From concept design to installation and eventual recycling, flat roofs provide a wide range of environmental benefits for building projects. SIG Design & Technology identifies the key areas of consideration for specifiers

In association with


Green roof with FDT Rhepanol hg PIB membrane at Meydan shopping centre, Istanbul, by Foreign Office Architects (photo: FOA)

In spatial terms, flat roofs can be highly efficient, reducing the apparent mass of a building and providing easily accessible space for services, such as HVAC plant, photovoltaic panels and solar thermal tubes. Terraces, green roofs and roof gardens can also enrich the ‘fifth’ elevation, providing an important amenity for building occupants. Green roofs provide additional advantages in terms of their ability to increase biodiversity, filter pollution, improve insulation values, control rainwater run-off, and require minimal maintenance.

Water attenuation is also a key feature of blue roofs, which are designed to retain some water during heavy rain, before draining in a controlled manner over a specified time period. Architects may also wish to consider a hybrid blue/green roof, which combines the benefits of a green roof with water attenuating properties of a blue roof.


Rock Salt restaurant at Folkstone Harbour, Kent, with FDT Rhepanol fk PIB roof

When it comes to the sustainability of the roof covering itself there are a number of factors to evaluate. First, what is it made from and can it be recycled? TPO membranes, for example, do not contain polymeric plasticisers, liquid plasticisers or chlorine, and are 100 per cent recyclable. Made from polyisobutylene (a synthetic rubber), FDT Rhepanol PIB is the only membrane currently available with a full Life Cycle Assessment that meets DIN EN ISO 14040 part ff. This means that it has no significant environmental impact at any time between its manufacture and eventual disposal. Added to this, Rhepanol fk specified in white can help maximise the reflectivity of solar PV systems.


IKO Permaphalt roofing at Westminster Cathedral, London

Bitumen-based mastic asphalt might initially appear unsustainable, however its material content should be balanced against four key environmental attributes. First and foremost it has an extremely long lifespan. In 1972 the BRE reported that “Asphalt roofing, properly designed and laid, should prove capable of lasting 50-60 years”. Second is carbon neutrality – in 2008 UK mastic asphalt became the first industry in the world to achieve the CarbonZero standard. Third, mastic asphalt has the potential to be 100 per cent recyclable, for example into road surfacing, and UK manufacturer IKO is currently developing a recycling programme. Fourth is local production – IKO Permaphalt PMA is manufactured in the UK using 85 per cent locally quarried limestone aggregate. The 13 per cent bitumen content is sourced from the Ellesmere Port bitumen refinery.


IKO Armouplan PSG roof at The Grove Hotel in Watford, Hertfordshire, by Purcell Architects

According to SPRA (Single Ply Roofing Association) the main source of recyclable produce from single ply roofing is in the post-service product. It expects volumes to rise as the rapid market growth of single ply in 1980s reaches the end of its service life. The potential growth will be offset by the popularity of overlays to the existing system – an advantage of single ply technology which avoids exposure of the building to the elements during refurbishment. Single ply membranes comprise high quality polymer, which is recyclable. While no more than 15 per cent used material can be added to new roofing membrane – due to issues relating to UV-resistance – it can nevertheless be processed into other high value products. SPRA has already run trials in the UK, and is currently working with a major waste handling and logistics partner to develop a nationwide take-back scheme.

St Anne School, Somerset, with IKO Armourplan PSG roof in slate grey

The sustainability of flat roofing can also be evaluated in terms of its manufacturing processes and waste minimisation. IKO for example has achieved environmental standards ISO 14001 and BES 6001 for its mastic asphalt and hot melt factory in Derbyshire. The company has also achieved BES 6001 on a range of products, including Permaphalt and Roofstar, as well as producing what it believed to be the first hot melt system with ‘zero wrapper waste’ – PermaTEC Ecowrap.


Hydrostop AH25 roof at Vue Cinema, Doncaster

Liquid waterproofing systems, such as Hydrostop AH-25, can extend the life-expectancy of flat roofs, minimising waste and reducing the need for virgin materials. Correctly installed and maintained they can last in excess of 25 years, and in refurbishment applications provide savings of up to 70 per cent on roof replacement costs. BBA-certified with low VOC content, AH-25 is both solvent- and fume-free, reducing the impact of a refurbishment cycle on building users and neighbours. It is applied wet-on-wet and is fully reinforced with a polyester fabric.

FDT Rhepanol fk PIB roof at Lymington Shores, Hampshire (photo: Adam Coupe)

Last but not least sustainability in flat roofing should be considered in terms of detailing practices and future maintenance. For architects, entering discussions with a roofing expert at the earliest possible opportunity can be a vital step towards specifying the right product for the application and identifying key detailing issues. The latter will invariably include the design of adequately dressed and sufficiently high upstands and parapets, as well as sufficient drainage falls that take into account building tolerances and mid-span deflection. For clients and end users, it is vital that flat roofs are subject to a proper and regular inspection regime. This will ensure drainage outlets remain unblocked and appropriate repairs or replacement can take place if needed. Adherence to these procedures should ensure flat roofs achieve and/or exceed their life expectancy.

Contact Details

For more information on sustainability in flat roofing visit the SIG Design & Technology roofing website.

Next month we’ll be publishing five case studies which illustrate some of the principles set out here.