RAAC: what property owners need to know

What is RAAC and should you be worried?

RAAC (reinforced autoclaved aerated concrete) is a lightweight concrete made into precast panels, mostly used for flat roofs and sometimes for floors and walls. It was used in UK construction from the 1950s to the mid-1990s. Most traditional London houses do not contain it, but it can turn up in blocks of flats, flat-roofed extensions, garages and the commercial parts of mixed-use buildings from that period. If you think a building may contain it, the right next step is an inspection by a structural engineer.

The concern is that RAAC is much weaker and less durable than ordinary concrete, and that panels in poor condition can fail with little warning. Knowing whether it is present, and what condition it is in, lets owners act proportionately rather than guess. In many cases the answer is simply that a building does not contain RAAC, and a short written confirmation is all that is needed.

Why RAAC is a structural concern

RAAC is made by adding a gas-forming agent to a fine cement mix, which fills it with small bubbles, and then curing it under high-pressure steam. The result is light and a good insulator, but it contains none of the coarse aggregate that gives normal concrete its strength and stiffness. The main issues are:

Many panels have now been in place for longer than their designers would have expected, and loads may have increased since installation. Added roof coverings, insulation, plant and solar panels all put more weight on panels that had little margin to start with.

• Lower strength and stiffness, so panels deflect more under load and continue to sag over time
• High porosity, so water from roof leaks soaks in readily
• Reinforcement that can corrode once wet, because the material offers it little protection
• Short bearings at panel ends in some installations, leaving little margin if panels move or crack
• Failure that can occur with limited visible warning

Where RAAC might be found in London properties

RAAC is best known from schools, hospitals and other public buildings, but it was also used in residential and commercial buildings. In the kinds of property we work on, the places to look are:

Do not confuse RAAC with the aerated concrete blocks used in the inner leaf of many cavity walls. Those blocks are made in a similar way, but they are small unreinforced units laid in mortar, and they are not the subject of the RAAC concern. Traditional Victorian and Edwardian houses with timber floors and roofs are very unlikely to contain RAAC at all, unless a later flat-roofed extension was added during the relevant period.

• Flat roofs of low-rise blocks of flats built between the 1950s and the 1990s
• Flat-roofed extensions, garages and outbuildings from the same era
• Commercial ground floors and rear additions in mixed-use buildings
• Former institutional or office buildings later converted into flats
• Occasionally floor or wall panels in system-built buildings

Signs that a roof or floor may be RAAC

From below, RAAC typically appears as a series of long planks spanning between beams or walls, with regular joints between them. Clues include:

Many RAAC soffits are hidden behind suspended ceilings or plasterboard, so the panels may need to be viewed through a ceiling void or a small opening. The building's age, any original drawings and previous survey reports all help narrow down the possibilities before anything is opened up.

• A smooth, light grey or white surface, often painted, with a bubbly texture where it is exposed
• V-shaped grooves or chamfered joints repeating at regular intervals across the ceiling
• No visible stones or coarse aggregate in a broken edge
• Noticeable sagging of the ceiling or roof
• Cracks across or along panels, particularly near the supports
• Water staining, rust marks or spalling that exposes reinforcement

What a RAAC assessment involves

An assessment has two aims: to confirm whether RAAC is present, and if it is, to judge its condition and the risk it poses. IStructE has published guidance on investigating and risk-assessing RAAC panels, and assessments generally follow that approach. The typical stages are:

Our RAAC identification and assessment follows this pattern and starts from £600 + VAT. All our structural inspection reports are signed off by a chartered engineer who is a Member of the Institution of Civil Engineers (MICE).

• Review of drawings, building records and previous reports
• Inspection of roofs, floors and walls, with access through ceiling voids where needed
• Identification of panels and a check for cracking, sagging, water ingress and corrosion
• Measurement of deflection and, where visible, the length of bearing at each support
• A risk rating for each area with recommended actions and timescales

If RAAC is found: what happens next

Finding RAAC does not automatically mean the roof must come off. Panels that are dry, in good condition and free of significant defects may be managed by periodic inspection and by keeping the roof watertight. Panels in poorer condition may need additional support, and those in the worst condition need urgent action, usually restricting use beneath and installing temporary propping until a permanent solution is in place.

Permanent options include fitting new supporting beams or bearings beneath the panels, or replacing the roof or floor with a new structure. The right choice depends on condition, how the space is used and the owner's longer-term plans. Our remedial works design and specification service turns the chosen option into drawings and a specification a contractor can price.

For freeholders, managing agents and buyers

If you own or manage a low-rise block of flats or a mixed-use building from the relevant era, lenders, insurers and buyers may ask whether RAAC is present. A documented inspection answers the question either way. RAAC can be checked as part of a wider structural safety review of a block of flats or mixed-use building up to 15 m and five storeys.

Leaseholders who notice sagging, cracking or water staining in a flat-roofed area should report it to the freeholder or managing agent promptly. In the meantime, keep loads off the roof and make sure any leaks are dealt with, because water is what drives most RAAC deterioration.