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CROSS Safety Report

Add-on timber trussed roof to flat roof causes failure

Report ID: 742 Published: 1 October 2018 Region: CROSS-UK

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A reporter discusses how, while visiting a property for an unrelated matter, they spotted something odd about the roof; the gable had bulged at one end and movement had clearly occurred.

Key Learning Outcomes

For construction professionals:

  • Consider appointing a competent single entity (or Chartered Engineer) to oversee the stability of structures particularly where more than one party is involved in the supply chain or the provision of stability

For civil and structural design engineers:

  • Where add-ons such as a timber trussed roof to a flat roof are required be aware of the load paths and whether diaphragm action to the bottom chord is achievable

Full Report

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The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.


The building was a flat roof modular system and the Operations and Maintenance (O&M) manual explained that the modules arrived with a waterproof roof, and that trusses were added above.

There was no access, so a hole was made for inspection. Inside the reporter found the bracing to the trusses fixed with a single nail and there was discontinuity in the bracing.

The main issue however was that as the truss did not support the ceiling (as the module came with a roof), there was no diaphragm under the trusses and all the force in the bracing went to the gable end through the final brace.

Figure 1 shows there is a nail pull out failure on the penultimate truss and it appears the brace was not even connected to the wall/sole plate. Following the inspection, bracing was added along with noggins at the areas of highest deformation and additional fixings were made. The reporter says that designers need to be careful putting trusses over an existing flat roof.

Figure 1: bracing deficiencies in trussed timber roof

Expert Panel Comments

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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

This appears to be a case where no one was in charge of the overall stability system or had any clear vision of what the load paths were intended to be. Having this vision, and assuring it is realised in practice, is always vital but is acutely so when there is more than one party involved in the supply chain or the provision of stability, as appears to be the case here.

The reporter makes a very good point and one to be aware of when using trussed rafters with no diaphragm arrangement to the bottom chord. How many engineers would have considered this aspect during design?

This appears to be a case where no one was in charge of the overall stability system or had any clear vision of what the load paths were intended to be

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