Daniel Bosworth, Design & Technical Manager at SIG Design & Technology, discusses how the new British Standard for roofing: BS 6229:2018 is changing best practice for different types of flat roof construction

In association with

Buildings.

Successful flat roof design and construction is reliant on many factors – not least the adherence to recognised codes of practice. One of the most useful and effective of these is BS 6229:2018 – ‘Flat roofs with continuously supported flexible waterproof coverings’. Introduced in November 2018, the new code incorporates a number of significant changes from the superseded document (2003), and is intended to reflect advances in construction systems, materials and methodologies.

In the first instalment of this special two-part feature, we interview Daniel Bosworth, Design & Technical Manager at SIG Design & Technology, about the reasons behind the update, what the main changes are, and how these affect best practice for different types of flat roof design. In addition, some of the organisations that helped to shape the code, including SPRA (Single Ply Roofing Association), and the NFRC (National Federation of Roofing Contractors), give their insights on the revised document.

Why has BS 6229 been updated and who was involved in the process?
The standard has been overhauled to reflect developments that have taken place within the flat roofing industry since the previous edition was published in 2003. It is also intended to ensure consistency with other related Building Regulations and Codes of Practice that have been updated over the same period. The changes were brought about by Committee B/546, which included representatives from a diverse range of organisations, including the RIBA, BBA, BRE, NHBC, RIBA, SPRA and NFRC. These respected and highly knowledgeable groups have ensured that the best practice advice is not only inclusive and comprehensive, but also relates to ‘real world’ design and construction issues.

Buildings.

We increasingly demand more and more of our flat roofs. Centrepoint, London (ph: Terence Smith).

What are the key changes?
Perhaps the most obvious change is in the name of the standard. The latest iteration introduces the words ‘flexible waterproof’ coverings. This is because the code no longer includes metal roofs, such as copper, lead and zinc. These are now the responsibility of technical committee B/542 and will be covered by a separate standard.

Another important change concerns the standard terms and definitions used in conjunction with flat roof design. Some of these have been altered to complement revised building practices for different roofing types and now include air and vapour control layer (AVCL) rather than VCL used previously; water-flow reducing layer (WFRL) for inverted and green roofs; and blue roofs designed for rainwater attenuation rather than storage.

Most significantly, a zero fall roof is now defined as a ‘roof with a fall between flat and 1:80’. The aim here is to eliminate back falls and other non-draining areas. This will be discussed in detail in part two of this feature, along with upstand heights for door thresholds, which have been reduced to 75mm in line with NHBC guidance.

How does the new standard affect different types of flat roof?
The types of flat roof covered by the code have been expanded from warm, cold, inverted and ‘others’ to include hybrid and uninsulated constructions. The latter relates specifically to canopies and unheated buildings, where the waterproof layer is applied directly to the roof slab or deck.

Overall, there have been few changes to the recommendations for warm roofs – the principal one being the specification of an AVCL instead of VCL. This acknowledges the growing importance of airtightness, and the effectiveness of incorporating it into the building envelope in this way. However, specifiers should pay particular attention to maintaining airtightness continuity between the AVCL and external walls. This is typically achieved using a proprietary double-sided tape, which should always be installed to the manufacturer’s recommendations.

Buildings.

A hybrid roof can improve thermal performance while simultaneously limiting the overall thickness of the roof, but a full condensation risk analysis is required. House in West Sussex (ph: AJ Lucas Roofing Contractors).

The difficulty of achieving adequate cross ventilation, especially on wide spanning, cold build up, flat roofs has led the standard to decree that this type of construction is now ‘not recommended’. It should be noted that the document does not go as far as to say cold roofs should not be used, rather, it has made the problems associated with them more explicit and revised detailing recommendations for instances where they cannot be avoided. The build-up now includes a breather membrane below a minimum 50mm vented void and an AVCL above a minimum 25mm service void to the underside. Added to this, the recommended maximum width for cross ventilation is limited to not more than five metres. The code also warns against the use of mushroom vents.

With respect to hybrid roofs, the standard recognises that there is an increased risk of interstitial condensation due to the presence of insulation both above and below the deck. As such, it now recommends that a full condensation risk analysis is carried out when designing roof systems of this kind. Advice on interstitial condensation was previously included in the standard, but is now covered by BS 5250:2011 – ‘Code of practice for control of condensation in buildings’.

Finally, for inverted roofs the new code advocates placing a WFRL rather than a simple filter sheet on the insulation layer to regulate the movement of water and prevent dirt and grit from penetrating the joints between insulation boards.

Industry comment
SPRA (Single Ply Roofing Association), NFRC (National Federation of Roofing Contractors) and LRWA (Liquid Roofing and Waterproofing Association) have issued the following statements on BS 6229:2018 for this feature:

“We expect a lot more from our flat roofs in the 15 years since the code was last revised, whether it be slowing down the flow of rainwater, collecting solar energy, creating green spaces or as platforms for rooftop structures. However, for our members, the three most important changes relate to the ventilation of cold roofs, door and threshold details, and fire safety using Safe2Torch principles developed by the NFRC. This last point is a crucial one, given the increased emphasis on fire safety after Grenfell, and architects should be considering Safe2Torch at design-stage.”
Gary Walpole, NFRC Technical Officer

“This latest BS 6229:2018 is deliberately simpler and more reader-friendly, so it should be referred to and used far more frequently. Our members welcome the clarification that for correct drainage, design falls of 1:40 are required to give a finished fall of at least 1:80. Zero-fall is now defined as ‘from zero to 1:80’. Back-falls or large deflections leading to ponding are simply not acceptable; it is the responsibility of the main contractor and concrete installer to present an acceptable deck for the roofer to waterproof. This will enable members to increase the quality of installed roofs, minimise corrective actions and reduce maintenance in the longer term.”
Paul Franklin FIoR, LRWA Technical Secretary

“The new standard reflects the many changes that have taken place in the design requirements for buildings since the introduction of the first edition. There have been changes to the scope of the document and the resulting terms and definitions. The new definitions include breather membranes, air and vapour control layer (AVCL), water flow reducing layer (WFRL), blue roofs and zero falls roofs. The new code attempts to take account of the extensive development of flat roofing materials and design requirements. It is a much needed and welcome addition to good flat roofing design and installation practice.”
Dr Ronan Brunton, SPRA Technical Manager

More information
Read part two of this article here.

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