Designing & Enabling the Roof to Work Harder

What is a roof for? That might sound a simplistic question, and the most straightforward answer is ‘To keep the rain out, of course.’ Any roof that cannot do that is certainly not fit for purpose. But today, in our crowded cities, very often the roof needs to do a great deal more.

Today, according to the United Nations, half the world’s population already lives in cities. And in Britain, our population, which is currently just over 60 million, is expected to reach 70 million by 2029. The number of households grows even faster than the population; according to the Office for National Statistics, the number of households in Britain grew by7.8 million between 1961 and 2004. That’s an awful lot of roofs to waste.

Or course, the first thing that our roofs need to do is to perform thermally. Their position on the top of the building is vital, since as we all know hot air rises, and so heat loss through a poorly insulated roof is even more severe than through a poorly insulated wall. Legislation is becoming ever stricter, and the roof has a key part to play in this. The next version of Part L of the Building Regulation, due in April, will ask for reductions in emissions of CO2 of 25 per cent relative to 2006 – the next step on our path to making all new domestic buildings zero carbon by 2016, and all non-domestic buildings zero carbon by 2019.

To achieve this we need not only better levels of insulation, but also higher standards of construction – cutting down on air leakage will become ever more essential to achieving standards as levels of insulation continue to rise. Already air-tightness testing is playing a vital role in the assessment of newly finished buildings. We need well designed, well insulated roofs, with reliable systems built by reliable people. SIG D& T launched an accreditation scheme last year, called DATAC (SIG Design & Technology Accredited Contractor scheme), which places heavy demands on installer companies.

In order to be accredited, roofing companies must have:

• an exceptional knowledge of roofing systems
• a proven record of installation
• suitable management skills and quality-assurance procedures and
• membership of a relevant trade association, with current insurance cover.

Then their installers have to undergo training and take tests in order to receive a DATAC licence, which will last for three years. SIG will only recommend DATAC contractors to install its roofs.

Within the DATAC scheme there are five levels of membership, ranging from ‘local’ to ‘blue-chip’ to these reflect the contractor’s track record in the type, scale and the complexity of roofing projects it has delivered.

Installation is only half the story, with the other half being the quality of the product. SIG is as proud of these as it is of its installers. It offers a range of single-ply membranes. Rhepanol single ply membrane is made from polyisobutylene (PIB), a synthetic rubber comprising 70% natural materials, and is the only membrane currently available with a full Life Cycle Assessment. It is available in two forms. Rhepanol fk is suitable for both new-build and refurbishment. Designed to be installed directly over bituminous roofs, it can be mechanically fixed with the unique Gripfix system, bonded or ballasted and requires no ‘hot work’. Rhepanol fk paint is a decorative, coloured two-coat accessory finish that weathers without cracking or peeling even on standing seam profiles and has the same longevity as the membrane.

The Rhepanol hg membrane system has been developed specifically from Rhepanol fk to meet the specification requirements of green roofs; it is hot-air-welded to provide the strongest possible seam. Rhepanol hg has full FLL certification as a root and rhizome barrier. And of course installation will be by members of SIG’s DATAC accreditation scheme.

In addition, SIG D & T offers Rhenofol PVC which is attractive, economical and easy to install. It is not as temperature sensitive as other membranes and has a larger weld window than other PVCs. It is available in three types:

• Rhenofol CV is reinforced with a polyester grid and is designed for mechanical fixing.
• Rhenofol CG is reinforced with a glassfibre mat and is suitable for ballasted and green roofs.
• Rhenofol CGV has a polyester fleece backing designed for adhesive bonding.

Achieving this level of integrity with the roof is essential, but it is still only a first step. Roofs have been described as the ‘fifth elevation’ with the idea that aesthetically they should provide as much interest as the conventional four other elevations. After all, if surrounded by taller buildings, a roof will be seen by as many people as the walls that support it. But many roofs are pretty disappointing, both to look at and in terms of what they can do. We need to move away from the dumb roof, to make our roofs work harder.

One of the important things that roofs can do is provide amenity. If with our increasing population and increasing number of households, we are not going to turn the entire country into suburban sprawl, we are going to start living, and working, more densely. This is vital not only to preserve our countryside, but also to make our cities work more effectively, and in a more energy efficient manner. Several studies show that carbon emissions per person rise in cities the less dense they become, because of the greater use of transport and, in particular, the private car.

But increasing density will mean, to a great extent, doing away with private gardens. Where then can people go to relax, to see the sun and to breathe some fresh air? The answer, in part, is on the roof. When Dickon Robinson was development director of the Peabody Trust, developer of some of the most innovative housing in the country, he publicly advocated putting all new gardens on the roof.

A roof garden can be a number of things. It can be a place to sit, a place to play, or a place to grow plants. Those plants may be decorative, but they could also be edible. More and more people are eager to grow their own vegetables and herbs.

Rosie Boycott, who is chair of London Food, was one of a number of pundits asked by the Observer newspaper to predict the nature of the future. She wrote, on 24 January: ‘Rooftops and spare places will be full of vegetables; not obviously, enough to feed the city, but enough to make people more resilient and more aware of where food comes from.’

All these roofs can be classified as green roofs – after all, even the paved roof for sitting is likely to have a few plants in pots. Although green roofs come in many forms, there are two basic types. These are the ‘intensive’ roof, which is carefully planted and tended and can be thought of, in some sense, as a garden. And there is the extensive roof, which is closer to a natural condition and allowed to a great extent to look after itself. Neither is an original idea. The intensive roof can be seen as going back as far as the Hanging Gardens of Babylon, which formed one of the Seven Wonders of the World. Nebuchanezzar II built the gardens – essentially a series of intensive roofs – for his wife who was homesick for the plants of her native Persia.

Much further north, at one of Europe’s most remarkable ancient monuments, the 5,000-year-old Skara Brae, Neolithic carvings beneath the glass viewing roof of one of its famous domed homes were threatened by an unstable environment. PIB was chosen by Historic Scotland to create a new watertight garden roof that blends in aesthetically and could be left undisturbed for up to 50 years.

In Scandinavia, farmers created one of the first extensive roofs, by stripping the sod from surrounding grass meadows, and placing it on the roof, supported by heavy timber beams. This provided an early form of roof insulation, in an unfriendly climate. Those Scandinavian extensive roofs were not really accessible, except for maintenance, and the same is true for many extensive roofs today. But the fact that you cannot sit in them, whether for reasons of overlooking, or safety, or just not to damage the plants, does not mean that they do not have a value.

The first is still one of amenity. Looking out of a window onto growing things lifts the spirits and puts the city dweller in touch with the changing seasons. And there are other benefits of green roofs as well, which obtain whether or not you can access them easily.

Like those Scandinavian farmers, contemporary designers have come to appreciate that having a planted roof can modulate temperature change. Researchers at Nottingham Trent University studied the fluctuations in temperature beneath the membranes of a traditional roof and a green roof. With the external mean daily temperature varying between 0C and 18.4C, the researchers found that the temperature beneath the membrane of a conventional roof fluctuated between 0.2C and 32C. In contrast, under the green roof the variation was much less – from 4.7C to 17.1C. So the green roof helped to keep the building warmer in winter and cooler in summer.

Heat is not the only thing that we worry about in our buildings. Noise is another, and there is research that shows that a green roof can significantly reduce the noise coming into a building – the greater the depth of the substrate, the more the sound is reduced.

Green roofs can also play a key role in SUDS (sustainable urban drainage schemes). Typically, a green roof will intercept at least the first 5mm of rain in any shower, reducing run off to sewers etc. With a significant number of green roofs, the existing Victorian systems will be able to cope for longer. This is a benefit not so much to the individual property, as to the city as a whole.

Another city-wide benefit comes from the role that green roofs can play in reducing the urban heat island – the effect that makes cities hotter than the surrounding countryside. The greatest benefit of all comes when a building is entirely enveloped in vegetation – with a green roof and also green walls.

Green roofs can also make a considerable contribution to biodiversity. In this case they can replace habitats that were destroyed when the building took place, or can provide oases for certain birds and invertebrates amidst a sea of building. All planted roofs will offer some benefits, but research shows that the best habitat consists of a mixture of sedums and native wild flowers.

With so many benefits to green roofs, it may be surprising that they have not become more widespread – as they are for instance in Switzerland and Germany. Certainly their use is being encouraged, with major cities including London and Manchester writing policies to encourage their adoption.

But there may be some reluctance, either because they are new and not entirely understood, or because of concerns about durability. In fact, a green roof protects the membrane beneath it, increasing its lifespan, but there is a concern that if a membrane were to fail, dealing with a leak could be an expensive and messy business.

The answer of course, is to go for a system that offers reassurance, and can make designing and constructing reliable, beautiful and appropriate green roofs as simple as possible. This is what SIG is offering with its new, lightweight modular green roof system.

Made up of 0.5 m module trays which slot together for a seamless finish and provide plant drainage, it allows the client to create either a sedum roof or a bespoke horticultural selection of sedum, indigenous grasses and wildflowers.

The modules are installed over a geotextile filter fabric, which sits on top of the waterproofing. They interlock and need no fixing, so there is no danger of puncturing the roof membrane. Waterproofing is again with SIG’s Rhepanol hg membrane system.

As mentioned above, some of the benefits of green roofs can be extended to walls as well. Increasingly popular, green walls give enormous visual pleasure, and can help to ‘humanise’ long stretches of wall that may be blank. They can take the eye from the mega-scale of a giant construction down to a much more intimate scale. We may know instinctively that we like being with plants, but there is also scientific research to prove that this is true. For example, research at the University of Texas shows that people who work in offices with plants are significantly happier.

And green walls have other quantifiable benefits as well. They help to remove dust and pollution from the air. And, as mentioned above, they can make a considerable contribution to reducing the urban heat island effect.

Although widely used in other countries, particularly in France, green walls are still a relatively new concept in the UK, and it is true that there have been some teething problems. But these are to do with management issues, not with the inherent properties of green walls. Indeed, there is a magnificent green wall at West London’s Westfield shopping centre that use’s SIG Design & Technology’s Living Wall system. Green walls do need irrigation – not a substantial amount as much of the water can be recycled, but the system does need to keep operating.

SIG Design & Technology’s Living Wall system is modular and easy to use, so giving the designer free range to use their imagination. It can be attached to either a wall or a freestanding structure, and it has a built-in irrigation system.

We have talked about green roofs (and green walls) but there are many other things that roofs can do. They can house important technical plant or, through the use of rooflights, they can help to bring light into deep-plan spaces. The more complex the demands on a roof, and the more penetrations there are through the membrane, the more vital it is that it be designed, manufactured and installed in a reliable manner.

One of the most exciting things that we can ask a roof to do is to contribute to the energy needs of the building, by using photovoltaics to generate electricity. SIG has teamed up with the developer of a new solar power system, designed especially for flat roofs on commercial and industrial buildings.

Solyndra® PV is a three-dimensional system, which consists of tubes sheathed in copper-indium-gallium-diselenide (CIGS) solar cells. It is one of the most efficient systems on the market, and has an excellent price to output ratio.

SIG has introduced a special membrane, White Rhepanol fk, to work with Solyndra, and make the most of its properties. White throughout its molecular structure, the membrane has an ultra-reflective surface resulting in a 90.4% solar reflective value. This helps to maximise the efficiency of the solar panel as the sun’s rays, through reflection, strike the generator from underneath, as well as from above.

White Rhepanol is permanently resistant to UV radiation and, due to its higher anti-slip friction co-efficient panels can be installed with a greater pitch – up to 9.5 degrees – than other membranes.

The decision to generate electricity from a roof need not rule out the possibility of having a green roof as well, in an adjacent area. Indeed, research shows that the two are complementary, since the cooling effect of the planting helps the photovoltaics to become more effective. That really is the ultimate in making the roof work harder.