CROSS Safety Report
Roof collapse at primary school
This report is over 2 years old
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.
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.
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Feedback
Regarding this report which I have just read in the latest copy of Building engineer magazine, it states that in current codes for flat roofs with access for maintenance only, should be designed for an imposed load of 0.6kN/m2. This is correct, (unless taken as a small roof for which there are different criteria), and then goes on to say that it is "considered that the dead load from both the existing and new coverings were within this allowance". This is incorrect as the roof construction loads are considered as construction, or dead loads, and therefore should be checked independently. The imposed loading allowance is for, it is generally accepted, snow or maintenance assuming spreader boards are used. If the weight of the new roof covering was taken as part of the imposed loadings, then this would mean that the overall allowable imposed loading was reduced and possibly that the roof could not accept its designed loads and was therefore liable to be overstressed. The information within the report is therefore, in my opinion, misleading and may possibly lead to further incidences such as this, if misread or misunderstood.
CROSS Response
The CROSS report has been updated to reflect the views of this feedback.
Expert Panel Comments
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.
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.