SCOSS alert: Failure of RAAC planks

Aerated autoclaved concrete close-up

Following the collapse of a flat roof in a school the Standard Committee on Structural Safety (SCOSS) have issued a safety alert on Reinforced Autoclaved Aerated Concrete (RAAC).

RAAC planks were predominantly used in the 1960-80s and despite being called concrete, you cannot treat them as traditional concrete. Due to the way they were made they were much weaker and have an expected life of around 30 years – so building owners will now need to consider work in replacing them.

Problems with RAAC roof planks have been known since the early 1990s and many have been replaced with alternative structural roofs or by introduction of secondary supports.

There is no central register of buildings with RAAC roof planks and the collapse of a school roof in late 2018 shows they're still out there. Therefore, those appointed to work on these buildings need to know what to look out for...

Warning signs

  • Significant cracking and disruption of the planks near the support
  • Any planks that have deflected more than 1/100 of the span, or a significant number of planks that have deflections approaching this magnitude
  • A number of the planks have very small bearing widths (less than 40mm)
  • The roof has been re-surfaced since original construction - this is particularly an issue if the load has been increased or the re-surfacing has a black finish and the previous surface did not
  • There is significant ponding on the roof
  • The roof is leaking or has leaked in the past

Full details on how to identify, inspect and manage RAAC planks can be found here:

Download the SCOSS alert


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