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Lightweight concrete roofs

Region: CROSS-UK Published: 29 March 2021

CROSS-UK has worked with the industry and government to raise awareness of and to help address safety issues associated with reinforced autoclaved aerated concrete (RAAC) planks.


What is RAAC?

Autoclaved aerated concrete (AAC) is different from normal dense concrete. It has no coarse aggregate, and is made in factories using fine aggregate, chemicals to create gas bubbles, and heat to cure the compound. It is relatively weak with a low capacity for developing bond with embedded reinforcement. It was used in two main forms of structural elements; lightweight masonry blocks and structural units, including roof planks.

When reinforced (RAAC) to form structural units, the protection of the reinforcement against corrosion is provided by a bituminous or a cement latex coating. This is applied to the reinforcement prior to casting the planks. The reinforcement mesh is then introduced into the formwork and the liquid AAC mix added.

RAAC roof collapse

In late 2018, the Local Government Association (LGA) and the Department for Education (DfE) contacted all school building owners to draw attention to a recent failure involving a flat roof constructed using RAAC planks. There was little warning of the sudden collapse.

Figure 1: RAAC roof collapse at a school in 2018

Raising awareness of the safety issue

Following the RAAC roof collapse at a school in 2018, CROSS-UK worked with the LGA and the DfE to publish a safety alert on reinforced autoclaved aerated concrete (RAAC) planks in May 2019.

The purpose of the alert was to help raise awareness of the safety issue among owners of schools and similar buildings which might have been constructed using RAAC planks, and professionals working on RAAC projects. It also provided guidance on how RAAC planks could be identified, inspected and managed.

The alert was widely disseminated to building owners and built environment professionals.

The consequences of an RAAC roof collapse are unthinkable. CROSS-UK’s support and expert advice has been invaluable in our attempts to raise awareness and provide advice to councils and other duty holders on this critical issue.

Lord Porter, Local Government Association’s building safety spokesperson

Helping professionals to share experiences

Following publication of the alert, a number of professionals working on RAAC projects used CROSS’s secure and confidential safety reporting system to share their experiences with RAAC to help others.

Report 874 was from a reporter who had surveyed a number of buildings with RAAC roof planks, while report 908 was from a structural engineer who had investigated a RAAC roof collapse in a separate school to the 2018 collapse.

CROSS-UK were contacted by numerous professionals who were interested in learning more about RAAC, including how others were managing issues with RAAC planks. There was a strong desire from professionals to learn from each other and share best practice.

As a result, a RAAC Study Group was established under the leadership of the Institution of Structural Engineers (IStructE). The Study Group provided professionals with the opportunity to collaborate with each other, to share best practice, and to investigate ways of ensuring continued safety to occupants of the affected buildings. If you would like to know more about the RAAC Study Group, you can contact

CROSS has allowed those people with experience of RAAC to share their knowledge and expertise

Martin Liddell, Chair of IStructE RAAC Study Group and Director at MLM Group

Working with government

It is believed that RAAC planks are present in many buildings constructed between the 1960-80s. These include some schools and hospitals. CROSS-UK have been working with government departments on the topic. We have engaged with the Department for Education (DfE), the Department of Health and Social Care (DHSC) and the Ministry of Housing, Communities and Local Government (MHCLG).

We provided expert and independent advice on the need for further research into RAAC planks. We also worked with the DfE to publish guidance for how schools can identify and take action if RAAC is found in roofs: reinforced autoclaved aerated concrete in roofing in schools.

We continue to work with government departments on managing the issue, including updating on any industry developments of which we are aware.

DfE has worked closely with CROSS to develop our understanding of the potential issues of RAAC for schools and the wider education estate. We expect that productive relationship to continue, and welcome the expansion of the CROSS remit to cover fire safety.

Crawford Wright, Head of Design at Department for Education

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Your report will make a difference. It will help to create positive change and improve safety.

Our secure and confidential safety reporting system gives professionals the opportunity to share their experiences to help others.