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Butyl is only skin deep

technical & practice: Flat roofing is here to stay. But, with so many waterproofing products, do you have to be a scientist to specify them?

It is now nearly 25 years since the introduction of British Standard BS747 and 20 years since BS6229, the formative standard documentation on flat roof detailing and workmanship. Both documents feel old and, although BS747 admits to no longer being current, it means there is a significant legislative gap in the regulatory framework on flat roofing.

More importantly, these two documents were written when built-up felt roofing was the sole player.While built-up bituminous roofing still retains the lion's share of the flat-roof market, the rise of single-ply membranes has not yet been met by progress on a British Standard - the European Standard will probably be published in a year's time.

Instead, a set of industry standards has been drafted. Though these are not statutory, they will form an important reference point.

Bill Jenkins, director of the Flat Roofing Alliance, says that given that the construction industry has 'grown up' with built-up bituminous products, 'people will not swap over to singleply solutions in great numbers'. He is confident that bituminous products will retain about 75 per cent of the flat roofing market. The bad publicity given to built-up 'felt', he believes, is now over. He says: 'In the '50s and '60s built-up felts weren't really an issue. It's only when we started insulating roofs in the late '60s and '70s that the problems started'.

He does not mean that we should not have clamoured for better insulation standards, rather, that many designers - and many contractors - did not know the implications of condensation build-up, which was the primary cause of failure. In the the '70s and '80s there was a public reaction against built-up felt roofing although, for the domestic buyer, there were few options. But, as Jenkins says: 'We studied the problem, understood it and solved it.'

Now, everyone in the industry is aware of the causes. He considers that there is no reason for built-up bituminous felts to fail within the 'normal range of thermal extremes'.

The Flat Roofing Alliance is now launching a mechanically fastened bituminous covering to rival the installer-friendly aspects of the single-ply market - screw fixing one end of the bituminous sheet and overlaying it with a torch-on edged follow-on sheet.

But even though elastomerics have benefited bituminous sheeting since the '70s - such that cold weather cracking should be a thing of the past - the bituminous joint is more likely to become the site of failure. The benefit of single-ply roofing, apart from its range of colours, is its thorough welded or solvent joints, which are as robust as the main body of the material.

Although it has been available in the UK for more than 30 years, it is only in the past 10 years that specifiers have realised its full potential.

Unfortunately it is a relatively poorly understood material.

While the built-up felt roofing market is trying to reclaim scientific credibility and product adaptability from the perception and practice of local jobbing roofers and DIY superstore suppliers, the polymeric manufacturers are battling against a lack of understanding about the variety of claims given to a range of complicated chemical constituents.

Whereas architects are willing to cede authority to experts in the field of landscape architecture, engineering and so on, they remain loath to extend responsibility to roofing manufacturers to designate the correct product for the circumstances.

If they do, they tend to compensate by demands for warranties and litigious mechanisms, rather than sensible specification drafting combined with trustworthy delegation.

Jim Hooker, secretary of the Single Ply Roofing Association, looks forward to the day when, rather than writing performance specifications, architects will write 'support specifications'.

These documents will allow the manufacturer - chosen through tender on the basis of basic performance requirements - to ensure the correct material for the conditions, confirm details and ensure materials compatibility.

Hooker regularly spot-checks association members' jobs, tracing the history of the installer and ensuring training records are up to date.

Since he is in the business of not allowing one poor contractor to jeopardise the credibility of the umbrella organisation, he is rigorous in his site inspection.

Nigel Blacklock, technical manager of Sarnafil, one of the main manufacturers of single-ply roofing in the UK, says that it is insulation upgrades, under the revisions to Building Regulations Approved Document Part L, that pose a challenge to the industry. By increasing the insulation in a warm roof, the mechanical fastener will have to be lengthened to reach the substrate with satisfactory purchase. Also, thermal bridging and air infiltration at the mechanical fixing penetration will have to be factored into the calculations and tested at the end of the job.

Blacklock is confident about compliance - fixings incorporating thermal breaks already exist - but he is concerned at the possible financial implications of the regulatory changes on the client and on his own company. Bigger fixings will be more costly and, theoretically, could shift the market to rely more on adhesive or loose-laid solutions.

Despite most single-ply suppliers providing a proprietary vapour-control layer as part of a packaged roofing system, the need for documentary records of air tightness and thermal resistance might make the support specification route a sensible way to proceed for all parties.

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