of our high-rise environment, given thelarge number of buildings here, there will likely be improvements in the

quality our physical environment, such as in the mitigation of the urbanheat island effect. Data collected in Singapore have for instance, shownthe potential of green roofs in reducing surface temperatures and henceheat trapped on walls or transmitted into building interior.

Faade greenery in a building Tokyo(Copyright: Organization for Landscape andUrban Greenery Technology Development).


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The infra-red image shows significant temperature differences between

greenery-covered surfaces of the green roofs compared to exposed sections.

Recent studies in Singapore have also shown that green roofs can helpcut down the amount of glare arising from reflection off a concrete roof,and even improve the quality of air in the immediate vicinity of the roof.Green roofs thus offer an innovative means to transform otherwise harsh

Concrete

Plantcoveredsurfaces

Increasingtemperature


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In addition to introducing flora into otherwise barren roofs, green roofs help tobring fauna back into the urban environment

and barren roofs into attractive roofscapes that benefit theenvironment, and introduce biodiversity back to the urbanenvironment.


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Plant Selection for Green Roofs inSingapore

In the use of green roof technology in Singapore, the biggest challengeand unknown factor influencing the success of the green roof is theliving component, i.e., the plants, especially for non-irrigated green roofsand those that employ shallow substrates of 10 cm or less. The othercomponents of the green roofs are largely physical elements that can beengineered to meet specific performance criteria such as drainage rate,water and nutrient retention, etc., to suit prevailing climatic condition.

On the other hand, since green roof research and implementationhave largely been limited to temperate regions, there is currently a lackof information on the types of plants that are best adapted for greenroof in the tropics. The use of appropriate plants is an important factorthat influences the amount of subsequent maintenance required, andwhich literally determines whether a green roof stays green. Plants alsoconstitute the most basic factor that determines the look and feel ofa green roof according to the imagination and creativity of landscapearchitects.

Despite our high rainfall and humidity conditions, analysis of the dailyatmospheric water deficit in Singapore will show that water availabilityfor plant is likely to be a key limiting factor for successful plant growthin non-irrigated green roofs. For instance, for the pilot green roof projectconducted in Punggol, it was estimated that the number of occasionswhen the green roof substrate experienced depleted moisture levelscontinuously for four days or more occurred in eight out of the twelvemonths monitored. This will invariably result in considerable water stressfor plants in the green roofs. Thus, despite our high annual rainfall andhumidity, the non-uniformity in rainfall distribution and the limited waterstorage in shallow substrates will mean that green roofs in Singapore doexperience xeric growth conditions.


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Plants that are drought-tolerant thus need to be used for non-irrigatedgreen roofs. Obviously, if irrigation is provided for the green roof, the typeof plants that can be used increases tremendously. The use of shallowsubstrate also imposes another limitation, in that because of limited

anchorage, ground cover or low shrubs, rather than tall shrubs need tobe used to prevent toppling of plants.

Given that water is the key limiting factor of growth, a useful starting pointfor plant selection is knowledge of the physiological behaviour of plantstowards water use. Plants that are drought tolerant would also tend to

have high water use efficiency, in that there is more carbon dioxide fixedby the plants per unit loss of water from the leaves. A group of plants thathas high water use efficiency is those that employ the Crassulacean Acid

Many native wild flowers or weeds on green roofs cannot tolerate 4-5 dayswithout rainfall, compared to the more drought tolerant succulents.

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Metabolism mode of photosynthesis, or CAM for short. These plants willclose the stomata during daytime, when evaporative loss of water fromthe leaves tends to be greatest, and instead keep stomata open at nightto absorb carbon dioxide for storage and use in photosynthesis during

daytime.

CAM plants should thus offer many potential candidates for green roofplants. Fortunately, CAM plants are very widespread, and are representedin thirty-three families encompassing 18,900 plant species. The hugediversity of CAM plants can be found in families such as Aizoceae,Agavaceae, Asclepiadaceae, Bromeliaceae, Cactaceae, Commelinaceae,Crassulaceae, Dracaenaceae, Lamiaceae, Orchidaceae, Piperaceae,Portulaceae, etc. For roofs that are less exposed, many tropical epiphytesand sclerophyllous plants in the families Orchidaceae, Bromeliaceae,Asclepiadeceae and Piperaceae will offer numerous possibilities.

Undoubtedly, continuous trials of plants on green roofs will also leadto the discovery of non-CAM plants suitable for green roofs in thetropics. Indeed, the tropics is blessed with such great plant diversitythat there are exciting opportunities to create of green roofs featuringtropical look that are distinctively different from green roofs commonly

seen in temperate climates. We can perhaps also draw inspiration fromobservations in nature on the types of plants that establish naturally onroofs, albeit these are roofs that generally suffer from a lack of regularmaintenance. Research into this exciting new area has just begun.


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Plants that establish naturally on roofs.

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03_Green Roofs for Singapore