RAAC Concrete in Homes, What It Is, Where It Was Used and What to Do

What is RAAC?

Reinforced Autoclaved Aerated Concrete (RAAC) is a lightweight precast concrete product manufactured by introducing air bubbles into a cement-sand-lime mix, which is then reinforced with steel bars or mesh and cured under steam pressure (autoclaved). The result is a cellular, low-density panel that is significantly lighter than conventional concrete but also considerably weaker.

RAAC was manufactured in the UK primarily between the 1950s and the mid-1990s. It was used extensively in public-sector construction, schools, hospitals, civic buildings, as a rapid, economical building material in the post-war period. In the residential sector, its use was more limited but not negligible.

Where RAAC Was Used in Housing

RAAC panels were used in domestic construction primarily as:

• Flat roof planks, the most common residential application; RAAC planks spanning between wall plates or beams to form flat or shallow-pitched roof decks
• Upper floor panels, in some low-rise residential and housing association blocks from the 1960s–1970s
• Wall panels, in a smaller number of system-built housing schemes, including some local authority housing programmes

RAAC was not used in cavity wall or timber frame construction. Properties most likely to contain RAAC include local authority–built homes, housing association stock, and post-war bungalows and maisonettes with flat or very shallow-pitched roofs.

Why RAAC Is a Structural Concern

RAAC has a design life that is typically assessed at 30 years. Many RAAC structures in the UK are now well beyond that threshold. The specific failure mechanisms are:

• Corrosion of reinforcement, the aerated cellular structure is highly permeable to moisture. Water ingress causes reinforcement steel to rust and expand, cracking the surrounding concrete (spalling)
• Deflection over time, RAAC has relatively low structural stiffness. Panels can creep (deflect progressively) under sustained load, particularly when reinforcement is compromised
• Sudden collapse risk, unlike conventional concrete, which typically shows visible signs of distress before failure, degraded RAAC can fail with little warning, as demonstrated by multiple school roof collapses that prompted the Department for Education's 2023 national survey

The BRE (Building Research Establishment) published a series of information papers on RAAC (including IP 11/96 and subsequent updates) and has recommended that any RAAC installation be formally assessed for its current structural condition.

Identifying RAAC in a Property

RAAC panels are distinguishable from conventional concrete by:

• A distinctive bubbly, porous cross-section visible at exposed edges
• A hollow sound when tapped (compared to a dull thud from dense concrete)
• Lower weight than conventional concrete panels of the same size
• Typically grey-white colouration

Identification should be carried out by a structural engineer with specific RAAC experience. A standard RICS Level 2 Survey will not typically identify RAAC unless the surveyor specifically investigates accessible roof or floor voids.

What Owners and Buyers Should Do

1. Establish whether RAAC is present, if the property has a flat or shallow-pitched roof and was built between 1950 and 1995, commission a structural engineer to inspect accessible voids

2. Commission a structural assessment, if RAAC is confirmed, a structural engineer should assess its current condition against current loading requirements

3. Check your mortgage lender's position, several mainstream lenders treat confirmed RAAC as non-standard construction and will not lend until the RAAC has been replaced or a structural engineer has confirmed it is in a satisfactory and safe condition

4. Budget for replacement, where RAAC flat roof planks are assessed as degraded, full replacement is typically the recommended remedy; repair is generally not considered adequate

Property Passport UK records construction year data which can help owners and buyers quickly assess whether a property falls within the period when RAAC was in use.