Key decisions driving sustainable roof designs, plus the challenges that need to be overcome.
The selection of roofing materials, insulation and the installation thereof all play an important role in putting and keeping a sustainable roof over your or your client’s head. On top of that, with each individual building element playing an important part in adhering to building regulations and achieving efficiency ratings, sustainability is probably the biggest consideration of all.
According to Sally Stromnes, marketing manager of the Safintra Group, sustainability in roofing is associated with using less raw materials and delivering a longer lifespan for the roof, whilst also being environmentally efficient.
Materials as decision driver
“Metal roofing has sustainability credentials that make it increasingly attractive for discerning commercial users and specifiers,” says Stromnes. According to Stromnes the use of modern aluminium-zinc coated steel roofing provides a substantially longer lifespan than alternative zinc-coated steel, which means that less raw material is used in the longer run.
“Metal roofing also has the benefit of being able to accommodate panels and water harvesting systems without compromising the weather-proof performance of the roof,” she says.
“It is versatile in terms of occupant comfort and requires less energy to heat or cool the occupied spaces than tiles that leak air. The creation of an insulated roof space covered with sheets from ridge to eave creates a well-sealed air cavity between roof and ceiling, which acts as a highly effective insulation barrier.
“And for building additions, the ease and speed of over-roofing or of retrofit thermal insulation in steel roofing structures will be an increasingly important decision driver for architects and building owners,” she adds.
Standing the test of time
The challenge lies in the use of materials which are not suited for the purpose. Stromnes points out that the investment in a structure’s roofing is substantial and forms a critical part of the total building’s durability and service life.
“Building owners and managers must ensure that each component in the roofing system has suitable quality and therefore the longevity to perform as they should. Manufacturers and suppliers should be able to provide warranties in this regard,” she advises.
“In particular, the use of a cladding or roofing material not suited to its specific environment is commonplace. Within half a kilometre of breaking water or near a coal mine, for example, one must use an aluminium material, or aluminium-zinc coated steel with a coating of at least AZ 200 (200 grams of coating per square metre).
“The use of substandard and unproven fasteners which are the only items holding the roof down further creates concern. These should be fit for purpose and have a lifespan equal to or greater than the roof cladding. Replacement of components during the lifespan of the roof is both expensive and usually completely unnecessary.”
Roofing is currently not recognised as a formal trade in South Africa and the necessary skills to install roofing properly is often lacking, leading to substandard installations. Seeking to address this challenge is the South African Metal Cladding and Roofing Association (SAMCRA), which was launched at the end of October last year.
SAMCRA is a sub-association of the Southern African Institute of Steel Construction (SAISC). According to SAMCRA director, Dennis White, the association will aim to ensure that there are comprehensive SANS standards for metal roof and side cladding as well as metal tiles and plans to establish an accredited testing facility. It will further provide information to specifiers, building inspectors and other decision-makers, facilitate artisan training and implement a programme to certify roofing contractors.
“Insulation regulations such as the SANS 10400XA legislation are further driving changes and innovation in installation practices, particularly insulation beneath, and green-energy structures above the roof,” adds Stromnes.
“SANS 10400 XA effectively requires far deeper sections of insulation to be installed to the roof space than before, in order to achieve the prescribed R-values. For insulation to perform to its optimum level, it is imperative that a continuous layer is installed with minimal thermal bridging,” she says.
Dion Marsh, general manager at Ashgrid South Africa, confirms that the installation of insulation blankets over purlin, without compressing the material below the roof sheet, has been one of the greatest challenges for roofing contractors since the introduction of the legislation.
Up to the challenge
“When insulation is installed without a spacer system, even when draped over purlins to maximise the loft, it will always be subject to severe compression at the purlins. This compression tapers off between purlins, but the material never recovers to its full design depth. Continuous timber or steel packers do little to reduce this compression and contribute to thermal bridging,” Marsh explains.
He notes that engineered spacer systems are specifically designed to eliminate compression of the insulation throughout the roof space, whilst providing a structurally engineered interface with the roof covering. The legs of the spacer system allow the full design depth of the insulation to be achieved, even at the purlins.
Ashgrid South Africa recently completed comparative tests to quantify the effects of compression on insulation installed with and without a spacer system. The results showed that the overall R-value and thermal performance of a 135mm fibreglass insulation blanket is reduced by almost 40% due to compression when installed without a spacer system, while when using a spacer system, there was no reduction since the insulation blanket maintained its design depth throughout the roof.
“Besides compliance with the South African National Standards, the additional cost of a spacer system is negligible in comparison to the savings made as a result of lower energy consumption over the lifecycle of the building,” Marsh states.
Taking energy-efficiency in roofing a step further, companies such as Safal Steel and Bluescope Steel, which produce modern aluminium-zinc coated steel for roofing, are keen promoters of the importance of the Solar Reflectance Index (SRI) values of roofing materials.
SRI is a numerical expression of a constructed surface’s ability to reflect solar heat, based on solar reflectance, thermal emittance and medium wind coefficient. Roofing materials with high SRI values helps to mitigate the Urban Heat Island (UHI) effect, resulting in less heat radiating into the home and offering greater thermal comfort.
While it is true that white or light coatings assist in cooling roofs and the building interior, merely painting a roof white turns out not to be the silver bullet it was once thought to be.
According to a study by Mark Jacobson and John Ten Hoeve, “Effects of urban surfaces and white roofs on global and regional climate” published in the Journal of Climate, white roofs have a local cooling effect of ~0,02K on the population-weighted ground and air temperature, but cause the global temperature to increase by a bigger margin (~0,07K). So while buildings might be cooler with white roofs, it comes at some expense to the planet.
The study ran a 20-year simulation in which all roofs worldwide were converted to white roofs. Urban land use, vegetation, albedo, reflection coefficient and soil data were combined in the global model to quantify the effects of urban surface and white roofs on the climate.
In addition, the producers of coating technologies are coming up with darker coloured products that boast thermal efficiency properties and absorb less energy with similar advantages to lighter shaded colours.
Sanedi and AAAMSA, together with the USA Department of Energy’s GSEP initiative, established the South African Cool Surfaces Association (SACSA) on 6 February 2014, with the aim to put cool surface technology on the South African map. SACSA is expected to educate built environment professionals on the topic of cool roofs in order to improve thermal comfort, energy-efficiency and sustainability of buildings in South Africa.
Full thanks and acknowledgement are given to Safintra, Ashgrid South Africa, Steel Construction, www.sanedi.org.za and the Journal of Climate for the information given to write this article.
– Material fit for its purpose.
– Insulation regulations.
– Cool roofs with high SRI values.
– Substandard fasteners.
– Installation skills.
– Clear information and guidance.