Rooftops can be rich in biodiversity benefits

The following article appeared in the Saturday Weekend Argus, 27 November 2010 (Photos courtesy of Penny Unsworth).

Roof gardens can be an important link in the environmental chain, by Alexandra Jongens and Lorelle Bell

The current challenge for cities facing the global climate crisis is how to design sustainably in ways that incorporate the creation of dynamic, inspiring and functional spaces for people to live, work, study, play and invest. In a previous article, the importance of public spaces was highlighted in the re-imagining of cities; pointing out that sustainable cities need higher densities coupled  with vibrant public spaces. Another form of public space, which is often overlooked, are roofscapes. This is a missed opportunity as not only can roof gardens contribute to stormwater management, they can also reduce biodiversity loss, heat island effect and noise pollution. In addition they act as natural oases which are very necessary for people living or working in cities. We are running out of space in our urban areas and need to use our space more intensively.

Roofscapes can vary in function and form – from green spaces used as gardens or social spaces to those that are planted with low-maintenance plants with a focus on climatic and biodiversity benefits rather than visual impact. Then there are those roofscapes that are integrated into the built structure as a whole, while also form part of the surrounding landscape.

The New Life Sciences Building at the University of the Western Cape in Bellville is an excellent example of how the roof and building as a whole have been incorporated into its immediate surrounds. The Life Sciences Building combines a number of interrelated departments under one roof in order to share laboratories and amenities. The roof garden provides an educational space which is an extension of the classrooms and laboratories. The campus is on the Cape Flats which is prone to drought in summer and flooding in winter, and where a number of species are under threat because of urban growth. As the building is located in a harsh environmental context, with gale force winds in summer, the challenge was to create a comfortable environment inside and outside the buildings.

In The Urban Green File Penny Unsworth of OvP Associates points out that because the new building is situated adjacent to a busy arterial road which is in public view,  a major objective was to create an appropriate landmark building which announced the presence and the new image of the university. This new image was achieved by landscape architects OvP Associates working closely with dhk Architects to create a learning facility comprised of two buildings which include a six storey laboratory and an organic learning centre with a café. These buildings are “held together” by the landscape through the linkages of open spaces around the buildings. Structural engineers Henry Fagan & Partners as well as Environmental consultants Ecosense ensured the structural integrity of the extensive earth embankments which ramp towards the roof gardens on top of the learning centre, creating the fluidity between built form and landscape.

Due to the environmental sensitivity of the site in close proximity to the nearby Cape Flats Nature Reserve, it was very important to plant mainly indigenous plants, chiefly endemic species found in the Cape Peninsula. The vegetation strategy for the roof gardens was to use succulent plants which are low-growing and mat-forming, and thus withstand gale-force winds. They are also extremely drought resistant, so less soil and moisture loss occurs with the heavy summer winds.

In addition, the selected plants do not require deep soil to survive so the structure requirements of the roof are reduced, making it more cost effective. Rainwater is not harvested off the roof as the roof gardens act as natural retention systems, reducing the impact of storm water.

Not only is the roof garden creating a more sustainable built environment, but it is also an educational landscape due to its strong connection to botanical studies and research at the university. The intention is to use the roof garden as a mother nursery to establish plants used elsewhere for rehabilitation and research purposes on campus and at the reserve.

On top of all the functions the roof garden is already fulfilling, it is also promoting the biodiversity of the area due to the use of indigenous plant species and as a result the roof garden forms an ecological island for plant, insect and bird species. This is extremely important for the sustainablitiy of cities especially in areas with little ecological value due to urban development such as the central city of Cape Town where there are few open spaces containing indigenous plants.

Gwen Theron of Golder Associates Africa (are they landscape architects) says it is critical that we look at the ecosystem value of roofs as an opportunity to bring biodiversity back into cities. This element is extremely important especially in an environment like Cape Town where the biodiversity loss rate is one of the highest in the world. The introduction of the Biodiversity Showcase Garden which is part of the Green Point Urban Park is an important step in the battle against biodiversity loss. Now imagine if plant, bird and insect species could live freely between the Cape Town roofscapes, the Urban Park and the Table Mountain National Park which forms the heart of our city. 

The installation of a vegetated roof is expensive but the roofs present saving in the long term building maintenance. Landscape architect Mark Saint Pol states that the real saving in green roofs is the roof itself (structure and waterproofing) as it is protected by the soil and planting which makes up a green roof. In a study done during the research for the proposed extension of the Cape Town International Convention Centre it indicated that in a retrofitted building, the roof could potentially be preserved by the new green roof for up to 25 years longer than the previous roof had been – and that is the major saving. With the green-building movement, a change in mindset is needed among professionals and developers to ensure the delivery of green roofs as a standard aspect of building development. This process will be hastened by the fact that planted roofs present a highly visible marketing strategy for ecological design; enhancing public perceptions and, therefore, the corporate image of buildings. Up to four points can be earned in the Green Star SA rating system when it can be demonstrated that the ecological value of a development site is not diminished.

Planted roofs effectively provide shading for buildings reducing the absorption of heat. This not only reduces the heat island effect associated with cities but also increases the efficiency of solar panels. Eric Noir of WSP Green by Design says green roofs can also impact on the effectiveness of photovoltaic cells which function more efficiently in cold environments. The reduction in the outside operating temperature of the area around the PV cells allows for up to 10% more efficiency out of the cells.

The Western Cape Department of Environmental Affairs and Development Planning introduced a roof garden at their offices at 3 Dorp Street. This was part of the 2wise2waste project which aimed not only to reduce the environmental impact of their operations but also to educate staff and broadcast the department’s commitment to go beyond advising others on how to be more sustainable, and to walk the talk. Only indigenous vegetation was used and planted according to vegetation types. This allows the staff to see the plants they are consistently reading about in environmental Impact Assessments (EIA’s). Not only is it an educational garden but it is creating a place of refuge for birds, insects and people. In a similar way to the UWC roof, it reduces storm water runoff which reduces the flood risk on roads and it regulates the temperature of the building, keeping it warm in winter and cool in summer, thus reducing air-conditioning costs. This is in line with the green building design which the department is promoting. The garden also provides an urban view for adjacent office blocks promoting the benefits of having spaces like these as an integral part of the urban landscape.

The Grand Daddy Hotel, in Long Street, is an example of using a roof as a social space that is commercially viable. The roof top not only has the world’s only roof top trailer park with six airstream trailers, but it is also a bar and open air cinema. The small roof top space has been designed efficiently using indigenous plants to define the different areas. The roof garden has a central stage area where local bands are featured every Friday night and during the summer months this space is converted into an open air movie house which includes comfy leather couches. For visitors who are looking for something different to a hotel room, the airstream trailers are perfect, each with their own themed interior.

With the need to design with more social-responsiveness, these are prime examples of how to re-imagine unused spaces. Creating green, vibrant and sustainable spaces within highly urban areas not only reduces the negative effects of urban development, but also provides more spaces for people to be able to take a deep breath of fresh air. Roofscapes compliment the theme of Cape Town’s World Design Capital 2014 Bid: LIVE DESIGN. TRANSFORM LIFE.