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CROSS Safety Report

Failure of RAAC planks in schools

Report ID: 908 Published: 1 April 2020 Region: CROSS-UK, CROSS-AUS, CROSS-US

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Overview

A reporter was asked to investigate a reinforced autoclaved aerated concrete (RAAC) roof which had collapsed in a school.

They are concerned that many schools are not aware if their roofs are constructed of RAAC planks and are therefore not aware of the risks.

They suggest that these planks are becoming more defective with time.

Key Learning Outcomes

For building managers/owners:

  • If a building has a flat roof, be aware of what it is constructed of

  • If you are unsure of the form of construction, it is advised to carry out an inspection. If RAAC is suspected, a structural assessment should be made.

  • If RAAC planks are present, Section 6 of the Alert on Failure of RAAC Planks provides advice on managing planks

For all those with experience of RAAC planks:

  • Consider confidentiality sharing your experience with CROSS for others to learn from

  • Contact technical@istructe.org if you are interested in joining the Institution of Structural Engineers’ RAAC Study Group

Full Report

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Having read the Standing Committee on Structural Safety (SCOSS) May 2019 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 (RAAC) 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.

Shear failure of RAAC planks

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.

Roof leaks can lead to the deterioration of RAAC planks

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. These 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 are not aware that their roofs are constructed using RAAC planks and are therefore not aware of the risks.

The reporter is now frequently encountering RAAC planks in school roofs and their experience suggests that these planks are becoming more defective with time.

Expert Panel Comments

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In a departure from our usual practice, this report was published in advance of Newsletter 58 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.

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. They have been advised to make inspections to ensure that they know what they own, and if RAAC is suspected, to have structural assessments made.

What is an RAAC plank?

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 in the information box below.

The Institution of Structural Engineers has set up a RAAC Study Group to monitor the situation and to recommend further research into the extent and nature of the problems. The Study Group will be interested to hear of further experiences and anyone looking for more information should contact team@cross-safety.org.

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