The new test measures the ability of an underlay to resist the wind. This is significant because of the role the underlay plays in maintaining the integrity of the roof structure. When wind blows over a roof, uplift forces act on both the underlay and the tiles. The primary purpose of an underlay is to reduce the force acting on the tiles by taking more of the wind load itself. However, if the underlay stretches too much when subject to wind pressure – or balloons – and touches the back of the tiles it ceases to perform this function.
This can cause the tiles to be blown off the roof in high wind conditions, such as those seen in the UK in the New Year.
The new test standard is expected to be implemented in July 2014 along with the publication of the new revised BS 5534 standard. This would mean that all third party accreditation bodies, such as the British Board of Agrément (BBA) and Building Research Establishment (BRE) will use the same test going forward when certifying underlays for their fitness for purpose.
Wind conditions vary considerably throughout the UK and the draft standard also includes provision for a new labelling system for underlays that will outline the suitability of the product for specific geographic regions.
Assuming that the standard includes these provisions when the final version is published later this year, we believe that the nature of the underlay supply market will be significantly altered. Cheaper, lightweight underlays that stretch excessively are likely to have restricted use and manufacturers will look to develop better quality underlays solution that will be suitable everywhere in the UK without restrictions. This development is likely to be driven by merchants who are unlikely to want to stock several different types of underlay to cater for regional variations.
In the new housing sector we anticipate that the new underlay standards, if implemented, are likely to be enforced by the National House Builders Council (NHBC), which continues to drive up its own standards to address the rising cost of roofing claims against its Buildmark Warranty.
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