CROSS Safety Report
Collapse of proprietary timber roof
This report is over 2 years old
Overview
A proprietary timber roof system spanning 8m over a school hall collapsed.
The roof was constructed in 1959, and formed from a ply-web beam system, with hardboard ceiling and ply deck and asphalt covering.
Key Learning Outcomes
For owners and operators:
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Ensure regular inspections and maintenance are carried out on flat roofs where ponding is likely to occur, especially after heavy rainfall events
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Regular inspections can help to detect issues which need addressing before they become hazardous
Full Report
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We were called, says a reporter, to investigate the collapse of a roof spanning 8m over a hall which was fortunately unoccupied, and no one was hurt. The roof was constructed in 1959, and formed from a ply-web beam system, with hardboard ceiling and ply deck and asphalt covering. The asphalt had been felted, and then a further layer of insulation and felt applied. The roof had a parapet wall running around it, 100 to 150mm high, with internal rainwater outlets. There was evidence of ponding at one of the outlets and a lot of water was on the roof at the time of the collapse.
Apart from creaking of the roof 15 minutes before the collapse, there was no warning. The roof was a proprietary system, which relied for support on end bearers glued and nailed to end stiffeners in the webs. The bearers sat on narrow ledges. This requires highly accurate construction of the supports; however, the failure mechanism in this case appears to be complete de-bonding of the end stiffeners and ply deck at one end, resulting in sudden shear failure. The end detail was a standard arrangement, as an alternative to sitting the beams directly on the supports.
The system seems to have been used from 1953 or 1954 onwards and continued for at least 5 years (when the hall was built). The system is noted as a patented design in the 1976 version of the timber designers manual so may have been used for considerably longer. The system was used for walls, floors and roofs in a wide variety of buildings. The manufacturer however went out of business some years ago. Spans varied from 15ft to 42ft (4.57m to 12.80m). Decks at particular risk of this type of failure, believes the reporter, (until the end supports are verified) will include floor beams, decks with trimmed openings and roofs up to 9.1m span, (longer decks are less likely to have the end board detail but may still be subject to other types of failure, particularly in warm, humid environments).
The only mechanical fastenings between the stiffeners and the web were small diameter nails, effectively panel pins, 3 or 4 each side, which they believe were only intended to provide gluing pressure for the joint. All such units with an end bearer detail should be regarded as dangerous and there may be hundreds in use. To identify these components a small hole and inspection with a borascope would hopefully be enough as the members are quite distinctive, with their inclined webs, but these of course may be hidden by ceilings. Figures below show the aftermath of the collapse, an exploded view of the form of construction, and a side view drawing of the bearing.
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This report raises a number of issues: water ponding as a result of blocked outlets which cannot be seen from ground level, deterioration of timber (and glued components) arising from wetting conditions through water ingress, and possibly details associated with the bearing such as the bearing length.
At the time of construction such plywood box beams were either structurally glued or structurally nailed. If structurally glued then tacking nails were used to hold the assembly together while the glue cured. After curing the nails were redundant and not considered in the design. In structurally nailed assemblies there was a designed dense nailing pattern along flanges and down stiffeners and 11 gauge galvanised nails were commonly used.
The glues that were typically used are water resistant if applied correctly and plywood web beams were good structural elements provided that manufacture and construction practice was controlled. It may be that the design and construction were satisfactory but that a lack of maintenance allowing rainwater ingress to occur. This could have affected the timber at the glueline, or the failure may have been related to support conditions, or it could have been a combination of the two. Whatever the cause this is a potentially serious issue and possibly there ought to be a requirement in building management to periodically check long span roofs.
As suggested by the reporter the use of a boroscope could be helpful when assessing conditions at bearings. In the section on robustness in Approved Document A there is a clear indication of higher risk when the consequences of failure are multiple injuries. In this case, and that of report 242, there were long span roofs over school halls where the risk of injury could be very high.
There have been previous examples of roof failures in schools which prompted large scale investigations. In 1973 at Camden School the beams made from concrete containing high alumina collapsed and many similar structures were checked and some strengthened. In 1976 the timber roof over the gymnasium at Rock Ferry School collapsed again resulting in widespread checks of such structures. For example see The Structural Engineer volume 78, Issue 1, A Century of Innovation: Structural Engineering 1900 - 2000.