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Reinforced Autoclaved Aerated Concrete (RAAC) planks in businesses

In 2018, a flat roof constructed using Reinforced Autoclaved Aerated Concrete (RAAC) collapsed without warning in a school. RAAC was commonly used in many buildings as a quick and cheap form of construction between the late 1950s and the 1980s. It was known that it has a life span of just 30 years. These RAAC planks are now well over their life expectancy and should be replaced as soon as possible.

Recently other cases of failed RAAC have made the news, leading to the closure of schools, hospital buildings, theatres and other buildings. It is not beyond possibility that it may be in your building too. It can be found in floors, walls and roofs.

RAAC looks like the inside of an Aero or Wispa chocolate bar, with bubbles throughout, and any water ingress could speed up any collapse or break up of the concrete.

Image source: Standing Committee on Structural Safety

Major warning signs are:

  • Cracking and disruption of the planks near to a support

  • If the roof has been resurfaced since original construction, especially if this has increased the load on the planks

  • If there is ponding in the roof

  • If the roof is leaking or has leaked in the past.

Seek advice of a Chartered Structural Engineer. Check your asbestos surveys, in case there is asbestos in the area. It is commonplace for RAAC and asbestos sheeting to be near each other.

Conduct a risk assessment on the activities around the RAAC.

Be prepared to cordon off the area or building and prevent all access if necessary. Inform your insurers. This is especially important if the RAAC is damaged.

Consider a long term plan to replace a RAAC roof. If the RAAC is black, it may be affected by thermal sensitivity. Ensure all staff know to report any leaks or cracks they spot.

Any sudden changes or audible cracking sounds, increased water ingress or observable deflection, close of the area immediately.

Do not take any risks with RAAC.

A useful safety alert on this, which goes into more detail, was produced by the Standing Committee on Structural Safety, and was published in May 2019. It can be found at


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