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CROSS Safety Report

Roof collapse at primary school

Report ID: 958 Published: 1 July 2020 Region: CROSS-UK


Overview

Blocked drainage outlets likely lead to the build-up of water on a school roof which collapsed.

Supporting timber joists failed at midspan when subjected to excessive loads from ponding of rainwater.

Key Learning Outcomes

For owners and operators:

  • Ensure regular inspections and maintenance are carried out on flat roofs where ponding is likely to occur, especially after heavy rainfall events
  • Regular inspections can help to detect issues which need addressing before they become hazardous

For civil and structural design engineers:

  • A holistic view of the design should be undertaken to ensure failures like this do not occur
  • If there is a risk of excessive ponding, the ‘leak before break’ safety concept should be considered

Full Report

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This concerns the collapse of a roof at a 1960s school (refer to Figure 1) that was constructed with the following:

  • Loadbearing masonry walls
  • A suspended concrete first floor
  • A flat timber roof which had been re-covered in 2015

The roof comprised of new felt covering over insulation. This was above the old felt and was supported on woodwool boarding. The boarding spanned between proprietary timber ply-box joists at approximately 1.2m centres. There was a suspended plasterboard ceiling supported by a timber framework.

Image
Figure 1: collapsed section of roof at school

Failure of timber joists

The joists to the main roof had fractured at their midspan, damaging the end supports as they did so. At the time, substantial quantities of water were seen pouring out from the ground floor. The section of roof in question was surrounded by a parapet with a hidden gutter at one end.

This had leaves within it and there was debris and vegetation in the gutters and outlets to other roofs to the school. It is likely that the outlet had become blocked and water had gradually filled the roof area without being noticed.

How an increase in deflection can lead to an increase in load

Current codes say that roofs with access for maintenance only, are to be designed for an imposed loading of 0.6 kN/m2. However, if the area above the roof did fill with water, say to a depth of 250mm (the height of the upstand), there would be a sustained uniform loading of 2.5 kN/m2.

Furthermore, under this loading, there would be significant deflection allowing for a further increase in loading should the area continue to fill from rainwater. The fact that it has been reported that so much water was seen passing out from the building suggests that the roof was holding a significant volume of water.

Elsewhere, there is only a small upstand surrounding the roofs and there are openings at outlet positions allowing water to overflow should there be a blockage of the outlet. The same scenario is therefore, unlikely to be able to occur on the other roofs to the school.

Expert Panel Comments

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There have been a number of historical major collapses especially on timber roofs caused by drainage blocking / ponding and potential rot. An example is the Bad Reichenhall ice rink collapse in Germany in 2006.

The hazard of drainage failure is common. Standard building practice, as in routine domestic plumbing, ought to be deployed to highlight overflow before ponding or undetected leakage can occur.

Safety concept to prevent excessive ponding

The safety concept which has wide application is known as ‘leak before break’. As in report 926, all structures are prone to degradation. Proper management requires routine inspection to detect the onset of damage before this becomes hazardous.

A holistic approach to design reduces risks

This report demonstrates that when designing for structural safety, a holistic view of the design should be taken. It may be that there was a maintenance issue with blocked outlets, but this possibility should have been considered and suitable overflows provided.

It seems to be yet another symptom of divided responsibilities on projects, with no overall view of the impacts of the building design on the structural response.

It seems to be yet another symptom of divided responsibilities on projects, with no overall view of the impacts of the building design on the structural response.

A similar report was published in CROSS-US Newsletter 1 in March 2020; Failure to maintain roof drainage during re-roofing leads to ponding instability collapse.

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