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CROSS Safety Report

Very slender props

Report ID: 34 Published: 1 July 2007 Region: CROSS-UK

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A reporter noticed prop that did not have ties installed while carrying out a site inspection.

Key Learning Outcomes

For construction professionals:

  • It is good practice to carry out a risk assessment and method statement (RAMS) for all construction activities. This can ensure the sequencing of work activities are properly considered and planned.

For civil and structural design engineers:

  • Speak to the contractor about your proposed construction methodology, highlighting any particular areas of risk or uncertainty that they should be aware of

Full Report

Find out more about the Full Report

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.


An unusual house was designed by a firm of avant guarde architects and its form required some interesting structural engineering solutions. Most of the structure was timber framed but part of the roof was held up by steel beams supported by two very slender (50x50mm) rectangular hollow section (RHS) posts which carried on down through a third storey loft and then through two floor levels to a concrete foundation – a total height of about 10m.

The contractor erected the timber frame around the edge of the building and the internal steelwork. Floor and roof joists were put in place and preparations were made for cladding the roof with tiles. At this stage an engineer saw that the props were continuous through the two floors with no ties or bracing and hence had unacceptably high slenderness ratios. 

The consulting engineer’s drawings showed ties, but their importance had not been appreciated by the builder who intended to install them as the floors were being finished. Once the contractor’s attention was drawn to the importance of the ties they were installed. Had this not been done before the roof tiles were added the additional loads on the props could have caused them to fail.

The consulting engineer’s drawings showed ties, but their importance had not been appreciated by the builder who intended to install them as the floors were being finished

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.

Unusual structures, whatever their size, need vigilance and frequent site inspections can be very important. These should be by the designer and by the local authority or approved inspector. A review of the drawings to together with local knowledge about the capability of the builder should form the basis of an informal risk assessment.

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