In the second of a two-part feature, Dr Kevin Ley, Technical Manager of Redland, looks how the requirements for tile fixings and underlays changed with the revisions to the British Standard Code of Practice for Slating and Tiling (BS 5534).
Despite the Code of Practice for Slating and Tiling in the UK – aka BS 5534:2014 – having come into full force in March 2015; many new roofs are being installed that do not conform either wilfully or through ignorance. Of the three main revisions to the Code, this month we will consider tile fixings and underlays.
When the wind blows over a roof, uplift forces act on both the underlay and the tiles. The roof system has to be designed to withstand these forces for the design life of the building, typically 50 years.
The revised code of practice introduces new design wind load calculations, resulting in greatly increased design wind loads, which means a greater requirement for tile clips in manufacturers’ fixing specifications. In anticipation of this Redland launched its innovative, award-winning tile clip, Innofix which is simply the quickest tile clip to use on the market today eliminating the need for nails and a hammer.
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!
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 subject to wind pressure.
If an underlay does stretch, or ‘balloon’, so that it touches the back of the roof tiles, then it ceases to perform as the wind load taken by the underlay is transferred onto the roof tiles. The tile manufacturers’ fixing specifications do not allow for this additional load and roof tiles can subsequently be blown off the roof during a storm even if they are fixed in accordance with the fixing specification.
A related problem concerns the bursting open of unsealed underlay laps when subject to wind pressure, which can also cause the removal of roof tiles. Both the old and new code of practice recommend that a timber batten be installed over the horizontal lap between courses of underlay to prevent this happening. In practice, this can be achieved either by increasing the horizontal lap of the underlay to coincide with an existing tiling batten or installing an additional batten also known as a ‘fly’ batten over the horizontal lap.
Many installers do not like installing these additional battens as they can cause a trip hazard when working on the roof so the preferred alternative is to increase the underlay lap. This has the disadvantage of increasing the amount of underlay required as well as requiring more care during setting out of the roof.
The 2014 version of BS 5534 introduces a new single test that measures the ability of an underlay to resist stretching when exposed to wind pressure.
These test results are then used to establish which types of underlay are suitable for the various regions around the UK. Before fitting, it would be wise to ensure that any preferred underlay has the necessary checks and testing in place to ensure it is fit for use for the project in question.
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