Devil in the detail is an architect's greatest ally

A roof's details are often the key to its success so the pressure is on to build cost effectively, conform to legislation and meet a site's unique challenges. Working with specialist partners who can help to design problems out of each element is the way forward says SIG Design & Technology's technical services manager, Steve Cleminson.

Designing in adequate roof falls is critical to long term membrane system performance. While British Standards stipulate that all single-ply roofs should "achieve" a fall of 1 in 80, the Standard goes on to say that in the absence of detailed calculations and taking into account mid span deflection, building tolerances etc. a "design" of at least 1:40 should be allowed for.

Get the water off a roof as quickly as possible. Membranes will withstand torrents but ponding can lead to roof failure. Accelerated aging of roof coverings can be caused by build up of leaf litter, moss, fungus and algae growth. Poorly drained roofs can lead to greater imposed loadings, increased levels of water ingress if coverings are damaged, problems of icing and associated slip hazards during maintenance.

Designing at 1 in 60, even, is all very well as long as your building tolerances are perfect. But buildings are never 100 per cent perfect so factor in some scope.

The best membranes are designed to last for up to 40 years as long as they are designed, installed and maintained in line with good practice.

Some 90,000,000 m² of our flagship single-ply membrane Rhepanol fk has been installed worldwide over the last 50 years.

Still hailed as the world's most advanced single-ply, it's also the oldest. First developed in 1938, recent innovations include membranes for green and photovoltaic roofs.

Based on polyisobutylene (PIB), it is the only single-ply membrane with the unique independent Life Cycle Assessment rating of DIN EN ISO 14040 part ff. It has a unique self-sealing edge and Gripfix system, so it can be installed quickly and with minimum fuss without naked flames in three simple steps to create an impermeable seam. Compatible with bitumen, it can be laid over almost any substrate which is structurally sound.

Durability is only as good as the design and installation. Sure, you can drive a Ferrari at 90mph in first gear but you wouldn't expect the dealer to pick up the tab when the engine blew up.

When value engineering a project, ask the manufacturer/supplier partner to look at where you can cut costs without designing in problems or compromising on material requirements.

SIG Design & Technology researches the products, provides a design complete with calculations of U values, condensation risk analysis, acoustic performance and wind uplift calculations. Together, these not only combine compatible materials within the build up but also ensure interfaces are correctly designed with other building elements.

You can deliver best practice to a budget, e.g. roof falls are or can be achieved through the use of tapered insulation with either multiple layer build ups or even pre-bonded boards. However on large projects, the use of screed to falls, firings or a slight alteration to the structural elements is often more effective and less costly.

We also plan how the falls interact with the rest of the roof geography or other building elements – e.g. parapets, windows, doors, or rooflights.

In one refurbishment project the best way to achieve the required falls was to recommend the removal of a run of existing ribbon windows and replace these with shorter ones to allow a tapered scheme to be installed whilst still achieving the minimum 150 mm recommended upstand height at this interface.

The client wasn't just interested in a “paper warranty”, but a sound solution that complied with best practice.

Location and building usage will also dictate the choice of materials used and sometimes there has to be a play-off between the two e.g. bonded membranes may be inappropriate for coastal areas because of the excessive wind uplift. This said, we wouldn't normally recommend mechanical fixings over something like a swimming pool, because in a building of high temperature and high humidity, penetrating the vapour barrier may lead to condensation problems. This aggressive environment may also highlight / exacerbate problems of bimetallic corrosion between the metal deck, the structural elements and the metal fixings. So perhaps a combined approach of bonding and ballasting may be a more risk reduced, cost effective option, or even a green roof? Each and every installation needs to be analysed individually.

There isn't always a one-size-fits-all solution but the sequence is always the same irrespective of the project: identify the design criteria and hone them down; design correctly, install correctly and secure a warranty which covers all products in the roof build up.

Steve Cleminson has been overseeing building contracts, managing construction projects and consulting on roof technology for more than 25 years. A member of the SPRA technical committee, he chairs the Falls & Drainage task group.

Telephone: 01509 505 714
Email: info@singleply.co.uk
Website: www.sigdandt.co.uk

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Rhepanol fk membrane is the watertight choice for Mansfield's £4.5m Rebecca Adlington Swimming Centre. A bespoke 2,000 m² roofing solution includes vapour barrier, flat and cut-to-falls insulation, maintenance walkways and full man-safe system.

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Rhepanol hg green roof membrane and advanced 21st century technological know-how are helping preserve one of Europe's most remarkable ancient monuments – 5,000-year-old Skara Brae, the Orkneys.

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No hot works and easy as 1 2 3 with Rhepanol's self sealing edge – installers just roll out the membrane, remove the tape to seal the joints and roll over the seam.