Implementing green roofs in South Africa could help to cool cities and increase biodiversity
Submitted by: Amanda Botes, Friday, June 21, 2013
The results from a green roof pilot project for eThekwini Municipality have shown that green roofs can help to cool cities down and increase biodiversity in a city. Clive Greenstone, owner of Green Roof Designs in Durban, together with Mike Hickman, implemented the green roof on the City Engineers Building in KE Masinga Road and has collected scientific data over the last four years to test the many advantages of green roofs in the South African context.
The pilot project was implemented in 2009 as part of the eThekwini Municipality’s Municipal Climate Protection Programme. A number of different growing mediums, indigenous plants, drainage layers, and edge types were tested as well as the two types of green roof systems, direct and modular. A portion of the roof was also left blank to serve as a control. The following advantages of green roofs have been explored by Greenstone.
Reducing the heat island effect and temperatures in buildings
In cities hard concrete surfaces absorb and retain heat, creating higher temperatures in cities than surrounding areas, this is called the heat island effect. Green roofs provide a layer of insulation and shading, and through evapo-transpiration of the plants can help to reduce building temperatures in summer and retain heat in the winter months. If more green roofs were implemented in the city of Durban this could help to reduce the heat island effect says Greenstone.
Greenstone’s data shows that the average recorded temperatures on a blank roof of the City Engineers Building were much higher than the average temperatures on the green parts of the roof. Average temperatures in the month of January 2013 showed at least a 13 degree difference at the hottest time of the day. Greenstone adds that the temperature was also influenced by the type of growing medium used, as the crushed brick growing medium tended to absorb more heat than the other types of growing medium.
Reducing storm water runoff
Impervious surfaces in cities result in accelerated storm water runoff. This water is directed into storm water drains and is let out to sea and in rivers. At accelerated rates this water can cause infrastructure damage. In Durban, more frequent rainfall storms are predicted in the future and green roofs can help to reduce storm water runoff by slowing down the rate that rainfall enters the stormwater drains. “Green roofs help to absorb initial downfall, and slowly over days some of my green roofs are still dripping, … that’s what we want, we want to stop the velocity,”says Greenstone.
The graph below shows the runoff from the City Engineers Building on the day of a storm event, 17 February 2010. The blank roof shows that over 6 litres of water a minute ran off the roof at the peak of the storm, whilst from the modular green roof 3 litres of water per minute came off the roof. The direct green roof type showed less than one litre of runoff per minute at the peak of the storm.
Growing a diversity of indigenous plants on a green roof can result in an increase in the variety of insects and birds visiting the roof. Greenstone advises that a variety of local indigenous plants are introduced on the roof in order to attract insects, “We use combinations of plants because we want to invite insects…you want a decent pallet.” Greenstone’s company only uses plant species found in a 50km radius of the site. In addition Greenstone warns against the use of only one plant especially grasses which can be a fire hazard in dry winter periods. “The whole area in Umhlanga has grasses which is problematic, a fire risk and high maintenance.”
Greenstone suggests the use of low growing succulents like Crasulas which require very little maintenance. He adds that a mildly creeping plant should be selected rather than an aggressively creeping plant. Regarding irrigation Greenstone states that irrigation is only required for the first two months if the correct plants are planted for that site and that a hand held watering system rather than an automated system is used to ensure that flooding does not occur. “Water points are key for the first three months then you shouldn’t be watering your green roof ever again if it is designed properly.” Weeding should be done and drainage should be kept clear.
The graph below shows the number of insects collected on the blank portion of the City Engineers Building roof, the “control” in comparison to the different greened portions. The greened parts of the roof show a far higher number of species collected and Greenstone also adds that an increase in the number of wasps, bees, ants and flies occurred “this is a good sign of a thriving ecosystem.”
In addition Greenstone explains that if a number of the flat roofs in Durban were turned into green roofs that this would help to increase the biodiversity in the city and the roofs would serve as “stepping stones for species including birds, insects and spiders.”
Other benefits of green roofs
Other benefits of green roofs include that they are aesthetically pleasing, they can help to reduce noise and increase the lifespan of the roof by 2-3 times. They can also serve as carbon sinks if the correct plants are used.
A useful guideline document on designing green roofs- “Guidelines for designing green roof habitats” has been developed for the eThekwini Municipality and can be accessed online.
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