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How to construct a roof which can withstand windy conditions

Architect Thomas Wiedmer, Technical Manger at insulation specialist Actis, advises on how to construct compliant roofs in the UK’s windier areas. With the UK experiencing more severe and unexpected weather conditions in recent years, building standards are being adjusted to ensure that new constructions can withstand these extremes.

The radically updated building standard BS5534:2014 slating and tiling for pitched roofs and vertical cladding code of practice, which was introduced this year, addresses every component of the construction of a standard roof to ensure it can withstand wind uplift. The code details the specification of mortar bedding, battens, flashings, structural sheathing fixings and underlays. And it is underlays which have recently become subject to a new labelling system designed to help Builders, Architects and Specifiers minimise the ‘ballooning effect’, which sees the underlay force tiles and slates off the roof in high winds. Underlay is not allowed to balloon more than 35mm.  

The labelling identifies the amount of wind uplift resistance each underlay has in five different geographical weather zones identified across England, Scotland, Wales and Northern Island. It will ensure that builders can easily identify the correct underlay for the region in which they are working. This is particularly relevant for roofing contractors in Scotland, the North of England and ultra windy parts of the UK. The higher the zone number the windier the region. Unsurprisingly the far north of Scotland scores a five. This labelling will be of particular help to builders working in the North of England, Northern Ireland and Scotland as not all underlays have sufficient wind uplift resistance to cope with zones 4 and 5.

Among the insulating breather membrane underlays which are suitable across all five zones is Actis Boost ‘R Hybrid which has recently passed the specialised wind uplift resistance test carried under controlled conditions at the BRE HQ. The Actis Boost ‘R Hybrid wind uplift resistance test saw the product trialled under strict laboratory conditions and it has been certified for general use across the whole of the UK including the windiest zones. Tests concluded that the wind uplift pressure achieved by Boost ‘R Hybrid was way more effective than that required under the revised BS standard – with an average of 3054Pa when used with taped lap alone and 5748Pa with the addition of a 38mm counter batten. The building standard BS5534:2014 slating and tiling for pitched roofs and vertical cladding code of practice  calls for a minimum wind uplift resistance of 1600 Pa for a 345mm batten gauged roof covering, rising to 2350Pa for roofs with no ceiling and a permanent dominant opening.

The new labelling system has been welcomed by the NFRC and will clearly identify in which zones each type of underlay can be used and how it is to be installed. NFRC guidelines inform members that underlay on open rafters should be laid with a nominal 10mm drape and can be fixed with clout nails, not staples. Underlay can be laid taut, providing a counter batten is installed over the underlay (12 – 50mm depending on specification). Laps should be secured by a naturally occurring batten course (additional battens are not recommended) or manufacturer approved glue or staples. For more information, visit  http://www.insulation-actis.com   map zonesImage: * The map shows the zones in which Boost R Hybrid can be used. It has been approved for use in zones 1- 5 while some underlays only achieve zones 1 – 3 approval. © IHS, reproduced with permission from BRE DG 489, 2014 edition

Thomas Wiedmer- Guest Blog

Author: Thomas Wiedmer- Guest Blog

This article was published by Thomas Wiedmer- Guest Blog on 19.10.2015.