RAAC Roofs: Why Flat Roof Guidance Doesn’t Cover Pitched Roof Risk

Most of what we know, or think we know, about RAAC risk has been shaped by one type of structure: the flat roof. Guidance from the Institution of Structural Engineers (IStructE) and the Health and Safety Executive(HSE) has been critical in helping duty holders assess risk. But when RAAC panels appear in pitched or inclined structures, those same frameworks may not fully apply.

The RAAC remediation project at Gun Wharf, Medway Council’s headquarters in Kent, is one of the first to demonstrate how RAAC works in a large pitched roof and what can be done to address it.

Medway: A RAAC Roof Case Outside the Guidance

Gun Wharf is a Grade II-listed civic building in Kent, designed by ARUP for Lloyd's of London in the 1970s and later acquired by Medway Council as its HQ. In 2023, a routine building inspection revealed widespread RAAC roof panels, but unlike the flat, rectangular panels that RCS has regularly found in schools or healthcare buildings, these were pitched trapezoidal panels installed beneath a tiled roof.

According to IStructE guidance, updated in 2023, RAAC panels fall into four risk categories: Low, Medium, High, and Critical. These are based on known failure modes such as bearing degradation, shear cracking, and water ingress. But the risk categorisation criteria are based on RAAC roof panels installed flat, and the geometry of the roof at Gun Wharf falls outside that scope.

When RAAC roof panels were discovered at Gun Wharf, the challenge for Medway Council was clear: how to assess the risk it posed, how to define the risk and liability for all stakeholders, and how to make remediation or demolition decisions, all with no comparable data available?

Building the RAAC Roof Dataset

Medway Council were advised that their only option was to replace the RAAC panels, with initial cost estimates ranging from £20 million to £70 million. Medway commissioned the specialist RAAC team at RCS to peer review the findings. Their theory was that the roof could be remediated, rather than replaced, but only if testing could prove it. To do that, the team needed to create the kind of data set that simply wasn’t available in existing RAAC guidance and legislation.

Working with the Building Research Establishment (BRE) and RAAC specialist Professor Chris Goodier at Loughborough University, RCS removed and destructively tested 30 full-size RAAC roof panels, including both rectangular and unusual trapezoidal sections. These sections were particularly important because, until this point, no testing had ever been done on panels of this shape. Testing was extensive and included both three- and four-point bending tests, compressive strength measurement, full reinforcement mapping of the panels and tested in pitch.

One of the key questions the BRE testing addressed was bearing length, often assumed to be a critical weak point in RAAC roof panels. So, the programme deliberately used reduced bearing widths to assess the panels' load capacity under worst-case conditions. Another focus was specifically on the performance of the trapezoidal panels from Gun Wharf. These are commonly assumed to be high risk because of their unconventional geometry and assumed cut reinforcement.

The results challenged some widely held assumptions. “The trapezoidal panels actually performed better than their rectangular counterparts in many cases,” said Philip Holden, Principal Structural Engineer at BRE. “They had more embedded steel and showed greater resilience, even under reduced bearing conditions.”

Implications of the Testing for Risk Managers and Duty Holders

The Medway project demonstrated very clearly the power of testing RAAC panels where it’s otherwise not possible to assess risk accurately.

Both the IStructE and HSE are clear on one key point: where a building with RAAC can’t be demonstrated to be safe, it must be taken out of use. But in cases like Gun Wharf, “cannot be shown to be safe” can often reflect a lack of understanding of RAAC as a material and how it behaves, rather than being a real safety risk. In this case, testing provided that understanding.

Having to take buildings out of use has implications not just for public-sector budgets but also for civil and criminal liability. RCS’s advisory note for private-sector clients outlines the responsibilities of duty holders when RAAC is present. Once RAAC is classified as ‘High Risk’ or ‘Critical Risk,’ continued occupation becomes a ‘foreseeable liability’, both legally and from an insurance perspective.

Where structures fall outside the current guidance, or the understanding of RAAC as a material is limited, making the correct classification with absolute confidence requires additional, systematic investigation. 

“RAAC isn’t inherently unsafe,”  said Professor Chris Goodier of Loughborough University. “What’s been missing is the scientific evidence to make real-time case-by-case decisions. The Medway project clearly shows what’s possible when research, data and practical expertise come together, and that collaboration, not fear, should drive the response.”

Testing Isn’t Optional for RAAC

Armed with the test results, Medway Council ultimately opted for a data-led, risk-managed remediation strategy that avoided full replacement. Using data gathered through testing, RCS designed a programme incorporating a range of remedial measures and load management, that enabled parts of the building to be safely and quickly reopened. The specialist testing carried out at BRE was a critical element of an approach which helped save the authority £millions and minimised disruption to normal operations.

Without testing, pitched RAAC roof structures remain in a grey area. For duty holders, that uncertainty carries operational, financial, and legal risk. For engineers, it presents a challenge: find ways to assess these structures that go beyond flat-roof assumptions.

We Need to Stop Treating All RAAC the Same

As more RAAC is discovered in UK buildings across a wider range of contexts, it is becoming increasingly apparent that a one-size-fits-all model simply isn’t sufficient. Testing and data collection can be a critical component of the risk management toolkit at any building where RAAC is discovered, especially when dealing with non-standard configurations like pitched or trapezoidal RAAC panels.

Read the full Medway case study here