Green Roofs and Flood Mitigation: An Irish Context – Lucy Carey


Landscape Architect, Lucy Carey reviews the recent Landscape Institute lecture on green roofs and flood mitigation in Ireland. The Irish Landscape Institute (ILI) runs a series of public lectures every year from September to May, with the aim of providing professional development for members and prompting the landscape professions.

On Wednesday 28th the ILI ran their last lecture of the season on Green Roofs and Flood Mitigation. Laurance Nesbitt, an architect with Nesbitt & Associates, presented his research into retrofitting green roofs and the potential for flood migration in Dublin. While Peter O’Toole, a landscape contractor with O’Brien Landscaping and ALCI President, presented case studies of some of the many roof gardens he has constructed.

The concept of retrofitting green roofs is not a new thing, nor are the benefits of storm water management. However there are no current figures for calculating their impact in Ireland. Laurance Nesbitt gave an impressive lecture clearly explaining the relevant history of green roofs, including Newgrange, the Dublin Docklands built on 526ha of land fill, and Dublin’s drainage system – typically managed by piping everything – which set the scene for understanding the impact that retrofitting green roofs would have on the Dublin Docklands. Within the study area Laurance calculated that 403 flat roofs (typically concrete) would be suitable for retrofitting green roofs, whether intensive or extensive, covering an area of approximately 175ha (33% of the Docklands).

Some of these roofs were suitable for intensive roofs while the majority where suitable from extensive roofs. Having looked at research Laurance chose standards set out by the FLL (Standard setting body for green industries) and those used by engineers in Ireland, to calculate the attenuation capacity of different roof garden depths. From there an average was determined that for a 30 year flood, potentially storm water run-off could be delayed by 30 minutes for extensive roofs and 50 minutes for intensive roofs. When compared with recent flash flooding data provided by Dublin City Council, Laurance found that it correlated neatly with where the flat roofs were. Studies in other countries have already shown the link between green roofs and reduced flash flooding and determined stress test for those countries. We base our averages for calculating attenuation capacity upon these studies. Laurance’s study concluded that if we want to have the benefit of storm-water attenuation from green roofs, we needs to develop our own stress test figures for Ireland. The research was privately funded so results are not in general use; it was was an outline study rather than a detailed examination. If fully funded, either privately or publicly, the potential knowledge generated for further studies could only result in improving the execution of green roofs and their function in helping to managing our urban environment.

“It was clear from the lectures that green roofs do not have to be complicated and expensive elements as they are just simply big flower pots”

Peter O’Toole’s lecture was insightful and full of knowledge. He looked at a number of case studies from Aviva Stadium, to Bord Gaís Networks’ headquarters, to the Mater Hospital. Peter’s intention was to give a clear indication of the challenges which can arise from building roof gardens and the ways in which they were resolved. Typical examples he gave were things like determining the weight loads, climatic conditions, working at heights, how to lift equipment and plants onto a roof, working with other contractors, and fundamentally, ensuring the roof doesn’t leak. The latter two examples highlighting how important it is to work with a team and to determine which professional is responsible for which aspect of the build.

At the Aviva stadium huge consideration had to be given to the weight load so they carefully calculated the weights of supplied trees as well as future weight, the substrate build-up, the planters and their locations on a roof, which in this case had to be positioned over the buildings structural columns. At Bord Gaís Networks it was the exposed element and the unusual angles of the roof which required specialised soil from Enrich to be used.

With the Point Village the issues ranged from working in confined spaces, to having to lift the plants into courtyards months ahead of planting when they had access, to being creative with lightweight fill material to raise the level. On one particular project the sedum roof failed in a courtyard but was completely successful on an exposed roof where it reflected the plants’ natural habitat. In general, Peter’s lecture put across that the simpler we build roof gardens and the more accurate the plant selection is for the locations, the longer they last.

It was clear from the lectures that green roofs do not have to be complicated and expensive elements as they are just simply big flower pots. If they are skilfully designed and constructed they can last for many years and do not have to be high maintenance. Their application would allow benefits for not just flood management and aesthetical pleasure, but also biodiversity, better air quality, improved health, physiological benefits, tourist attractions, recreationally facilities, wildlife habitats, local climatic control, and the list goes on ….. The 2013-2014 lectures were sponsored by the Department of Arts, Heritage and the Gaeltacht.



LUCY CAREY is a Landscape Architect with Cunnanes Stratton Reynolds and a member of the ILI Council.