Green roof systems are living vegetation installed on roofs that can provide many environmental and social benefits for achieving low-carbon high-performance buildings (Hui 2013). They could contribute positively to more liveable cities by providing green spaces, mitigating urban heat island effect, reducing air quality problems, enhancing stormwater management and biodiversity (Dunnett and Kingsbury 2008; Hui 2006). Table 1 gives a summary of the benefits of green roof systems (Hui 2011a).
However, the market for green roofs is still developing in Hong Kong and more information on their technical design, effectiveness and actual benefits are needed (Hui 2011b). Although green roof technology is relatively straightforward, it is possible for people who are unfamiliar with it to make mistakes. Green roof planning, design, installation and maintenance on new developments and existing buildings must be carefully considered and evaluated to avoid mistakes.
Modern roof greening has two main approaches: intensive and extensive. Table 2 shows three major types of green roofs and their characteristics. Depending on site-specific factors such as location, structural capacity of the building, budget, client needs, and material and plant availability, each individual green roof will be different.
According to the basic construction method, green roof systems can be divided into two categories: built-in green roofs and modular green roofs. Table 3 compares these systems. Figure 1 shows the integrated green roof systems on top of the assembly hall of a primary school in Hong Kong.
There are some advantages to applying green roof systems to new buildings (Urbis Limited 2007). First, costs can be saved in the design stage since the systems can be part of the existing contract. Second, roof slabs can be designed to take heavier soil depth loads. Third, irrigation and water supply can be built into the roof from the start. Fourth, utilities can be arranged to maximise green roof area. Fifth, barrier-free access (such as lifts) can be incorporated into the design. Finally, extended side walls to protect green roofs from excessive wind may be incorporated at the design stage.
It should be noted that green roofs on existing buildings is an important consideration for urban cities because existing buildings constitute a major portion of the building stock. Roof greening is an effective way to maximise the greenery of built-up areas. When applied to these buildings, the green roof design will be limited to the loading capacity of the existing roof unless a higher initial cost is paid to upgrade the structure. Thus, a suitable green roof system (usually lightweight) and proper design are needed. When considering a retrofit project, the age and condition of the existing building and roof will affect the feasibility of a green roof. Roof-top utilities and plant space can constrain the possible greening area. The current structural loading and building requirements may limit growing medium depth and type of vegetation. Additional rooftop water points and new drainage points may need to be installed.
Germany and Japan are technically more advanced than Hong Kong and have done a lot of pioneering work in support of roof greening. When the green roof market boom first took off in Germany, many unqualified green roof companies surfaced, leaving behind a legacy of poorly constructed green roofs. The FLL guidelines (FLL 2008) have been highly successful in setting quality standards for green roof systems throughout Germany. The FLL guidelines and related standards have formed the basis of similar standards in many countries that have adopted green roofing standards.
Like Germany, Japan has a mature green roof market with some green roof suppliers and companies prepared to provide planning guides and information to practitioners. The local provincial governments have also developed requirements and standards, together with incentive schemes. These help to ensure the ultimate quality of the green roof systems and to promote their use. In recent years many other countries and cities have developed guidelines, manuals and codes for green roof systems to suit their own needs, such as Singapore (NPB 2013), Taiwan (ABRI 2015) and the UK (Green Roof Organisation 2014).
Structural load-bearing capacity will determine the access, function and type of green roof to be adopted as well as growing medium type and depth, plant selection, and replacement and repair strategies. The structural analysis should consider the waterproofing membrane, plant weight at maturity, fully saturated growing medium and drainage layers, and weight of all components including dead and live weights for all phases of the green roof. Spot loadings generated by large-scale bushes, trees and structural components will need to be calculated separately. The weight of every layer in a green roof system is determined at the point of maximum water capacity, including materials and stored water. For existing buildings, the structural integrity of the building must be verified before considering retrofitting the building with a green roof.
Green roof systems are considered an extension of the roof system, so must comply with requirements for structural loading and moisture protection. It should be noted that green roof policies are important for encouraging applications and promoting the healthy development of the green roof market in Hong Kong. Green roofs offer an important way forward in urban greening, bringing natural wildlife back into the urban realm and providing much-needed ecosystem services to the built environment. Our cities and towns need to embrace green roofs to ensure that we adapt and mitigate our impact on the environment to achieve a liveable quality and sustainable built environment.
- ABRI: Roof Greening Technical Handbook, Architecture and Building Research Institute (ABRI), Ministry of the Interior, Taiwan, 2015. (in Chinese)
- Dunnett N and Kingsbury N: Planting Green Roofs and Living Walls. Revised and updated edition. Oregon: Timber Press, 2008.
- FLL: Guidelines for the Planning, Construction and Maintenance of Green Roofing, 2008 edition, Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e V (FLL), Bonn, 2008.
- Green Roof Organisation: The GRO Green Roof Code: Green Roof Code of Best Practice for the UK 2014, Groundwork Sheffield, UK, 2014.
- Hui, S C M: "Guidelines for the Design and Application of Green Roof Systems", Chartered Institution of Building Services Engineers, London, 2013.
- Hui, S C M: "Green roof urban farming for buildings in high-density urban cities", invited paper for the Hainan China World Green Roof Conference 2011, 18-21 March 2011, Hainan (Haikuo, Boao and Sanya), China, 2011.
- Hui, S C M: "Mechnical Guidelines for Green Roof Systems in Hong Kong", Department of Mechanical Engineering, the University of Hong Kong, Hong Kong, 2011.
- Hui, S C M: "Benefits and potential applications of green roof systems in Hong Kong", in proceedings of the 2nd Megacities International Conference 2006, 1-2 December 2006, Guangzhou, China, pp 351-360, 2006.
- NPB: A Concise Guide to Safe Practices for Rooftop Greenery, National Parks Board (NPB), Singapore.(www.skyrisegreenery.com/index.php/home/guidelines), 2013.
- Urbis Limited: "Study on Green Roof Application in Hong Kong: Final Report", Architectural Services Department, Hong Kong, 2007.
About the author: Ir Dr Sam C M Hui is a lecturer of the Department of Mechanical Engineering, the University of Hong Kong. Further information about the green roof research can be found on http://me.hku.hk/bse/greenroof/.
Table 1: Public and private benefits of green roof systems
Table 2: Major types of green roofs and their characteristics
Table 3: Comparison between built-in and modular green roofs
Figure 1: Integrated green roof systems on top of the assembly hall of a primary school