Engineering Led Thinking Redefines RAAC Remediation

CSP delivers what is now being recognised as a landmark example of evidence-based RAAC remediation

Reinforced Autoclaved Aerated Concrete (RAAC) has become one of the most challenging and emotive issues facing building owners and public authorities today, often prompting urgent, high-cost decisions made under intense pressure.

Perry Stewart, Managing Director of Clark Smith Partnership (CSP), working together with RAAC Consulting and Solutions (‘RCS’), draw on first-hand experience from a landmark local authority project to explain how science-led thinking, collaboration, and proportionate engineering judgement can cut through uncertainty, challenge assumptions, and deliver safer, more sustainable outcomes for RAAC remediation.

Science-Led Thinking Redefines RAAC Remediation at a Local Authority Landmark

Clark Smith Partnership (CSP), the civil and structural engineering practice founded in 1987, has delivered, together with RCS specialists what is now being recognised as a landmark example of evidence-based RAAC remediation at a Grade II-listed local authority headquarters.

 This project demonstrates how rigorous science, calm decision-making, and collaborative delivery can challenge long-held assumptions around Reinforced Autoclaved Aerated Concrete (RAAC), while saving building owners millions of pounds and setting a new benchmark for sustainability and best practice across the UK.

 “This project shows what’s possible when decisions are driven by evidence rather than assumption.” Angus Drummond, Partner at RCS

A High-Profile Discovery

The project began when RAAC was identified within the roof of a major 1970s council headquarters housing both County and Family Courts. Unlike the flat-roofed school and healthcare buildings that dominate national RAAC guidance, this structure featured pitched trapezoidal RAAC panels beneath a tiled roof — a configuration for which authoritative structural data was limited. Following the discovery, the council acted swiftly, partially evacuating the building to ensure public safety. The challenge was immediate and stark: should the roof be fully replaced at enormous financial, environmental, and operational cost, or could a more measured, evidence-based solution be developed?

“The question wasn’t whether to act — it was how to act intelligently.”

RAAC exposed rebar caused by water ingress

Calm Collaboration Under Pressure

CSP and RCS, supported by Loughborough University and the Building Research Establishment (BRE), began with stabilisation, understanding, and data generation — not assumptions. A bespoke temporary mitigation system using engineered timber supports was designed and installed, allowing large parts of the building, including the family courts, to reopen safely.

This approach enabled essential council services to continue while CSP carried out detailed structural assessments across multiple zones, each with differing load conditions, usage patterns, and tolerance for disruption.

“Too often, RAAC projects are driven by fear rather than facts.”

Our role is to bring clarity, structure, and science into what can otherwise become an emergency-led response, allowing clients to make informed decisions rather than reactive ones.

Evidence Before Assumptions

A structured options appraisal guided the council through complex trade-offs around safety, environmental impact, programme risk, disruption, and cost. The unusual pitched-roof geometry placed the building outside existing national RAAC guidance, prompting CSP and RCS to undertake one of the most comprehensive investigations of trapezoidal RAAC construction to date.

Thirty full-size RAAC panels were carefully removed under controlled conditions and sent to BRE for destructive testing. The objective was clear: generate robust, building-specific data where none previously existed.

“We needed real data from real buildings — not generic assumptions.”

RAAC damage with exposed rebar

Panels-in-an-Industrial-pitched-roof-scaled

At BRE, the panels underwent extensive laboratory testing, including four-point and five-point bending tests to assess flexural strength, failure modes, and load-bearing behaviour. Panels were tested across varying spans and bearing widths, including deliberately reduced supports to simulate worst-case scenarios. Additional compressive strength testing and reinforcement surveys provided further insight into the contribution and condition of embedded steel.

“This programme allowed us to understand how these panels actually behave in situ.”
Alan Pipe, Director – RAAC Technical Strategy, CSP

Challenging the RAAC Narrative

One of the most significant findings related to bearing width, long considered a critical vulnerability in RAAC systems. While narrower supports did reduce capacity, the reduction was far less severe than commonly assumed. Even under significantly reduced bearing conditions, the trapezoidal panels retained meaningful structural capacity. The testing programme also captured deflection and ductility data, reinforcing the value of long-term monitoring rather than binary pass-or-fail judgements.

Key findings included:

  • Sufficient structural capacity in all tested panels, meeting relevant codes with appropriate safety factors
  • Enhanced performance of trapezoidal panels due to higher embedded steel content
  • Lower-than-expected sensitivity to reduced bearing widths
  • A robust evidence base supporting continued use with targeted remediation and ongoing oversight

“RAAC is not inherently unsafe — the risk lies in misunderstanding it.”

Delivering Value Through Science

Armed with this data, CSP and RCS developed a bespoke remediation strategy that delivered full compliance and safety without resorting to wholesale roof replacement. The approach significantly reduced environmental impact by minimising new materials, lowering embodied carbon, and limiting construction waste. Operational disruption was minimised, and the historic integrity of the building preserved. By applying proportionate engineering judgement informed by real evidence, CSP and the team have saved this client — and others facing similar RAAC challenges — millions of pounds.

“Science-led engineering delivers better financial, operational, and environmental outcomes.”

A Model for the Future

The project represents a shift away from emergency response and towards strategic refurbishment. As Professor Goodier of Loughborough University observes:

“RAAC isn’t inherently unsafe. Until now, we haven’t had sufficient laboratory testing data from real buildings to sit alongside inspections and calculations. This project fundamentally changes that.”

The insights generated are already shaping CSP’s wider RAAC portfolio, informing remediation strategies for public authorities and private-sector organisations alike, including blue-chip clients. With high-quality, building-specific data now available, CSP can interpret national guidance with greater precision, providing evidence-based recommendations rather than precautionary assumptions.

By combining leadership, collaboration, and technical rigour, CSP — alongside BRE, and Loughborough University — has demonstrated that RAAC challenges can be addressed safely, intelligently, and sustainably. In doing so, the firm has not only safeguarded a vital civic building but has also set a new standard for RAAC management across the UK.

CSP takes a proportionate, science-led approach to structural design that helps clients make confident, defensible decisions. By avoiding default, high-cost replacement strategies and instead balancing safety, sustainability, operational continuity, and value for money, CSP consistently delivers outcomes that protect buildings, budgets, and reputations — a mindset that proved critical on this complex and high-profile project     

Contact us us for an insightful discussion covering:

 * Understanding the Scope of the RAAC Issue: Expert perspectives on the prevalence and potential risks associated with RAAC.

 * Investigation and Assessment Techniques: Sharing best practices for identifying, surveying, and assessing the condition of RAAC structures with a focus on extending the lifespan of the building for the client

 * Remedial Strategies: Exploring innovative and practical solutions for addressing RAAC concerns, from site strengthening/remediation to avoidance of complete replacement.

 * Navigating Regulatory Frameworks: Understanding current guidelines and anticipating future policy developments related to RAAC.

 * Collaborative Pathways: Fostering open dialogue and establishing networks for effective information sharing and joint problem-solving

  • RAAC with longitudinal cracking.
  • RAAC panels with water stain damage
  • Panels-in-an-Industrial-pitched-roof-scaled
  • Damp due to RAAC