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The verge of despair

January 31st 2018 saw the introduction of BS 8612: Dry-fixed Ridge, Hip and Verge Systems for Slating and Tiling, which frames minimum performance requirements for dry-fix roofing components: establishing that they are both fit for purpose and are delivered with sufficient information to ensure correct installation. One year on, Dr Kevin Ley, Head of Technical Services & Residential Technical Standards, BMI UK & Ireland; briefly reprises the detail of this important standard.

The publication of BS 8612 was a response to an increasing number of dry-fix systems failures, particularly dry verge. The introduction of BS 5534: 2014 led to an increase in the move from traditional pitched roof wet-fix to dry-fix systems for ridge, hip and verges; with many benefits to both the finished roof and quicker installation time for the contractor.

Yet, the lack of standardised product performance allowed products of poor quality to enter the market which, coupled with the supply in many cases of inadequate installation instructions, has led to roofs with problems. Problem examples have included ridge and/or hip tiles blown off by the wind; dry verges hanging from roof edges, or unsightly black staining down gable-end walls due to verges not shedding rainwater properly.

The publication of BS 8612 was, therefore, an essential and welcome landmark for the industry. Not only does it ensure that dry-fix design and installation standards are raised; but also that developers, contractors and building owners get the product quality and performance they deserve.

The rollout of this important standard continues, with the NHBC recently making it a requirement for compliance with their influential technical standards from 1st January 2019. Going forward, BS 8612 is likely to be the first of many such dry-fix roofing product standards since there are other dry-fix components on the roof currently not covered by British Standards.

Dr Kevin Ley is Head of Technical Services & Residential Technical Standards, BMI UK & Ireland

BMI UK & Ireland is part of the BMI Group, the largest manufacturer of flat and pitched roofing and waterproofing solutions throughout Europe with a significant presence in parts of Asia and Africa. With 128 production facilities and operations in Europe, parts of Asia and Africa, the company brings more than 165 years of experience and employs over 9500 employees. BMI Group is headquartered in London.

With a UK manufacturing heritage in roofing dating back to 1837, BMI UK & Ireland produces and supplies a comprehensive range of concrete, clay, metal and reconstituted slate tiles, reinforced bitumen membranes, single-ply systems and liquid waterproofing solutions as well as an extensive collection of fittings, components and building membranes. In the UK and Ireland, these are sold under some of the market’s best-known brands including Redland, Icopal, Cambrian, Decra, Rosemary Hydrostop and Wolfin.

There’s something in the air

BS 5534:2014 for slating and tiling now recognises that air permeable underlays can take the place of ventilation in a cold roof, if that underlay meets minimum performance standards. Nick King, part of the training team at Klober, explains.

The update to the standard published last February, now states minimum air permeability figures included for underlays, allowing them to be used without any other ventilation in a cold roof. This is potentially good news: pick the right underlay, and there’s no need to spend time and money on additional ventilation at the ridge.

Breathable underlays had been historically problematic, with new homes being found to have roof space condensation. The problem was understanding the definition of ‘breathable’ – just because an underlay was permeable to water vapour; it doesn’t mean it is air permeable too.

In 2011, the NHBC added a clause that said that high-level roof void ventilation equivalent to a 5mm continuous slot at or near the ridge should be used when installing vapour permeable underlays. In 2012 they went further, saying underlay certified as ‘both vapour and air open’ could be used without ventilation, providing manufacturers could prove it enabled ventilation levels equivalent to the 5mm gap.

2018’s amendment to BS 5534 set down a specific value for air permeability: a low resistance to water vapour underlay must have a water vapour resistance not more than 0.25 MNs/g, combined with an air permeability of not less than 20m³/m²h at 50 Pa. Happily, Klober’s Permo air meets this definition. It has a water vapour resistance of 0.045MNs/g, and when it comes to air resistance, tests at the BRE delivered results of 63m³/m² hr at 50 pascals.

All in all, the choice of a vapour and air permeable underlay will often be the right choice for the specifier. As well as time and materials saved, it means a house can be watertight sooner, with all the benefits to a project this brings.

Nick King is part of the training team at Klober

"Breathable underlays had been historically problematic, with new homes being found to have roof space condensation. The problem was understanding the definition of ‘breathable’ – just because an underlay was permeable to water vapour; it doesn’t mean it is air permeable too."

References: BS 5534:2014 + A2:2018 Slating and tiling for pitched roofs and vertical cladding. Code of practice.

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