At last year’s official Green Building Council of South Africa (GBCSA) Convention and Exhibition in Cape Town, green roofs emerged as one of the ways to transform buildings in an environmentally sustainable way. Even though the concept of green roofs is relatively new in SA, these attractive features are being successfully integrated into buildings the world over. We chatted to one of the speakers, local horticulturist Zayaan Khan, about the significance of this new form of gardening and how to create one yourself… What is a green roof and what different types are there? There are many definitions as to what a green roof is, but simply put a green roof system is ‘an engineered roofing system that allows vegetation to grow on top of buildings, while protecting the integrity of the underlying structure.’ This description can be found in Green Roofs: Ecological Design and Construction by Earth Pledge. There are two types of green roof systems: extensive or intensive. Extensive green roofs are shallow and lightweight, and are conducive to growing heat-tolerant, low-growing plants. They are generally, although not always, inaccessible. That said, they require minimal maintenance and survive on rainwater – much like a naturally occurring system, they are left to their own devices. Intensive green roofs have a deeper substrate which may accommodate flowering shrubs, vegetable production, trees, and even ponds. An intensive green roof is more intensive in maintenance, hence its name. It is also more interactive and accommodates the user, unlike an extensive green roof system. In intensive systems one would find bigger plants with a deeper root system, such as trees and larger shrubs, and even vegetable gardening which all require more in-depth planning and after care. What’s the difference between a green roof system and roof gardens? The main difference is that the plants on roof gardens are contained and not integrated into a system, whereas in green roofing the entire area is covered and utilised for planting. That is where the green benefits come in. Where did green roofs first originate? Green roofing is centuries old – in northern Europe, specifically Scandanavia, sod roofs were layed on birch bark and incorporated into the building structures of dwellings during the Viking era. Also, there are the mythical Hanging Gardens of Babylon which existed around 600 BC. Regardless, these gardens and their description have inspired many other green roof movements. More recently, Germany has formally been green roofing for about a century and have founded a guideline system for green roofing. So green roofing is essentially a northern hemisphere design with its base in Europe. Much of this has had to be translated into southern hemisphere realities, as our plants, climate, essentially our environment, are vastly different. Are green roofs sustainable and how do they benefit the environment? Green roofs are essentially sustainable to building design and environmental integrity, as long as they are designed and installed in a sustainable way. For example, if you just use straight lawn on the roof, it won’t be sustainable, as it has limited biodiversity, and requires much irrigation and maintenance. In my opinion, modern landscaping has led to the production of ornamental monoculture, where suppliers produce the typical Agapanthus, Tecomas, Tulbaghias, Plumbagos and so on, increasing the population of these species, and neglecting the thousands of other species which are integral in maintaining our country’s biodiversity. Although these plants are water-wise, indigenous and hardy, in essence, the bigger picture decreases their ‘sustainable’ value. Some environmental benefits of green roofing are:
- re-establishing indigenous planting systems
- rehabilitation of spaces
- filtering the air in the city
- providing thermal insulation
- audioproofing (against noise pollution)
- providing a stormwater management technique
Perhaps not directly an environmental benefit, green roofing also provides an aesthetic value and appeal to the city’s roofscape, when viewed from neighbouring buildings and viewed from above (especially in a city that sees aerial activity). They also improve the quality of life of city users and dwellers by decreasing stress, and creating the environment for recreation and respite from the hustle and bustle of city life. Quite importantly, green roofs have been noted to decrease the impact of the urban heat island effect. Impermeable, dark and reflective surfaces all absorb heat energy and effectively radiate it back into the atmosphere, as opposed to absorbing and dissipating much like a natural system would. An urban area effectively blocks these natural system properties with the methods and materials used in creating a city or urban area. The overheating hikes electricity usage for cooling devices such as air conditioners and fans which, in turn, generate more pollutants by their chemical processes. This also affects quality of life for the inhabitants in these urban areas as it aggravates heat-related discomfort and illness. Green roofing reduces these effects by re-instating the natural systems the city has: the plants incorporated into a green roof system are active air coolers due to evapotranspiration (the transport of water into the atmosphere from the soil), efficiently cooling the surrounding air pocket. A great part of green roofing is that it essentially allows the local ecosystem to regenerate, which naturally means wildlife conservation and preservation too. The greatest benefit would be to see a network or system of green roofs within the urban environment, as then we would see a greater functioning of these benefits as the roofscapes link in creating a vast green lung, an ecological landscape, where concepts such as urban forests could be reality. It is a viable way to link the natural environment with the built environment. Green roofing has also proven to prolong the life of roof structures, as roofing materials and waterproofing membranes are protected against weathering from hail, storm conditions and UV degradation. Temperature induced expansion and contraction is also moderated with a green roof system. For more information you can visit Zayaan’s blog or contact her on email@example.com. You can also find more information about the Green Building Council of South Africa by visiting their website here. Text: Kim Grové Images: Zayaan Khan; Tom Gray of Good Hope Gardens Landscaping, firstname.lastname@example.org. Why not browse through some of our other featured gardening trends?