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CROSS Safety Report

Timber roof truss bracing problem

Report ID: 354 Published: 1 July 2014 Region: CROSS-UK

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Overview

Lateral movement of the roof tiles on a two-storey mid-terrace house built approximately 35 years ago with a trussed rafter roof led to the discovery of broken bracing.

Key Learning Outcomes

For the construction team:

  • Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design

For civil and structural design engineers:

  • If possible, attend site to inspect works to ensure they are installed in accordance with the design

  • If you are unable to attend site, ask the contractor for site photos of the installation of critical structural elements

Full Report

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A report has been received regarding a 2-storey mid terrace house built approximately 35 years ago with a trussed rafter roof. Lateral movement of the roof tiles of some 35mm was visible on the front and rear elevations as shown in Figure 1.

Image
Figure 1: distorted tiles on roof

Clearly, says the reporter, the roof structure is inadequately braced (Figure 2) which has allowed the trusses to move laterally and other properties in the terrace are similarly affected. Additional bracing was required. 

Image
Figure 2: snapped timber bracing

Expert Panel Comments

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Even if there are no external forces, there will always be a tendency for a heavy mass to move sideways: the mass will always be eccentric to the vertical supports. This was a common defect in early trussed rafter roofs often caused by slight out of plumb during erection and inadequate provision of diagonal and longitudinal bracing.

BS 5268 Part 3:1985 ‘Structural use of timber. Code of practice for trussed rafter roofs’ provided the necessary specification and guidance. This was promoted by BRE in their Defect Action Sheets including two in December 1987 dealing with remedial measures:

  • 110 (Design) Dual-pitched roofs: trussed rafters – specification of remedial bracing

  • 110 (Site) Dual-pitched roofs: trussed rafters – installation of remedial bracing.

Now that BS 5268 Part 3 has been withdrawn, the guidance can now be found in Annex E of BSI Published Document PD 6693-1:2012.

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