From solar panels and rainwater collection to wind power and rooftop gardens, roofs are steadily becoming more than just a shelter from the elements. Harnessing the elements and putting them to functional use, a roof is now an essential, integrated part of how homes and businesses function in the long term, and a platform for an off-grid solution.

Today roofs take on many shapes and sizes, and can be made from virtually any material. For many years the specific choice of roof materials and forms were dictated by the materials available at the time, and as such roofs are one of the hallmarks of architectural history. With the advent of rooftop solar installations and rainwater harvesting systems, amongst others, perhaps the hallmark of current times will be a roof that provides more than shelter, but also a multi-functional space, which supports the essence of a sustainable lifestyle.

Selecting solar solutions

A roof is an ideal place to install solar panels to supplement a building’s energy supply, as well as solar geysers for water heating. Panels should be orientated correctly in relation to the sun’s path to yield optimum power output. Conventional wisdom dictates that this means north-facing panels in the southern hemisphere and south-facing in the northern hemisphere, with an optimum vertical pitch of 30º. While tiled roofs are pitched typically at around 15º-30º, sheet metal can be pitched a lot lower depending on the roofing profile, which may be Kliplok, IBR (inverted box rib) or plain corrugated. “It’s very seldom that the pitch of the roof is perfect,” says Anja Visagie, head of project acquisition at Sustainable Power Solutions (SPS). It’s easier to optimise the installation for a new roof but for retrofits a mounting structure for the panels can be used to correct the orientation. Tilted panels take on a higher wind load and mounting structures add to the cost. Therefore installing panels in the plane of the roof adds to long-term economic feasibility.

Visagie says contrary to the conventional notion that panels should always face north, it is often more optimal on buildings to face solar panels east-west, as this captures the full force of the peak morning and afternoon sun. This is advantageous to both commercial and residential installations as peak energy demands (as well as Eskom peak tariff charges) also match those times of day. Special east-west panels are available and are the same size as north-facing panels, however, on a flat roof, due to the spacing of the east-west modules, which are different in structure to the north-facing modules, more panels can be fitted. The east-west panels will be affected by seasonal variations but not as much as north-facing panels.

Commercial buildings tend to have flat roofs, whereas residential roofs are more likely to be pitched. SPS has been involved in the installation of a phased R20million solar project for the roofs of 14 buildings at Cape Town’s Victoria and Alfred Waterfront (V&A). Once the installation is complete (to date 1.3MWp have been installed on 12 buildings), it will provide 1.45MWp of solar power for the V&A, explains Visagie. At roughly 1KWp of power equal to eight square metres of space, this amounts to over 11 000m2 of panels over this previously unused roof space.

Solar panels on flat concrete roofs are secured with ballasts to eliminate penetration of the roof itself. Special UV-resistant double-insulated DC power cables are used and are further encased in trunking to protect the cables from direct UV, and thus ensure longevity, says Visagie. For other kinds of roofs cables are fed through a gap in the roof tiles, under the roof sheeting or through sidewall penetrations.

Roofs for rain

Roofs also provide an opportunity for harvesting rainwater from a large surface area. Particularly in South Africa’s current drought-stricken times, collecting rainwater from rooftops when it does rain is likely to become an obvious necessity. Water technology is improving at a rapid rate as people respond to the crisis. “Rainwater harvesting is where solar PV was two years ago,” says Grové. Pitched roofs provide the best opportunity for rainwater harvesting as water can be gravity fed into gutters and downpipes and into rainwater collection tanks. From here it’s mainly used for irrigation as it would need further purification to become drinkable.

Most efficient rainwater harvesting systems have a first flush system installed at the head of the downpipe. The first flush device traps roughly the first 1mm of rain that falls on the roof – the layer that contains the most impurities –and allows only the cleaner water to enter the tank. Metal roofing, particularly those that have a “self-cleaning” coating (like Colorbond), is generally the best material for rainwater harvesting because it retains the least amount of dust and pollutants compared to other roof material types.

The sky’s the limit

In built-up cities, where space is at a premium, it makes sense to use an unused roof space for social and functional activities. In late 2016, the City of Johannesburg launched a pilot project for a hydroponic rooftop vegetable garden in the inner city, to facilitate food security for city inhabitants. This is intended to become part of a greater network of rooftop gardens and the concept is being used to train up locals with entrepreneurial and agricultural skills.

By Mary Anne Constable

See earthworks Issue 37, Apr-May 2017 for the full feature.