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CROSS Safety Report

Collapse of folded plate timber roof at a school

Report ID: 1160 Published: 18 October 2023 Region: CROSS-UK


Overview

This report is about a critical safety issue concerning folded timber roofs, in various settings, including over school halls.

Key Learning Outcomes

For owners and persons responsible for the safety of buildings including schools:

  • Inspect and assess existing buildings, particularly those that are of a significant age, to see if they contain unusual forms of construction, including roofs similar to the reported failure
  • If so, or if there is doubt, arrange for structural inspections and risk assessments to be undertaken by engineers who are suitably qualified and experienced persons (SQEP) – normally Chartered Structural or Chartered Civil Engineers

For inspecting engineers:

  • Undertake a risk assessment of old and unusual structures where there is a life safety risk should they fail
  • Consider what combination of causes could lead to a structural failure
  • Understand where structural elements may be beyond their reasonable service life
  • Look out for signs of distress, including those in hidden components or locations
  • Be aware of the risks associated with moisture build-up, particularly where timber is a structurally significant component

 

Full Report

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Our secure and confidential safety reporting system gives professionals the opportunity to share their experiences to help others. If you would like to know more, please visit the reporting to CROSS-US page.

This report is about a critical safety issue concerning folded timber roofs, in various settings, including over school halls.

In 2011, the reporter told CROSS (known at that point in time as SCOSS) of a sudden failure of a proprietary timber roof system over a school hall that had been constructed in 1959. In response to the failure, SCOSS issued Report 273 - Collapse of proprietary timber roof. The reporter believes that local authorities shared that information to help identify similar roofs. However, the reporter, who is a Chartered Structural Engineer, has had another enquiry concerning a roof suspected to be of the same type of construction. In researching this enquiry, the reporter has come across a news report of a collapse of another school roof in England in 2019 which appears to be of the same type of construction as that which collapsed in 2011. The construction of the roof appears to be similar to that shown in Figures 1 and 2 below. The age of the roof is not known to the reporter.

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Figure 1: sketch of roof system
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Figure 2: side view at bearing

The reporter considers that if the cases are similar, the end blocks of the roof, which were likely just held with glue and panel pins, may have become detached, as they did in the previous case, resulting in a sudden shear failure and the collapse of the roof.

If this second case does indeed concern the same proprietary system, continues the reporter, then it has been involved in two sudden collapses, either of which would have had catastrophic consequences if the buildings had been occupied at the time. The reporter considers that a significant number of these roofs, which may not have had failsafe remedial work undertaken, could exist, and users of such buildings may be at significant risk. The reporter is of the opinion that the failure of these roofs is almost inevitable as they age, and that a reminder concerning these structures would be very timely.

two sudden collapses, either of which would have had catastrophic consequences, had the buildings been occupied at the time of collapse

The reporter also makes the point that a robust system for local authority surveyors to share safety information concerning their buildings is essential, given that such system builds were widely adopted and may, by now, be showing their age. However, given the erosion of in-house local authority services, the reporter is not sure if any such reporting mechanisms that existed may have atrophied.

Expert Panel Comments

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An Expert Panel 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-US Expert Panel page.

This roof collapse was reported as a sudden failure. Such types of failures should be guarded against as they often come with no warning. Very fortunately, the failure happened during a school holiday otherwise the outcome, as the reporter says, could have been catastrophic. The reporter is right that this incident should have been shared widely. This would especially be the case if the roof is a proprietary system where generic weakness may exist.

The cause of the collapse was not clear, however any structure that relies upon glue and panel pins is unlikely to be robust if water damage occurs at critical connections. It may be that a lack of maintenance permitted water penetration which impacted critical connections and precipitated the collapse. Any roof with suspected water penetration or water shedding problems should be inspected and repaired as a priority, as water degradation can cause structural damage and failures. Water and moisture generally are contributing factors to much deterioration and failure of buildings. Good detailing, construction, and maintenance of weatherproofing systems are essential.

Any roof with suspected water penetration or water shedding problems should be inspected and repaired as a priority

Deterioration can contribute to the collapse of structures. An ice rink roof collapsed onto skaters in Bad Reichenhall, Bavaria, Germany, in 2006, killing fifteen people. Investigations found no single cause for the collapse, but rather a series of contributing defects and damage. The design capacity of the failed elements was found to be inadequate. This already inadequate capacity was then further reduced over time due to deterioration in the timber box girders. The structure was about 34 years old at collapse.

CROSS published Report 1227 - Collapse of unusual hybrid concrete and steel strand truss on school roof in May 2023. This new failure in North West England, and the failure of the unusual hybrid truss, have remarkable similarities in that both are unusual forms of roofing structure, both were used in school roofs, and both roofs were of significant age.

For brevity, the findings of the unusual hybrid truss report, as well as CROSS Report 273 - Collapse of proprietary timber roof (concerning the collapse in 2011 mentioned by the reporter) are not repeated here. However, readers are advised to read both as the issues and concerns are wholly related to this latest report.

This report markedly reinforces the importance of robust and timely inspection and maintenance strategies as outlined in both earlier reports.

This report markedly reinforces the importance of robust and timely inspection and maintenance strategies

The reporter is concerned that previous efforts to identify similar plywood folded box timber roofs may not have been completely successful. This failure in North West England appears to support that concern, and responsible bodies of buildings potentially containing such roof structures, are urged to take notice of this latest failure.

responsible bodies of buildings potentially containing such roof structures, are urged to take notice of this latest failure

The reporter also makes the point that a robust system for local authority staff to share safety information concerning buildings is essential, but the reporter is not sure if any such reporting mechanisms that existed may have atrophied.

This concern emphasises the importance of the voluntary reporting system provided through CROSS and if any readers have experience of such roof systems then will they please submit a CROSS report.

In addition, CROSS understands that the Department for Education, seeks to make bodies responsible for education facilities in England aware of building issues of concern. Similar arrangements could be in place across other devolved administrations.

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