RAAC roofs, planks and roof panels

How a RAAC roof deck is built

RAAC roof planks are reinforced along their length with steel bars, often about 600 mm wide, and span between walls or beams. They are laid side by side to form the roof deck, with joints between them, then covered with insulation and a waterproof membrane.

What goes wrong with RAAC planks

• Water. A failed roof covering or blocked drainage lets water in. Wet RAAC is heavier and weaker, and the reinforcement corrodes.
• Deflection. RAAC planks bend more than ordinary concrete, and excessive deflection is a warning sign.
• Cuts and penetrations. Roof lights, ducts and services cut through planks.
• Added loads. Plant, new coverings and extra layers of insulation add weight the planks were not designed for.
• Bearings. Planks that sit on very little support, or whose supports are damaged.
• Reinforcement. Bars that were placed in the wrong position when the planks were made.

RAAC roof collapse

In 2018 a school roof made of RAAC planks collapsed with little or no warning, which is what brought RAAC to national attention. Failures like that are why RAAC roofs are surveyed and assessed rather than judged from below. Every roof is different: the answer for yours comes from its own survey.

What to do about a RAAC roof

Survey it, assess it plank by plank against the current IStructE RAAC guidance, keep water out, and price the options. Many RAAC roofs can be kept and strengthened in place with bespoke CFRP. RAAC strengthening.

Source for the roof facts above: the MoD Defence Infrastructure Organisation safety alert on RAAC planks (2023).