Garden Roofs: International Inspirations explored the overarching benefits of garden roofs by touching on some of the most inspirational garden roof projects across the globe. It essentially raised a rather open-ended question surrounding the potential role of garden roofs in creating a more sustainable and liveable Cape Town. I approached Jaco Ferreira from Advanced Green Roofs (AGR), an organisation specialising in the distribution of garden roofs in and around Cape Town, US and Canada, to shed some light on the subject.
The only formal incentive for garden roofs in Cape Town at this stage is the Green Building Council’s Green Star SA credits. This voluntary system gives a development a score based on its environmental performance. The total amount of credits are represented by a “star”-rating, with the highest rating of six (6) stars being between seventy five (75) and one hundred credits (100). The maximum number of credits a garden roof can contribute towards a development’s Green Star score are twenty-four (24). Given that the current trend in the built environment is predominantly green-driven, obtaining Green Stars could be used to generate investment or enhance status. However, further benefits seem to be limited.
The Green Building Council consists predominantly of private rather than public sector participants. It is therefore of great concern that the only effort made by the public sector is the creation of the City of Cape Town Smart Building Handbook (2012), which merely mentions the use of garden roofs as a possible option. The current system in Cape Town, as well as in the rest of South Africa, is still a long way from incentives along the lines of the subsidies, tax rebates and density bonuses prevalent in other countries.
Scientific research in a South African context is key to the public sector taking garden roofs seriously and realising its potential benefits. The below diagram (Blank et al 2013) supports Jaco’s statement. It shows just how far behind Africa is in terms of the number of research publications in this particular field between 2001 and 2012. The USA, UK, China, and Canada are the forerunners and are evidently those countries that implement far more advanced regulations and incentives.
Another important factor depicted here is the difference in climates. In South Africa specifically these vary between Tropical, Subtropical, Deserts, Shrublands and Mediterranean. This is of utmost importance as every climate will have its own challenges that will need to be taken into account when regulations and incentives are formulated. For instance, wind breaking structures or green houses will have to be incorporated in Cape Town given the extreme seasonal wind. AGR is currently in communication with various Universities in the Western Cape, together with Michigan State University, where the AGR-system was first developed, to engage in garden roof research projects with a specific focus on the local context and climate.
The system comprises seven key components. Numerous irrigation trays are connected via an irrigation channel that services a drip line which feeds the irrigation mats that distribute the water directly to the plants’ roots. It is one of the only systems that can be used for a sloped application of up to 45 degrees. When evaluating whether an existing roof is suitable, determining factors include its loadbearing capacity as well as the degree of accessibility.
The average cost of a semi-intensive bio diverse roof is between R800 to R1000 per square meter. This includes a waterproofing layer and a lightweight growing medium at twenty-four plants per square meter. If a green roof is properly installed, then minimal annual maintenance or extra cost is required. Services such as soil analysis and plant evaluation are encouraged to determine fertilizing and watering requirements, detect leaks and identify weeding or replanting needs. Visit www.agrgreen.com/videos/ to see the system in action.
AGR is continually researching ideas and techniques to improve their system. Two of the most prominent interests at this stage are those of food production and nature conservation. The system is currently being used by a multinational company in a food growing project in Saudi Arabia. Pursuant to this, AGR is currently looking to embark on a commercial-scale local project to test garden roofs’ ability to supply produce to local restaurants and the public, as well as donate produce to those in need. They are also looking to promote initiatives to reintroduce plant species, that were originally found where the city is built today, some of which are currently at an all-time low prevalence of only 13%. Another project entails incorporating Owl nest boxes into the AGR projects to assist the Endangered Wildlife Trust (EWT) with their Urban Raptor Conservation Project.
“I do believe that Cape Town has huge potential to implement green roofs across existing city buildings and new buildings and become the greenest city in Africa.”
Blank, L., Vasl, A., Levy, S., Grant, G., Kadas, G., Dafni, A. and Blausten L. 2013. Directions in green roof research: A bibliometric study, Building and Environment, vol. 66, p. 23-28.
Special thanks to:
Jaco Ferreira (Advanced Garden Roofs)
– Article by Karla Booysen