The RAAC concrete ‘ticking time bomb’ that schools were warned about years ago

In 2017, a committee was asked to investigate the suitability of RAAC after a school roof collapsed

Alexander Butler
Saturday 02 September 2023 11:18 BST
A school roof built with RAAC collapsed in Kent, 2018
A school roof built with RAAC collapsed in Kent, 2018 (NO CREDIT)

Last year, construction experts warned RAAC was a “ticking time bomb” and estimated around “half” of the four million non residential buildings in the UK were affected by the material.

Now, more than 100 schools, nurseries and colleges in England have been told by the government to close classrooms and other buildings that contain aerated concrete that is prone to collapse.

So when was the material first used, when was it first flagged as a danger, and what was done about it?

When was the material first used?

Reinforced autoclaved aerated concrete (RAAC) was first used by municipal architects between the 1950s and 1990s for the construction of offices and schools, as well as other buildings, like hospitals, according to the Local Government Association.

As of 30 August 2023, there were 156 educational buildings built with RAAC, records show. Only 56 of these had “mitigations” in place, meaning engineers had deemed sites were not a risk due to areas being closed down or reinforced with other materials.

When was RAAC flagged as a danger?

The lightweight concrete, which is a “porous” material, has long been recognised as having “limited durability”, according to the LGA. The Government has been aware of public sector buildings constructed with the material since 1994.


The Standing Committee on Structural Safety was asked to investigate the suitability of the material after a school roof collapsed, although it is not clear which school this was.


Another roof collapsed at a Singlewell Primary School in Kent. It happened above the school staff room, also damaging toilets, ICT equipment and an administration area. The collapse prompted Kent Council to write to other local authorities warning them to check for RAAC in their schools.


A structural engineer investigating on behalf of SCOSS began to “frequently” encounter RAAC planks that weren’t fit for purpose and warned all those installed before 1980 should be replaced.

RAAC planks were becoming “more defective with time”, the report found. It said: “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.”

What did the Government do?


The Government published guidance for schools about the need to have adequate contingencies in place for the eventuality that RAAC-affected buildings need to be vacated at short notice, according to the DfE.

But the information for schools applying for grants for construction improvement funding round said that “not all RAAC is dangerous”, according to some reports.


The Department for Education sent a questionnaire to all responsible bodies, asking them to provide information to help us understand the use of RAAC across the school estate.


Now, the Government is “less confident” that buildings constructed with RAAC should remain open without extra safety measures. New guidance advises schools and colleges to close any buildings known to contain RAAC.

The DfE has contacted 104 “settings” where RAAC is confirmed to be used in construction without mitigations in place, and has asked them to vacate these buildings.

‌Around 24 schools will have to fully close their doors and find alternative teaching spaces, it is understood. In total, more than 100 schools have been told to find emergency accommodation until classrooms are made safe.

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