RAAC, or ‘reinforced autoclaved aerated concrete’ is having its moment in the spotlight, but for all the wrong reasons.

Over the summer, a panel made of RAAC that was not thought to be at immediate risk collapsed at a school, bringing about a dramatic change in government policy and stopping pupils from returning to 104 schools across England. This was not the first such incident at a school — a RAAC roof collapsed at a Kent primary school in July 2018. Luckily, this happened on a Saturday when the school was empty, but it was clearly a close call and prompted the issue of alerts the following spring.

The 2018 incident alone should undoubtedly have been enough to justify extreme caution, and the two incidents together are clearly being taken very seriously, particularly given the catastrophic potential of a structural collapse in a building full of children.

However, the political furore and media hype, driven by understandable anger and concern over the delayed timing of the warnings, has stirred up an extraordinarily high level of anxiety over RAAC, with very little accompanying information to educate the public when a serious hazard is present and when it is not. Articles fail to mention that while all instances of RAAC need to be carefully and quickly assessed, in many cases RAAC poses little immediate risk.

A guide to the crumbling concrete

RAAC was widely used in public sector buildings from the 1950s to the early 1980s. It is used as an alternative to concrete and contains gas bubbles, which give it an aerated texture, making it lightweight and well-insulating.

It has long been known that the bubbles within RAAC that make it so light have the knock-on effect of reducing its strength under pressure and increasing its elasticity, thereby creating the potential for sagging. The air pockets inside also stop it from bonding well with steel reinforcements and can allow water to seep in.

In the early ’80s concerns about its short lifespan were raised, and it began to fall out of favour. However, it was still used in some properties until the late 1990s.

It is now accepted that the life span of RAAC is around 30 years, meaning that most of the buildings that used RAAC are now beyond their life expectancy.

Fortunately, almost all those buildings are still intact. This shows how, in a well-maintained property, the lifespan of RAAC can be significantly extended. Indeed, in many buildings, RAAC is present in good condition and is well-supported and well-designed. In these cases, it is considered safe for it to remain in place in the short term, as long as regular inspections are carried out and the building is carefully maintained to avoid anything that could worsen the condition of the panels.

Behind the headlines

Unsurprisingly, given the recent headlines, many people are not happy with a ‘watch and wait’ approach.

If RAAC was a material that failed gradually it wouldn’t be such a problem — the warning signs would be evident long before the material failed and caused any damage or danger to occupants. Unfortunately, RAAC is susceptible to abrupt shear failure. This means, as the Standing Committee on Structural Safety put it, that there can be ‘little warning of … sudden collapse.’ It can also sag (deflect) relatively easily, allowing moisture into the steel reinforcement, which can then corrode, resulting in cracking of the concrete.

While collapse can occur without any obvious signs of deterioration around the panels, that does not mean that we cannot tell which buildings are at high risk and which are not. If RAAC is present, government guidance states that a qualified building surveyor or chartered structural engineer should carry out an inspection and risk assessment. They will check that the panels are correctly designed and installed, examine them for signs of deterioration, assess how much pressure they are under and review the surrounding areas of the building for anything that would put the structure at risk.

While it is never appropriate to simply sign off RAAC as safe in the long term, for panels in good condition, ongoing monitoring will be a valid recommendation. Nevertheless, in the past few years, there has been a noticeable move towards strengthening works or removal. Given recent events, this shift is expected to continue. Indeed, some public bodies, including the NHS, have committed to eradicating RAAC entirely from the NHS estate.

Should I worry?

Firstly, if you’re not spending any time in a building that was constructed or extended between the 1950s and late 1990s, you can relax — RAAC will almost certainly not be present.

Secondly, if the building was constructed during the RAAC years, but was not built as a public sector building, office, shop, factory or warehouse, it is very unlikely to contain RAAC.

So far, the only residential roof sighting has been in a block of flats that was converted from a commercial office block. It is highly unlikely to be present on the roofs of properties originally built for use as housing, so residents in such buildings need not be concerned.

If your property does have RAAC and there are immediate safety risks, you will be told to vacate by the responsible body. However, if your property has been assessed by a qualified building surveyor or structural engineer and they recommend ongoing monitoring without the need for propping, strengthening or removal, you can rest easy that your roof is not about to collapse.


By Kim Allcott, Allcott Commercial