RAAC Concrete Explained: How to Identify Reinforced Autoclaved Aerated Concrete

What RAAC is

Reinforced Autoclaved Aerated Concrete (RAAC) is a lightweight precast concrete made by mixing cement with aluminium powder, which produces hydrogen bubbles during curing. The result is a porous, lightweight panel that was used as a structural roofing and flooring product in UK construction between roughly 1950 and 1990.

RAAC is significantly lighter than dense concrete and was popular for flat roofs in schools, hospitals, public buildings, and some commercial premises. It was also used in some social housing and a small number of private dwellings, although domestic use was much less common.

Why it became a public issue

In 2023, the UK government identified that RAAC roof panels in dozens of schools were at risk of sudden structural failure. Several schools were closed or partially closed at short notice. The risk arises because:

• RAAC has a design life of around 30 years, but most installations are now 35 to 70 years old
• The bubble structure absorbs water, which corrodes the embedded steel reinforcement
• Long-term creep and deflection can lead to sudden brittle failure with little warning
• The lightweight structure has limited reserve strength compared to dense concrete

The Institution of Structural Engineers and the government have published guidance on identifying and managing RAAC. Many public buildings have been surveyed; private dwellings have not been systematically surveyed.

Where RAAC is found in homes

RAAC in private homes is rare but not unknown. It was occasionally used in:

• Flat roof panels of post-war estates and prefabricated houses (1950s to 1970s)
• A small number of low-rise blocks of flats
• Garages and outbuildings of homes built between 1950 and 1990

Most pitched-roof family houses do not contain RAAC. The risk is concentrated in flat-roofed properties from the relevant era.

How to identify it

RAAC is hard to identify without a specialist survey because it is usually hidden behind ceilings, screeds, or insulation. Visual indicators that warrant further investigation include:

• A flat roof with no visible joists or trusses on the underside
• Lightweight grey panels visible in a loft space or garage
• Cracking, sagging, or staining on the underside of a flat roof
• A property built between 1950 and 1990 with a flat roof

Definitive identification requires a specialist survey, which usually involves removing a small section of ceiling or roof covering to inspect the panels directly. A specialist RAAC survey typically costs £500 to £1,500 for a small property.

Mortgage and insurance impact

Most lenders will refuse to lend on a property with confirmed RAAC unless a structural engineer's report confirms the panels are safe and have remaining life. Buildings insurance can also be affected, and some insurers may decline cover.

If you are buying a flat roofed property built between 1950 and 1990, a Level 3 RICS Building Survey is essential. The surveyor should specifically address whether RAAC is present.

What to do if you find it

If RAAC is identified in a property you own:

1. Commission a structural engineer's report from a specialist

2. The engineer will assess remaining life and recommend monitoring or replacement

3. Replacement options range from sister-beams and propping (£10,000 to £30,000) to full roof replacement (£20,000 to £60,000+)

4. Notify your insurer and mortgage lender

5. Take any propping or load reduction measures the engineer recommends as an interim safety measure

What to check before buying

For any property built between 1950 and 1990, especially with a flat roof:

1. Check the construction age on Property Passport UK at [/search](/search) (the platform shows the EPC construction age band)

2. Get a Level 3 RICS Building Survey

3. Ask the surveyor to specifically check for RAAC if there is a flat roof or garage roof

4. If there is any suspicion, commission a specialist RAAC survey before exchange

5. Build the cost of any remedial work into your offer

Check the property on Property Passport UK before you buy

Property Passport UK aggregates verified data on every one of the 19.35 million properties in England and Wales, including listed building status, conservation area, EPC, flood risk, and HM Land Registry tenure. Search any address at [/search](/search) before you make an offer or commission a survey. Construction-related risks often flag through one of these data points before they show up in the building survey itself.

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