CPD: How to make the most of your roof

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Whether it’s installing a sedum roof or PV panels, Stephen Cleminson of SIG Design & Technology says more use needs to be made of our roofs

In today’s crowded cities, we often have to ask the roof (the fifth elevation) to do more than simply keep the rain out and help to insulate the building. As our cities become more crowded, we cannot afford to treat the roof as dead space, making no further contribution. This will become more urgent as both our population and household formation continue to rise.

As we demand more from our roofs, it is essential that they are designed, specified and installed to perform efficiently now and throughout their design lives.

This CPD article looks at some of the key aspects of today’s roofs: how to guarantee the overall process; the specific issues raised by green roofs; and advances in the use of photovoltaic technology on the roof. It will show how, in addition to keeping the rain out, a roof can make the best possible contribution to the environment.

Altogether now

For a roof to work effectively, all the elements have to be specified appropriately, so that they are not only the best choices themselves, but can also perform in harmony with each other. The best place to start is with the choice of the roofing membrane. These can be PVC, thermoplastic polypropylene (TPO), flexible polyolefin (FPO), ethylene propylene diene monomer (EPDM), or polyisobutylene (PIB).

SIG Design & Technology’s Rhepanol hg product is a PIB membrane that is bitumen compatible. PIB uses the same base material that goes into chewing gum, so evidently it is not environmentally harmful. In fact, of all the commonly available roofing membranes, it is the only one to have a full lifecycle assessment, completed by German ecology consultancy CAU. This concluded: “During the entire lifecycle of the roofing and waterproofing membrane Rhepanol hg, there are no significant environmental impacts or health hazards.”

It is important that the membrane and the insulation used are compatible. Foil-faced insulation boards will have a higher thermal performance than an equivalent thickness of board with glass tissue-faced insulation, so making it possible to specify a shallower board. But if the membrane is to be glued down, then tissue-faced insulation must be used, as the glue will not adhere to the foil.

There are also issues regarding fastenings. Part L 2006 introduced a requirement to take thermal bridging into account in terms of mechanical fixing (Part L 2010, just published and due to come into force in October, will be even more onerous). This means that, even if levels of insulation appear to be appropriate, if thermally broken fasteners are not used, the roof, and therefore the building, may fail in terms of its U-value.

It is the importance of such issues that makes it desirable to have one supplier, such as SIG D&T, in charge of the overall design and specification of the roof.

Going green

There is a growing appreciation of the benefits that green roofs can bring, from improving biodiversity to reducing rainwater runoff (see graph right) and having a beneficial impact on the urban “heat island” effect. They also help to keep the building cool in summer, and offer protection that makes the roof membrane last longer. But if they are to work effectively, they must be properly specified and installed.

As there is currently no British or European standard for green roofing, it is vital that the roof membrane used is certified to the German FLL standard – a widely-used test drawn up by the government research agency Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau) for green roofs.

FLL testing is a process where materials are placed in a box in such a way that there are both internal corners and a T-junction in the base. This is then filled with soil and plants are grown for two years. Any penetration of the materials is monitored. The species used in the testing are agropyron repens (couch grass) and pyracantha coccinea (orange charmer).

There are two levels of FLL certification – one that the roof membrane is resistant to penetration by roots, and the other, more onerous, that it is resistant to both roots and rhizomes – thick underground stems which sprout new growth. Having this resistance is essential, because if roots or rhizomes do penetrate the waterproofing layer, integrity will be lost and the roof will leak.

SIG D&T’s Rhepanol hg satisfies the higher requirement of the FLL standard. In addition, SIG D&T offers a modular solution for green roofs, which makes installation both simpler and more reliable without the need for penetrations. In a traditional green roof there will be a complex build-up of five components above the waterproofing layer. Typically this will consist of a fleece layer, then a cuspated (eggbox-shaped) drainage layer, a filler layer, the soil medium, and finally the sedum roll.

In contrast the SIG D&T modular green roof requires just two elements: a fleece layer which is rolled out, and the pre-grown and pre-established planting, grown within the cuspated layer and simply placed upon it.

In addition to the simplicity of this approach there are a number of other advantages. Because the plants are already established, and have grown roots down into the growing medium, they are much less susceptible to wind uplift, and hence to loss, than a newly placed sedum mat would be.

Sedum roofs are designed so that watering is unnecessary once the plants are established, but watering is needed while this establishment is taking place. With a conventional green roof, this watering has to take place on the roof. With the modular system, the plants are already watered and established in the nursery, so there is no need for any watering on the roof itself.

The nursery which SIG D&T works with always has “standard solution” modules available. But special solutions are also available, for example, different sedum mixes, decorative grasses and local flora can be included in the modules.

Roofing suppliers can also advise on crucial issues, such as the weight of the green roof. This will vary according to the depth of substrate used. Designers need to be aware of the maximum weight of the green roof when it is fully saturated as it is vital that the structure can support this load. But the minimum weight, ie the weight in a dry period when the roof is almost entirely desiccated, is also important for wind uplift calculations, to ensure that the roof can resist the maximum loads to which it is likely to be subjected.

PV innovation

Photovoltaic installations on roofs are gaining popularity, and with the introduction of feed-in tariffs, this trend is likely to continue. SIG D&T is offering an innovative new solution, replacing conventional PV panels with tubular generators from Californian manufacturer Solyndra (see opposite page and overleaf), and installing these in conjunction with a special white reflective membrane.

Intended for use on large roof areas, the Solyndra tubes, which are covered with small copper indium gallium selenide (CIGS) solar cells, can capture sunlight from 360 degrees. They generate electricity in response to direct, reflected and refracted light.

As well as offering effective conversion of solar energy to electricity, CIGS panels are not dependent on orientation (conventional panels are fixed in the best possible orientation, with peaks and troughs in performance as the sun moves). They can also be placed more densely on a roof. With conventional panels, there is the risk of one panel shading its neighbour, while staggering panels to avoid this reduces their density on the roof. But the Solyndra units can be packed more densely on the roof than conventional panels, and there is still room for air to circulate around and between them. This helps to cool the panels, and PV works more effectively at lower temperatures.

To take maximum advantage of the shape of Solyndra panels, they should sit on a reflective membrane. SIG D&T has therefore introduced Rhepanol fk white, which is both white and shiny, giving a solar reflectivity of 90.4%. It should also stay white throughout the building’s lifetime, since the PIB membrane is less susceptible to solar degradation than some other membranes. At the correct fall – a minimum active fall of 1 in 80 as specified in BS 6229:2003 Flat roofs with continuously supported coverings – rainfall will simply wash off any accumulated dirt.

Another advantage is that the system does not require any fixings through the roof membrane. Conventional solar panels are very susceptible to wind uplift, and must therefore be fixed to upstands. These, in turn, are fixed to the roof structure, and the waterproofing has to be brought up and detailed over these upstands, which means there is potential for the integrity of the waterproofing to be undermined. The alternative of ballasting the panels adds to the load on the roof, possibly requiring structural strengthening (see Eurocode EN:1991-1-4: 2005 for further details). In contrast, Solyndra panels are fixed to simple carriers that sit on the roof with no penetrations. In this set up they have been tested and certified at wind speeds up to 130mph (208km/h).

So as well as offering an increased electrical output, this approach is simpler than the conventional approach and protects the integrity of the roof. 

Stephen Cleminson is technical manager at SIG Design & Technology

More information at www.single-ply.net

What’s in a modular green roof?

1 Rhepanol 300g/m² fleece

2 Rhepanol hg waterproofing layer

3 Tapered insulation

4 Bitumen vapour control layer

5 Bitumen primer

6 Concrete substrate

7 Rhepanol laminated metal drip

8 Rhepanol stainless steel gravel guard

9 20-40mm river washed gravel

10 Pre-grown roof modular tray including cuspated layer

CPD test paper

Sustainable roofing

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