Written and reviewed by the Apolodor LTD concrete repair team

Reinforced autoclaved aerated concrete, better known as RAAC, became a national concern in 2023 when schools in England were told to stop using spaces with confirmed RAAC unless safety measures were in place. Since then, building owners in every sector have been asking the same question: do we have RAAC, and what should we do if we do?

This guide explains what RAAC is, where it is usually found, how to recognise it and why it must be handled differently from conventional concrete. It also explains where a concrete repair contractor such as Apolodor LTD can help, and where the decisions must rest with a structural engineer.

Quick answer: RAAC is a lightweight, aerated form of concrete used mainly in roof planks, and sometimes in floors and walls, in UK buildings constructed from the 1950s to the mid-1990s. It can be recognised by its light weight, its uniform bubbly texture with no visible stones, and smooth panels with regular joints when seen from below. Any suspected RAAC should be assessed by a structural engineer, because it cannot be repaired in the same way as dense concrete.

What Is RAAC?

RAAC is made from cement, lime, sand, water and an aerating agent that creates gas bubbles throughout the mix. It is cured under high-pressure steam in autoclaves, producing a lightweight product that was easy to handle and quick to install. Reinforcing steel was included so that it could span between supports as planks or panels.

It is very different from normal reinforced concrete. It has no coarse aggregate, it is much weaker, and its high porosity makes it vulnerable to water. Over time it is prone to deflection, cracking and, in the worst cases, sudden failure.

Where Is RAAC Found?

RAAC was used widely from the 1950s until the mid-1990s, particularly in flat roofs. It is most often found in:

• Schools, colleges and university buildings
• Hospitals and health buildings
• Public and civic buildings
• Offices, factories and commercial buildings
• Some housing, particularly where flat roofs were used

Why Is RAAC a Concern?

• Limited service life. Many RAAC planks have now exceeded the life originally expected of them
• Water sensitivity. Leaking roofs let water into the porous material, which can corrode reinforcement and weaken the planks
• Short bearings. Some planks sit on very narrow supports, so small movements or edge damage can lead to failure
• Reinforcement protection. The steel was usually protected by a coating rather than by dense concrete cover, and its bond with RAAC is weak
• Deflection over time. Long-term sagging can lead to ponding water on roofs, which adds load and accelerates deterioration

How Can You Identify RAAC?

From Below

In ceilings, RAAC planks often appear as smooth, light grey or white panels with regular V-shaped grooves where planks meet. Many have been painted or hidden behind suspended ceilings, so it may be necessary to look above ceiling tiles.

Exposed Material

Where RAAC is exposed, for example at a damaged edge or in a roof void, it has a distinctive bubbly, aerated texture with no visible stones. It is also much lighter than normal concrete and can be scratched relatively easily.

Warning Signs

Signs that RAAC may be in poor condition include:

• Visible sagging or deflection of roof or ceiling panels
• Cracking across the width of planks or along their length
• Water staining, damp or leaks through the roof
• Spalling or crumbling near supports
• Exposed or corroded reinforcement

If you see any of these, restrict access to the area beneath and seek engineering advice promptly. If we identify possible RAAC during an inspection, we flag it to the client immediately and recommend an engineer’s assessment before any work in that area goes ahead.

What Should You Do If You Suspect RAAC?

1. Do not attempt repairs yourself. Patching RAAC with ordinary repair mortars does not address the underlying weakness.
2. Appoint a chartered structural engineer with experience of RAAC. The Institution of Structural Engineers has published detailed guidance on assessing and managing RAAC.
3. Manage water ingress. Keeping the roof watertight is one of the most important ways to slow deterioration while longer-term decisions are made.
4. Follow the engineer’s recommendations, which may include monitoring, temporary propping, strengthening, partial replacement or full replacement.

How Does RAAC Differ From Conventional Concrete Repair?

Conventional concrete repair, as described in our guide to planning a concrete repair project, relies on breaking out damaged concrete and reinstating it with compatible materials. RAAC does not lend itself to this approach because the material itself is the weakness.

Buildings containing RAAC usually contain conventional concrete too, in frames, columns, beams and stairs. Those elements can and should be maintained and repaired in the usual way. A thorough inspection for hidden concrete damage can establish the overall condition of the building alongside the RAAC assessment, and understanding what concrete damage is more broadly helps owners prioritise.

Where Apolodor LTD Fits In

We are clear with clients about what we do and do not do. RAAC assessment and decisions about strengthening or replacement belong with a structural engineer. What we bring is support around that process: inspections that help identify the construction type, and repair of the conventional concrete elements in the same building, such as frames, columns, beams, stairs and external areas.

Many buildings with RAAC also have years of deferred maintenance elsewhere. Dealing with that properly, while the engineer manages the RAAC, keeps the whole building safe and avoids two separate rounds of disruption.

Areas We Serve

We support building owners with concrete repairs in:

• London
• Manchester
• Birmingham
• Liverpool
• Leeds
• Nottingham
• Bristol
• Brighton
• Cardiff
• Plymouth
• Luton
• Reading
• Norwich
• Swindon
• Portsmouth
• Oxford
• Ipswich
• Maidstone
• Cambridge
• Southampton
• Slough
• Warrington
• Sheffield
• Leicester
• Coventry
• Milton Keynes
• Northampton
• Derby
• Stoke-on-Trent
• Wolverhampton
• Hull
• Exeter
• Gloucester
• Sunderland
• York
• Peterborough
• Chelmsford
• Newcastle
• Bradford
• Bournemouth
• Swansea
• Newport
• Middlesbrough
• Preston
• Blackpool
• Bolton
• Stockport
• Huddersfield
• Lincoln
• Worcester
• Colchester
• Canterbury

 

Key Takeaways

• RAAC is a lightweight aerated concrete used mainly from the 1950s to the mid-1990s
• It is found most often in flat roofs of public and commercial buildings
• Sagging, cracking and water staining are key warning signs
• RAAC should always be assessed by a structural engineer
• Keeping RAAC dry is one of the most important interim measures

Frequently Asked Questions

Is all aerated concrete RAAC?

No. Aerated concrete blocks are widely used in walls and are a different product. RAAC refers specifically to reinforced planks and panels.

Can RAAC be strengthened?

In some cases engineers specify strengthening or additional supports, but many owners choose replacement. The right answer depends on the condition of the planks and the building.

Does RAAC only affect schools?

No. Schools received the most attention, but RAAC was used in many other public, commercial and residential buildings.

Need Support?

Apolodor LTD can support engineers and building owners with the repair of conventional concrete elements in buildings where RAAC is present. Call 07808 709670 or contact us here.