Reinforced Autoclaved Aerated Concrete (RAAC) is a lightweight form of precast concrete that was frequently used in public sector buildings in the UK from the mid-1960s to the 1990s. It was invented in Sweden in the 1930s and is mainly found in roofs, occasionally in floors and walls. Visually, RAAC planks may look the same as pre-cast concrete, and may be hidden above false ceilings. RAAC is less durable than traditional concrete and has a far lower structural loading capacity than other generic reinforced concrete products. Its condition deteriorates further if water is present, due to leaks from roofs, which can compromise the reinforcement bars contained within RAAC planks. The lifespan of such planks has been estimated to be around 30 years. RAAC is considered a safety risk because it is beyond its lifespan and may "collapse with little or no notice".

RAAC was used in roof, floor, cladding, and wall construction due to its lighter weight and lower cost compared to traditional concrete, and has good fire resistance properties. It does not require plastering to achieve good fire resistance, and fire does not cause spalls. RAAC was used in construction in Europe, in buildings constructed after the mid-1950s. RAAC elements have also been used in Japan as walling units owing to their good behavior in seismic conditions.

RAAC is still present in the UK in public buildings such as schools and hospitals. The material is considered a safety risk, and there have been reports of roofing collapses in schools made from RAAC. The Welsh Government has been working with the UK Government and other devolved governments since 2018 to manage buildings with RAAC. The Department for Education (DfE) has requested all of the country’s 20,000+ schools to check if they contain RAAC, but some still have not. The LGA is advising its members to check as a matter of urgency whether any buildings in their estates have roofs, floors, cladding, or walls made of RAAC.