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908 Failure of RAAC planks in schools

Report ID: 908

Published: Newsletter 58 - April 2020

Report Overview

Further issues with defective RAAC planks in flat roofs are highlighted in this report.

Report Content

Having read the May 2019 SCOSS Alert on Failure of RAAC Planks, a structural engineer has contacted CROSS to share their experience of working on projects with Reinforced Autoclaved Aerated Concrete planks. In 2017, they were asked to investigate an RAAC roof which had collapsed in a school. Luckily, there was no one in the classroom at the time of the collapse.

According to the reporter, the cause of the collapse was a shear failure due to inadequate bearing following some structural alterations made by the school. The failure was triggered by outfall gutters becoming blocked which allowed ponding of water on the roof to quickly build up during a storm. The reporter carried out a full structural survey of the school and found numerous other signs of progressing defects similar to those highlighted in the SCOSS Alert.

In 2019, the reporter was asked to investigate the partial failure of an RAAC plank at another school. Temporary props were installed to prevent collapse of the RAAC planks. The reporter carried out a full structural survey of the school and again found numerous defects in the planks, which were mainly related to historic roof leaks which caused the reinforcement in the planks to corrode and thus lose bond with the concrete.

The reporter is now frequently encountering RAAC planks in school roofs and their experience suggests that these planks are becoming more defective with time. They have also found that many schools do not even know that their roofs are constructed using RAAC planks and are therefore not aware of the risks.

Comments

In a departure from our usual practice, this report was published in advance of the Newsletter due to the possible urgency of the issues.

It is one of several that was received following the publication of the SCOSS Alert on Failure of RAAC Planks in May 2019. These confirm that there are considerable areas of roofing consisting of RAAC planks in use in public buildings in the UK. It appears that not all of these have been identified, so structural engineers and building professionals need to be aware of the situation and, when possible, check for RAAC on large flat roofs built around the 1960s-80s.

The Local Government Association, the Department of Health and Social Care, and the Department for Education have advised owners to check their premises and make inspections to ensure that they know what they own, and if RAAC is suspected, to have structural assessments made.

It is not surprising that schools do not know the composition of the structures in their buildings. By way of explanation, a description of RAAC is shown below.

An interest group has been set up to monitor the situation and to recommend further research into the extent and nature of the problems. The group will be interested to hear of further experiences and anyone looking for more information should contact structures@structural-safety.org.

 

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View other CROSS reports published in Newsletter 58

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