NHBC will accept traditional steel LRS (30mm x 5.0mm thick x 1200mm long) fixed to the roof construction by one of the following optional methods: The test results indicated that eight 25mm x 4mm steel screws evenly distributed along the length of a standard 30mm x 5mm steel strap x 1200mm long would adequately resist the test tensile force when fixed to 100mm x 25mm timber longitudinal bracing members. Tension straps of 30mm x 5mm are accepted by NHBC Standards Chapter 7.2 and building regulations guidance for provision of lateral restraint on 3-storey residential construction. Tests were undertaken on a range of fabrications using different quantities of 4mm screw fixings in order to determine a suitable arrangement to resist the test tensile force applied to the samples.Ī tensile capacity of 8.0kN was used as the target for the test samples, as this value is given in the UK National Annex to BS EN 845-1:2013 as the minimum declared tensile load capacity for tension straps that provide an equivalent performance to the prescriptive 30mm x 5mm lateral restraint straps. Trussed rafter roof bracing systems are designed to provide adequate stability to the entire roof structure and it is recognised that, providing the straps are located at suitable positions and centres to accommodate the requirements for wall restraint, the longitudinal bracing members, generally comprising 25mm x100mm sawn timber, together with their fixings to the trussed rafters should be adequate to transfer the forces from the restraint straps to the roof construction.Ī programme of testing was undertaken on sample assemblies comprising standard 30mm x 5.0mm thick x 1200mm long steel restraint straps fixed to lengths of 25mm x 100mm sawn timber members, as supplied for bracing timber to trussed rafter roofs. In 2015 therefore, NHBC initiated an investigation into the capacity of longitudinal roof bracing and in particular the fixings between the LRS and the bracing members, to resist horizontal forces at roof level. Frequently, builders fix LRS to roof longitudinal bracing timbers rather than to noggings fitted tight between the trussed rafters at rafter and ceiling chord levels, as illustrated in the foregoing guidance documents.ĭespite the emphasis placed on the importance of correctly fixing restraint straps in the Standards Extra 43 article, the results of the NHBC survey revealed that the use of the longitudinal bracing system persists particularly in England and Wales.Īlthough the practice does not necessarily result in any detrimental effects to restrained walls, it is difficult to prove the adequacy of the detail by calculation in compliance with British Standards. This article highlights situations where it might be considered acceptable to fix LRS in this way and describes proposed amendments to NHBC Standards 2017 edition to accommodate an alternative fixing arrangement.Įvidence from an NHBC internal survey in August 2012 indicated that LRS, used for tying walls to trussed rafter roofs, are not always fixed in the manner recommended in Building Regulations, NHBC Standards, Standards Extra 43 article on ‘Gable walls – the importance of restraint’ (December 2008) or other authoritative documents.

Rafter straps