Flat roofs and Reinforced Autoclaved Aerated Concrete: keep calm and carry out an inspection

When a classroom roof made of ‘crumbly concrete’, better known as reinforced autoclaved aerated concrete, collapsed in 2018, fortunately, no one was hurt, as it happened at the weekend. But the risk of similar collapses was such that the Government issued a structural alert to check all flat roofs in public buildings.

There is no deadline for carrying out inspections but, while the implications of finding RAAC in your estate are concerning, Richard Love and Jack Colgate explain why assessing the presence and condition of reinforced autoclaved aerated concrete in flat roofs might not necessarily be opening pandora's box on replacement costs if found. 

What is RAAC, and why is it a problem?

Reinforced Autoclaved Aerated Concrete (RAAC), to give its full product name, was developed in the 1920s and used after the Second World War, primarily in public buildings as pre-cast panels for flat roofs. It was lighter and cheaper than traditional reinforced concrete.

When issues with its durability came to light in the 1980s, the use of reinforced autoclaved aerated concrete stopped. Interest was reignited when the school roof collapsed, and pressure to address it mounted. 

While there is no deadline for assessing public sector estates for reinforced autoclaved aerated concrete, it’s not unfeasible that one could be introduced. And it is important to mitigate the risk of failure and the potential damage that would cause. 

“Prevention is better than cure,” says Richard Love, National Head of Building Consultancy: “Addressing it before you're forced to gives you a chance to be in control of the timescales and any costs.” And carrying out assessments might not necessarily result in an expensive problem to solve. 

Data analysis and using satellite imagery can quickly rule out a lot of buildings by looking at when they were built and if there is a flat roof. But even if a building was constructed when RAAC was being used, it doesn’t necessarily mean it is present; it wasn’t the only material used for flat roofs. A process of inspections, and a RAAC survey, can narrow down the list further.

For example, Hertfordshire County Council commissioned Carter Jonas to assess the presence of RAAC across its education portfolio. After narrowing down the list of properties based on the date of construction and building design, inspections were carried out on 1,270 buildings and three were found to have RAAC panels. After further investigations, these slabs were found not to be at immediate risk of failure and a programme of maintenance and inspection was created to mitigate risks. 

These findings however are not reflected with other public sector organisations. In the case of West Lothian Council, the Whitburn Community Centre had to close parts of the building for six months, after structural surveys assessing the condition of the RAAC roof, found that the roof was at risk of failure. The Council estimates that remedial works will cost over £1 million.

The Lanthorn Centre has had to be closed for two years, when cracking was found around the roof supports. Not only community centres in West Lothian Council’s property portfolio have been affected, with a primary school also having to undergo remedial works during the summer break after RAAC was found within the teaching block roofs with the replacement works estimated to cost around £1.3 million.

West Suffolk Council and other public sector bodies in Eastern England have also experienced disruption to essential services with 27 roof supports having to be put into place at West Suffolk Hospital with 200 structural supports being put into place at Queen Elizabeth Hospital in Kings Lynn, causing disruption to operations and costing tens of millions of pounds to rectify. Despite this, but it’s not necessarily a reason for concern.

“Because of the nature of the types of buildings it was used in, many became obsolete and have been demolished. So, there's only a little bit comparatively small proportion left,” says Love.

What should you do if you find Reinforced Autoclaved Aerated Concrete?

The risk with reinforced autoclaved aerated concrete is that it doesn’t show much sign of failure before it goes wrong, so it’s important to act sooner rather than later. However, it doesn’t automatically mean a roof will need replacing. 

Jack Colgate, Building Surveyor, says: “We found reinforced autoclaved aerated concrete in a variety of places, such as above a classrooms, staff areas and assembly halls. If it’s in good condition, it just needs monitoring rather than replacing at this stage.”

Keeping up with maintenance is important. Colgate explains: “Make sure coverings are kept in good shape, reducing the chance of water ingress and remove any dead loads on flat roofs, such as chippings.”

Remedial work may be needed in the form of temporary propping or additional strengthening, but wholesale replacement may be required in certain circumstances. 

Love says: “I think people panic about deleterious materials because they think they're a problem, but it’s just something you need to know about and plan maintenance around as part of an estate strategy.” 

There are different ways of addressing reinforced autoclaved aerated concrete, but the first step is to know if it is there.

For further information on this subject, please get in touch with one of our experts: 
Richard Love
Partner, National Head of Building Consultancy
T: 07780 667010
richard.love@carterjonas.co.uk 

Jack Colgate
Graduate Surveyor
T: 07734 192411
jack.colgate@carterjonas.co.uk