Science-Led RAAC Remediation at Medway Council’s Gun Wharf

A pioneering approach to RAAC that challenges assumptions, delivers value, and sets a benchmark for building owners across the UK.

When RAAC was discovered in the roof of Medway Council’s Gun Wharf headquarters in Chatham, Kent, in late 2023, the immediate question was whether to evacuate and replace the whole roof or take a more measured route.

What followed is now being recognised as a landmark example of evidence-based decision-making, multi-sector collaboration, and the value of applying science, not guesswork, to the challenges posed by RAAC in the built environment.

Gun Wharf is a striking Grade II-listed civic building, originally constructed for Lloyds of London by ARUP in the 1970s and acquired by Medway Council in 2008 as its primary offices. The County and Family Courts for the Medway towns are also housed there. Its distinctive geometry, with pitched trapezoidal RAAC panels beneath a tiled roof, set it apart from the flat-roofed schools and healthcare settings that had previously dominated the RAAC debate. And when a routine RAAC survey unexpectedly revealed widespread use of the material in the roof, the council acted swiftly, initiating a partial evacuation of the building within hours.

“When the surveyor first confirmed the extent of RAAC in the roof, it was a sobering moment. We had to make quick decisions to ensure the safety of staff and building users, while also avoiding knee-jerk reactions. The scale of the challenge became clear very quickly, but so did the opportunity to do things properly,” says Robert Banks, Capital Programme Manager at Medway Council. “It was a dramatic Friday afternoon. One moment we were preparing routine surveys, and the next we were being told our main building had significant RAAC. It would’ve been easy to panic, but we knew we had to pause, understand the science, and take a structured approach. That decision, to stop and ask the right questions, has made all the difference to the outcome.”

Rather than rushing to a default replacement roof strategy (estimated to cost between £20m and £70m), Medway Council instead commissioned a multi-disciplinary team from RAAC Consulting Solutions (RCS) to investigate whether the roof could be safely retained. Their hypothesis: that the roof could be saved and safely remediated, and that science could prove it.

Collaborative, Science-Backed Strategy

RCS conducted an initial reconnaissance of Gun Wharf at no charge. Following that, in close cooperation with the council’s Capital Projects and FM teams, they were able to reopen closed-off areas of the building, including the family courts. They then implemented a bespoke temporary mitigation system using engineered timber supports, carefully designed to allow continuity of council operations. At the same time, a more thorough structural assessment was carried out.

“The scale and complexity of the structure at Gun Wharf made it an especially demanding engineering challenge”

Dr Pete Winslow, FICE and board director at RCS

“There were multiple zones with different loading conditions, different user needs, and a range of stakeholder priorities to balance, from long-term operational needs and financial implications to environmental performance and disruption tolerance.”

Dr Winslow was involved in the project early to build a scientific understanding of the structural risks, but also to develop a more holistic view of what remediation needed to achieve. “We ran a structured stakeholder workshop that brought together the council, facilities teams, legal teams, and even regional economic voices,” he explains. “That gave us a much broader picture of how the building was used, what the organisation needed from it in the future, and what constraints mattered most. It meant we could evaluate every option against not just engineering safety, but also Capex, Opex, long-term business use, and even local development priorities.”

The structured options appraisal enabled RCS to guide the client through complex trade-offs, such as the balance between disruption and sustainability, or between speed and embodied carbon. “Too often in projects, there’s no clear framework for choosing between repair and replacement. What we were able to do - with input from Loughborough and BRE - was offer actionable, evidence-based guidance that moved the conversation to a clear strategy.”

The result was a bespoke remediation plan that delivered compliance and safety, but with minimal use of virgin materials and significantly lower environmental impact than a full roof replacement. “It’s a sustainable option, not just in materials but in mindset,” says Winslow. “It challenges the status quo assumption that RAAC must mean a rebuild. And it shows that a science-led, multi-competency approach can unlock better, more responsible outcomes.”

Critically, Gun Wharf’s pitched roof and inclined RAAC panels placed it outside the scope of existing national guidance on RAAC safety, which was primarily based on horizontal or flat installations. There were no existing standards or datasets for what RCS had uncovered. That knowledge gap prompted a comprehensive investigation supported by Professor Chris Goodier of the School of Architecture, Building and Civil Engineering (ABCE) at Loughborough University, and the Building Research Establishment (BRE), that would make Gun Wharf “the best understood RAAC structure on the planet,” according to RCS.

Destructive Testing and New Data

Thirty full RAAC panels were removed under controlled conditions and sent to BRE for detailed destructive testing, overseen by the research team at Loughborough University. The aim was to generate the kind of structural data that simply hadn’t existed before for this type of RAAC construction, particularly pitched and trapezoidal installations of this scale.

At BRE, the panels were subjected to a rigorous laboratory programme to determine their load carrying capacity, their flexural and material strength, and their bending behaviour. “Our role at BRE was to work closely with RCS and Loughborough University to carry out structural testing on a representative sample of RAAC panels removed from Gun Wharf,” explains Philip Holden, Principal Structural Engineer at BRE. “The panels were tested using different conditions and were loaded to their limits.”

The test programme included both four-point and five-point bending setups, enabling the team to measure flexural strength and failure modes across different panel geometries. Panels were tested at varying spans and bearing widths, including deliberately reduced supports to simulate worst-case scenarios, and both rectangular and trapezoidal panels were assessed. BRE also undertook a series of compressive strength tests and reinforcement surveys better to understand the condition and contribution of the embedded steel.

A key focus of the testing was on bearing width, which has long been considered a critical vulnerability in RAAC systems. “We deliberately tested panels with reduced bearing lengths to examine the effect on the load carrying capacity,” says Holden. “The expectation was that narrow supports would result in a steep drop-off in performance. In practice in this scenario, we saw a reduction, but it was far less dramatic than may be predicted. The panels retained good capacity even with significantly decreased support areas.”

Another surprise came from the performance of the trapezoidal panels in the test lab. Traditionally regarded as high-risk due to their irregular end geometry and uncertain reinforcement placement, these panels were revealed to have been intentionally over-designed by ARUP, the building’s original structural engineers. “The trapezoidal panels actually performed better than their rectangular counterparts in many cases,” Holden notes. “Once we mapped the reinforcement using cover meters, it became clear that they contained more steel and were stiffer as a result.”

Beyond simple pass/fail thresholds, the tests also captured deflection and ductility data, supporting the view that the long-term monitoring of movement is critical. “The testing provides confidence in the load carrying capacity of the planks as reclaimed, in the performance of the material and in the viability of a targeted, proportionate remediation strategy,” says Holden.

Professor Chris Goodier of Loughborough University echoed this assessment, calling the configuration at Gun Wharf “structurally unique,” combining lightweight pitched RAAC panels with a concrete vaulted ceiling below.

“Even if the RAAC were to fail, it would land on a substantial slab of reinforced concrete, which effectively acts as a massive protective crash deck,” he explains. “But the testing showed that the RAAC panels were dry, under historically low loads, and hence remarkably well preserved. They consequently met modern structural performance codes even without that added layer of structural protection.”

Key Findings from the BRE testing and Loughborough include:

• Sufficient structural capacity in all tested panels, meeting codes and standards with appropriate factors of safety.
• Superior performance of trapezoidal panels, thanks to higher levels of embedded steel reinforcement from the original ARUP design.
• Reduced sensitivity to bearing width reductions, with load capacity decreasing less than previously assumed.
• A robust evidence base supporting continued use of the panels in situ, with appropriate remediation and oversight.

This detailed testing regime helped shift the project from an initial emergency response into a science-led programme of selective intervention and strategic refurbishment, challenging long-held assumptions about RAAC and setting a precedent for similar buildings nationwide.

Practical Outcomes and Targeted Mitigation

RCS used these findings to model and assess a comprehensive range of remediation options, in collaboration with Medway stakeholders, including political leaders, legal teams, procurement teams, and building users. Each option was evaluated against structural performance, cost, disruption, and heritage considerations.

The preferred route, which is now underway, involves:

• Targeted interventions using bespoke support solutions.
• Remediation of water ingress identified during survey phases.
• A future-proofing programme addressing both the panels and broader envelope repairs.

The Ministry of Justice, which sublets parts of the building, has already been able to reoccupy previously closed-off courtrooms and offices. Around 25% of the building is now safely back in use, with wider phased reoccupation to follow as work progresses.

Learning Outcomes

The knowledge gained at Gun Wharf is already shaping the way RCS approaches other RAAC projects. The volume and quality of data generated, both on-site and through lab testing, have given the team a much deeper understanding of RAAC performance and the variables that influence it. Crucially, it reinforces the principle that effective solutions must be nuanced and tailored to the specific conditions of each structure.

This deeper insight is now being used to inform assessments and remediation strategies for a range of clients, including major public bodies and private sector organisations. Crucially, it allows RCS to interpret existing guidance in a more forensic and evidence-based way, supporting clients with decisions grounded in data rather than default assumptions.

For future projects, this Medway model, a model that combines structural expertise, full options appraisal, and science-led decision-making, offers a repeatable framework that balances safety, sustainability, and operational need.

Setting a New Standard

The Gun Wharf project represents more than just a well-managed structural challenge. It demonstrates what can happen when the public, private, and academic sectors work in true partnership. As Professor Goodier puts it, “RAAC isn’t inherently unsafe. But until now, we haven’t had the significant structural lab testing data on a real building to go with the on site inspections and structural calculations.”

“Medway Council asked the right questions and waited for the answers to enable them to make sensible science and engineering-informed decisions based on real data from their actual structure.”

Professor Goodier

The whole process has allowed Medway Council to reinvest in its estate, using the opportunity to address long-standing issues at Gun Wharf like accessibility, energy performance, and workspace modernisation. The building has been declared Medway’s ‘forever home’ for council operations, an outcome made possible by what Professor Goodier calls a great example of “collaboration, curiosity, and a commitment to evidence.”