Roof surfaces can positively impact the environment in many different ways, whether serving as bases for solar installations or vegetated covers, reflecting solar heat or collecting rainwater.
Roofs are big areas with great potential to contribute to the sustainability of the overall building as well as the surroundings.
Roofing manufacturers are constantly working to improve materials and profiles to be stronger and last longer, their manufacturing processes to be more ecologically friendly and for the end-product to contribute to the sustainability of the building while having as little as possible negative impact on the environment.
On top of using sustainable roofing materials, roof surfaces can serve many functions and positively impact the environment in many different ways, be it to generate clean energy through solar panels, help cool the building by reflecting heat, serve as a vegetated garden or one that captures stormwater.
Black roofs: Solar rooftops
Solar power, both from photovoltaics (PV) and concentrated solar power (CSP), offers mature technology solutions well suited to the excellent solar radiation resources in South Africa.
According to the State of Renewable Energy in South Africa 2015 Report, published by the Department of Energy, the country has an average of more than 2 500 hours of sunshine per year and average direct solar radiation levels which range between 4,5 and 6,5kWh/m² per day, placing it amongst the top three in the world.
Combined with rising electricity costs, strain on the national grid, a decline in technology costs and advances towards the development of energy storage systems, solar rooftops are becoming increasingly attractive.
The report further shows that the solar photovoltaic (PV) rooftop market has been steadily growing without much government incentives. A voluntary database of small-scale rooftop solar PV installations established around 2011, had by May 2015 recorded an established capacity of about 43,8MW.
A solar PV baseline study completed in 2013 as part of the South African Solar PV technology roadmap, considering trends and different penetration rates, predicted the combined commercial, industrial and residential installations of rooftop PV in the country likely to be between 3,5GW and 11,6GW by 2035.
Clearwater Mall solar rooftop
Operating as South Africa’s largest solar rooftop, the Clearwater Mall in Strubens Valley, Johannesburg, is one example of retail development capitalising on self-generated solar power.
The 1 500kW photovoltaic system on the rooftop of the shopping mall consists of KACO new energy Powador 60.0 TL3 solar inverters, installed by Solareff using Safintra S-5 clamps, which are specially designed for PV attachments to metal roofs.
The installation supplies electricity to over 250 shops, restaurants, cinemas and sport centres, covering about one third of the mall’s 5MW installed load.
V&A Waterfront installation
Another major solar rooftop installation of more than 7 500m² is currently taking shape at the V&A Waterfront in Cape Town. The installation is spread over eight rooftops, including those of the Watershed, the Breakwater, the Clock Tower, Granger Bay, the Two Oceans Aquarium, Ports Edge, West Quay and Portswood House.
Sustainable Power Solutions is installing 4 207 SolarWorld SunModule Plus SW260 Poly PV modules with an output of 1093,8kWp, supplied by SolarWorld Africa, with the project scheduled to go live early in 2016.
This rooftop installation is estimated to produce 1 640 000kWh of clean energy, and reduce 1 610 tons of carbon emissions per year. The average daily production of this system will be 4 495kWh, which is the energy needed to supply one day’s worth of electricity to 310 average-sized households.
Developer aims high
The V&A Waterfront installation forms part of Growthpoint Properties’ objective to increase its solar energy generation capacity to over 6MWp by mid-2016. Together with installations at 33 Bree and De Waterkant, Airport Industria, and The Constantia Village in Cape Town, as well as Northgate in Johannesburg and the Kolonnade and Brooklyn Mall in Pretoria, 4,827.5kWp will be generated.
Solar installations at the developer’s Lincoln on the Lake in Umhlanga, InfoTech in Pretoria, Waterfall Mall in Rustenburg and Bayside Mall in Cape Town are already generating 1,215kWp.
White roofs: Reflective, cool surfaces
Reflective roofs are designed to mitigate the effect of urban heat islands in major cities across the world, and they are not only white but come in a variety of “cool” colours.
“With the solar reflectance technology available today, the thermal performance of any colour can be optimised without changing appearance, providing a wider choice of thermally efficient colours besides white,” explains Arno Hanekom, business development manager at BlueScope Steel Southern Africa. “This technology also reduces heat stress on the roofing system resulting in greater durability,” he adds.
According to an article by Dr Jim Hoff, research director at the Centre for Environmental Innovation in Roofing in Washington, over 99% of the heat in our atmosphere is a result of solar radiation. As a result, almost all temperature effects associated with urban heat islands can be traced to increased absorption of solar radiation by urban surfaces.
Since roofs account for about a quarter of the surface area of cities, they offer a good opportunity to reduce urban heat absorption.
Compared to non-reflective roofs, which only reflect about 5% of the total heat energy, reflective roofs can redirect up to 80% of the total heat energy that would impact the heat island back towards space, so that only 8% heats the city air, 10% heats the atmosphere above the city and 2% heats the building.
When considering a reflective roof system, it is crucial to investigate the product’s Solar Reflectance Index (SRI) rating, which combines two measures that influence the reflective potential of a product. The solar reflectance (SR) refers to the amount of solar energy that is directly reflected by the roof, while thermal emittance (TE) quantifies the amount of solar energy that is indirectly heating the atmosphere and adjacent spaces.
Roof Slope SRI
Low slope ≤ 2/12 78 or greater
Steep slope > 2/12 29 or greater
SRI values required by LEED & GBI.
Courtesy of BlueScope Steel Southern Africa
Initial reflectivity values as well as measurements after an aging period should be available to show the degree of degradation due to aging of pigments and discolouration caused by the accumulation of dirt, pollutants and possible biological growth on the surface. Field studies, however, have indicated that roof reflectivity tends to stabilise over time.
With the availability of both high-performance commercial roofing systems and cool paints with reflective molecular structures, it is also important to consider the warranty that comes with the product to ensure a long-lasting solution.
Green roofs: Vegetated covers
The idea of a green roof goes way back to the construction of the Hanging Gardens of Babylon and has over the years manifested in architecture in many different forms.
Modern systems engineered for living, breathing planted roofs typically include a waterproofing layer, a filter fabric, a growing media or soil and a variety of plants. Green roofs can also be created by using modular systems such as individual trays set side by side on top of a watertight roof, or thin, lightweight mats which are fully vegetated with low-maintenance sedums and moss plants, which are rolled up and installed like sod.
Green roofs mitigate the negative effects of a building’s footprint by recreating lost natural green space to some extent at roof level. They provide both thermal and acoustic insulation, lower ambient temperatures, help to manage stormwater and create a healthier environment by absorbing CO2 and other pollutants, filtering air and binding dust particles. They further protect structural elements from ultraviolet (UV) rays, wind and temperature fluctuations, which can double or triple the life of the roof.
Blue roofs: Collecting rainwater
Stormwater run-off has traditionally been managed from ground level, but roofs can also play a part, especially in highly urbanised cities with limited space for stormwater detention.
A fairly new practice, blue roofs are designed to capture and temporarily store rainwater in a detention system on the roof to slow the run-off rate and reduce the potential of flooding. During a rainstorm, water can be contained immediately and afterwards be released slowly into municipal drainage systems, or it can be reused for irrigation or even cooling purposes.
Blue roofs work best on long and flat roofing styles and normally have wide gutters with sturdy watertight liners. Weirs at roof drain inlets facilitate temporary ponding and control the rate of the flow of the water to ensure a gradual release. Ideally, the ratio of captured runoff should closely mimic the pre-construction hydrology of the site.
A viable retrofit option for existing buildings, it is important though to determine structural capacity and the maintenance required. On new developments, they reduce the need for underground storage systems and therefore construction cost and time.
A roof system’s sustainability isn’t based on just a single characteristic. Modern sustainable roofs involve a comprehensive approach and more complex design, which on the one hand poses more challenges for design professionals, but also opens up many more opportunities for architects to design multifaceted roofs that combine green strategies and sustainable options.
Full thanks and acknowledgement are given to the Department of Energy, KACO New Energy, Sustainable Power Solutions, SAPVIA, BlueScope Steel Southern Africa, sourceable.net, www.greenroofs.com and www.arwarchitect.com for the information given to write this article.
In a nutshell: Sustainable roof applications
– Solar rooftops – self-generate energy for building use.
– Reflective roofs – mitigating the urban heat island effect in cities.
– Green roofs – extensive, intensive, semi-intensive and biodiverse systems.
– Blue roofs – collect and gradually release rainwater.