BUILDING FABRIC & EXTERIORS
Get on top of the issue
Despite British Standards regarding slating and tiling being updated, sadly practice is still lagging behind, resulting in voided warranties and significant roof failures. Dr Kevin Ley of Redland explains
T
he first iteration of BS 5534, the ‘Code of Practice for Slating and Tiling in the UK’ was CP 142,
published in September 1942. It set the standards for recommended best practice for slating and tiling, covering all aspects of pitched roofing above the rafters. It was first codified as a British Standard in 1978 and variously updated since. Most recently revised in August 2014, changes included the introduction of mechanical fixing of all hips and ridge tiles, new wind uplift resistance requirements for underlays, and a considerable increase in the tile fixings particularly for interlocking tiles which are the most widely used. This increase has been brought about by a simultaneous increase in the minimum tile fixings required for tile security and safety along with increased design wind loads, by up to 100 per cent in some cases. Consequently, all single lapped, interlocking tiles on a roof now need to be nailed, clipped or both depending on exposure.
BS 5534:2014 applies to all pitched
roofing, whether new build, re-roofing or repair work, and came into full force in March 2015. Although not legally mandatory, compliance with BS 5534:2014 is considered not only best practice, but also provides the best defence in the event of failures or disputes. In the self- and new build sector, the NHBC is especially vigilant in respect of the standard and, since July 2015, all new projects, regardless of size or phase; must comply with BS 5534:2014 to qualify for its 10-year Buildmark Warranty.
Despite the new code of practice being
three years old, many new roofs are being installed that do not conform to the standard. For those designing, commissioning or installing pitched roofs, there is a need to be aware of the main provisions to ensure specifications are being met.
MECHANICAL FIXING The revised code of practice states that the use of mortar alone can no longer be
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relied on as a method of fixing as it has been deemed to provide no reliable adhesion. Mortar can still be used but only if accompanied by mechanical fixings. This means if mortar is used then additional materials are needed including a ridge/hip batten with fixings to rafters, and mechanical fixings for securing the ridge/hip tiles to the ridge/hip batten. While the above measure secures the ridge/hip tiles it does not eliminate the risk of mortar failure, resulting in roof leakage and subsequent repair work as the mortar can still crack and drop out of the mortar bed. A simple way of avoiding all the hassle associated with mortar is to use the alternative of modern dry-fix ridge/hip solutions instead.
TILE FIXINGS
When the wind blows over a roof, uplift forces act on both the underlay and the tiles. The revised code of practice introduces new design wind load calculations based upon Eurocodes, resulting in greatly increased design wind loads – up to 100 per cent in some cases – which means a greater requirement for tile clips in
manufacturers’ fixing specifications. Since fixing specifications – the location, number and type of nails and clips required – vary so greatly and are based on many factors, including geographical location and type of building, it is even more important that every roof is fixed in accordance with manufacturers’ recommendations, ideally with a site-specific fixing specification. Historic experience should not be relied upon!
UNDERLAYS
The issue of wind uplift resistance of roofing underlays, or resistance to ‘ballooning’ as it is sometimes called, and its consequences for underlays is addressed in the new standard. The primary purpose of a roofing underlay is to reduce the wind load acting on the roof tiles by taking a significant proportion of the wind load itself. To do this successfully, it must not stretch to the point where it can touch the underside of the roof tiles when subjected to wind pressure. If a roofing underlay does stretch, or ‘balloon’, so that it touches the back of
march/april 2018
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