Roofing: Choosing cover

Roofs can be divided into two categories: lightweight structures characterised by trusses, beams and purlins with some form of cladding; and slabs, which take the building’s structural form to the roofing level. Both offer their own set of opportunities and challenges.

When questioned about a hypothetical project involving a loft conversion to a small residential house in Cape Town, Paul Carew, founder of  PJCarew Consulting, said wisely: “The question you need to ask is whether you want to cover your house in a blanket or an umbrella.”

In other words: do you want to keep warmth in, or shade the building against solar gains? This very basic but important principle is the ideal starting point for roofing design at any scale.

Ventilating roof cavities

One of the best examples of Carew’s “umbrella” notion are the roofs designed by Kéré Architecture for application in the tropical belt in Africa. Francis Kéré was born in Burkina Faso and was educated in a classroom under a hot tin roof. After winning a scholarship to study in Europe, he made it his mission to address the hot roof problem in rural Burkina Faso. He has now completed several school projects in the region.

The system is designed with an open roof canopy that covers a ceiling that has some thermal mass. In one building this manifests as an arched vault; in another, the design holds dry-packed bricks in a rebar “net” strung between opposing beams. The thermal mass provides a heat sink and the ventilated roof allows free air flow across the surface, removing heat build-up.

Unlike Burkino Faso and Mali, winter in most parts of South Africa is cold enough to complain about. To prevent heat losses from below in lightweight roofs, insulation above the ceiling and below the roof cavity must be introduced: Paul Carew’s “blanket”.  In addition, ventilating the roof cavity can prevent overheating of the services.

Lightweight roofing – impact of colour

Two of the environmental challenges arising from the position and exposure of roofing are the urban heat island effect and overheating of the space below. For the internal space of the building, the temperature of the ceiling surface is the dominant factor in determining the level of comfort experienced by building occupants.

In a study conducted by PJC in Mauritius, they found that changing the roof to a colour of high reflectance reduced indirect solar gains by 45% for colours that have a light reflectance value (LRV) of 80% (the typical value of pure white). They concluded that simply changing the colour of the roof could produce a marked improvement in the performance of the building space.

Urban heat island (UHI) is a phenomenon that built-up areas can be as much as 6ºC warmer than the surrounding hinterland. To mitigate this, a number of solutions are possible, the most simple of which (in terms of roofs) is also, to paint the roof a light colour – as close to white as possible.

For slab roofs, the interventions suggested for optimising thermal comfort are a little different. Suggestions from PJC include insulating the top of the slab, maximising the shading of slab where possible, or covering the roof with an extensive roof garden, sometimes referred to as a sod roof.

Green roofs

Green roofs have a particularly important role to play in responding to sustainability issues. ESD consultants consistently mention them as an effective mechanism for reducing the UHI effect and providing a natural insulation and shading layer to the top of the slab. Clive Greenstone of Green Roof Designs has been measuring the effects of green roofs as part of his PhD studies.

In comparing a bare roof with a newly planted roof of the City Engineering Building in eThekwini, he found the immediate impact was a 10ºC average drop in peak surface temperature when the newly installed green roof was compared to the bare roof. Underneath the modular trays, he noted an additional 10ºC drop in peak temperatures.

Once the plants had become established, the difference between the shaded surface temperature and the temperature below the trays dropped to relatively insignificant readings.

A critical mass of green roofs throughout a city can create a patchwork of habitats that work together to create biodiversity corridors and facilitate species dispersion. A green roof is also an important component in a sustainable urban drainage system.

Slab roofs and thermal mass

An exposed slab will absorb heat when air is warmer than the slab and release it when it is cooler than the air around it. This process creates a thermal lag that provides an opportunity to use the slab as a cooling and heating mechanism for passive control of the building temperature.

Thermal lag of concrete and masonry can be taken advantage of in buildings with exposed structures as a part of a thermal comfort strategy. According to Etienne Terblanche, energy specialist at PJC, using thermal mass as part of the building comfort control strategy is “especially useful in climates where there is a large diurnal shift between day and night. It can be particularly effective in these circumstances when the strategy is partnered with night flushing.”

To bolster the thermal mass effect strategy at WITS University Science Stadium, PJC, in partnership with Orenge Environeering and Spoormaker & Partners, designed the slabs throughout the structure to create a thermally activated building system (TABS).

At WITS this system uses the mass of the structure and is heated or cooled by water that circulates through pipes embedded in the exposed coffered slabs. No ceilings are included in the design so there is a thermal exchange between the slabs and the occupied spaces.

According to Mthethwa, who was involved in the conceptual modelling of the project’s design, the TABS system was to be incorporated in and inform the design of the coffered roof slab.

Pitched Roof Structures and Dematerialisation

Minimisation of material requirements, or dematerialisation, is based on the cardinal principle that by using less, the environmental impact is reduced. MiTek Industries South Africa is a direct subsidiary of MiTek USA, a roof system supply company that initially developed and patented the gang-nail timber truss system with metal connectors in 1956, which is now used worldwide.

In 1989 they developed and patented the ultra-span light gauge steel roof system using the least possible material.  Uwe Schluter of Mitek says by adding solar panels or geysers to the roof, it increases the structural loading and can add to the cost of the structure by as much as 15%. However, the additional structural requirement can be mitigated by considering the position of the loading on the roof.

“Place any extra load near the ridge area at the apex, because the trusses are deeper and therefore stronger in this area,” he says. According to Schluter, there are two reasons why truss systems might fail. The first is that the roof is designed with a certain type of cladding in mind, and additional loading on top can be disastrous.

The second is when the actual roof erection is not done in accordance with all the provided system information and drawings. “For this reason only Institute of Timber Construction approved and certified roof truss suppliers and roof erectors should be used,” he says.

By Peta Brom

The full feature appears in the June/July 2015 issue of earthworks magazine.