Mimicking natural ecosystems to improve green roof performance

Funder: Australian Research Council Linkage Grant

Partners: Melbourne Water, Inner Melbourne Action Plan Group of local governments (Cities of Maribyrnong, Melbourne, Stonnington, Yarra, Port Phillip)

Researchers: A/Prof Nick WilliamsProf Tim Fletcher, Prof Lu AyeDr Claire Farrell, Andrea Pianella, Zheng Zhang, Joerg Werdin, Carine Desbois, Zhanna Grebenshchykova

Year: 2014-2018

Background

In this project we aimed to:

  1. develop new green roof substrates to improve water and nutrient retention, focussing on the use of waste and recycled products
  2. assess whether ecosystem mimicry can be used to identify species that improve green roof stormwater retention
  3. determine how plant functional trait and species diversity affects green roof performance
  4. model green roof thermal and hydrology performance with different substrate and plant combinations
  5. validate experimental findings on a full-scale green roof.

Key findings and recommendations

Water

  • Rainfall event size is the main driver of stormwater retention on green roofs.
  • Under Melbourne conditions, green roofs can retain 84-95% of rainfall on an annual basis. Although it varies slightly with plant species, the substrate is the most important factor. This is because in Melbourne, most rainfall occurs as small rainfall events (<2 mm) and these are fully absorbed by the substrate.
  • For big events (>11 mm) plants do matter! High water users retain more rainfall than succulents, but we also found that plants with thick roots can result in more runoff due to preferential flow through the substrates. This is a novel finding as previously everyone thought that plant roots would have no impact on the flow of water through substrates which are designed to drain very fast and prevent potential flooding.
  • While we know that plants may be less important for retention, we still need them to provide the other benefits of green roofs!

Energy

  • R-values of green roof substrates depend on the substrate thermal conductivity and water content. High water content reduces the substrate insulation as heat transfers increase due to convection.
  • Scoria substrate has the lowest thermal conductivity of those we have developed.
  • R-values of scoria and bottom ash were higher than most green roof substrates studied overseas.
  • Plant selection helps improve thermal performance of green roofs, particularly in summer. Plants with greater leaf area and high albedo drive the temperature reduction at surface level in summer because intense and long duration solar radiation is absorbed and reflected by the plants.
  • Compared to non-vegetated substrate only roofs, plants reduce the temperature by between 5 and 22%.

Outreach

PhD student Zheng Zhang at work on a green roof.
PhD student Zheng Zhang at work on a green roof.

Publications

Journal papers

Clarke, R. E., A. Pianella, B. Shabani, and G. Rosengarten. 2017. Steady-state thermal measurement of moist granular earthen materials. Journal of Building Physics 41:101-119.

Pianella, A., L. Aye, Z. Chen, and N. S. Williams. 2017. Substrate depth, vegetation and irrigation affect green roof thermal performance in a mediterranean type climate. Sustainability (Switzerland) 9.

Pianella, A., R. E. Clarke, N. S. G. Williams, Z. Chen, and L. Aye. 2016. Steady-state and transient thermal measurements of green roof substrates. Energy and Buildings 131:123-131.

Szota, C., C. Farrell, N. S. G. Williams, S. K. Arndt, and T. D. Fletcher. 2017. Drought-avoiding plants with low water use can achieve high rainfall retention without jeopardising survival on green roofs. Science of the Total Environment 603-604:340-351.

Szota, C., T. D. Fletcher, C. Desbois, J. P. Rayner, N. S. Williams, and C. Farrell. 2017. Laboratory tests of substrate physical properties may not represent the retention capacity of green roof substrates in situ. Water (Switzerland) 9.

Zhang, Z., C. Szota, T. D. Fletcher, N. S. G. Williams, J. Werdin, and C. Farrell. 2018. Influence of plant composition and water use strategies on green roof stormwater retention. Science of the Total Environment 625:775-781.

Conference papers

Pianella, A., L. Aye, Z. Chen, and N. S. G. Williams. 2016. Effects of design and operating parameters on green roof

thermal performance in Melbourne.in A. Hashemi, editor. 5th International Conference on Zero Energy Mass

Customised Housing – ZEMCH Kuala Lumpur, Malaysia.

Pianella, A., J. Bush, Z. Chen, N. S. G. Williams, and L. Aye. 2016. Green roofs in Australia: review of thermal performance and associated policy development Pages 1-10 in J. Zuo, L. Daniels, and V. Soebarto, editors. Fifty years later: Revisiting the role of architectural science in design and practice: 50th International Conference of the Architectural Science Association. The Architectural Science Association and The University of Adelaide, Adelaide.

Contact

For more information please contact Nick Williams from the Green Infrastructure Research Group at nsw@unimelb.edu.au.