Is RAAC Only a Flat Roof Problem?

RAAC is most commonly associated with flat roofs. And this association is understandable: early RAAC failures, public guidance and inspection programmes focused heavily on flat roof construction, particularly in schools and hospitals.

But this focus has created the misconception that RAAC is only a flat-roof issue. In practice, RAAC has been used in a much wider range of structural forms, and applying flat-roof assumptions elsewhere can lead to inaccurate risk assessments. Understanding where RAAC sits within a building and how it behaves structurally is just as important as knowing that it is present.

Why RAAC became linked to flat roofs

Flat roofs were among the most common applications of RAAC panels, particularly since the 1960s. Their lightweight nature made them attractive for large-span roof construction, and early guidance quite reasonably prioritised these systems when concerns emerged in recent years.

As awareness grew, flat roofs became the reference point for RAAC risk. Over time, this narrowed focus has shaped perception, sometimes at the expense of a broader understanding of how RAAC was actually used across the built environment.

Where else is RAAC being found?

Site investigations carried out by RCS have identified RAAC in a variety of locations beyond flat roofs, including:

• Pitched and sloping roof structures
• Roof decks with complex load paths
• Wall panels and façade elements
  
• Hybrid RAAC systems

These configurations will obviously behave differently from flat roof systems. Load transfers, restraint conditions, and exposure to moisture can vary significantly, influencing both performance and risk.

Why structural form matters

The existing RAAC guidance on flat roofs often assumes uniform loading, predictable support conditions and relatively simple structural behaviour. Pitched roofs and vertical elements rarely conform to those assumptions. For example, load paths in pitched roofs may introduce additional stresses, and end bearing and restraint can differ from flat roof arrangements with uneven or concealed moisture exposure. Applying flat-roof-based risk models to other structural forms can therefore either overstate or understate the true level of risk, and an accurate assessment depends on understanding how the RAAC element works within the wider structure.

The risk of one-size-fits-all assumptions

Treating all RAAC as a flat roof problem encourages a binary response: safe or unsafe, retain or remove. In reality, risk sits on a spectrum and must be evaluated in context. Risk is influenced by multiple factors, including age, condition and exposure over time. Understanding how RAAC lifespan is determined is therefore an important part of assessing overall structural risk. Overly simplified assumptions can lead to unnecessary restrictions on building use, inappropriate remedial measures, and significantly increased costs without any real or proportionate safety benefit. A properly designed RAAC remediation strategy ensures that interventions are proportionate to the actual level of risk. That’s why it’s vital to use specific, tested, science-led surveys and assessments that allow risk to be managed in a targeted way, focusing attention where it is genuinely needed.

What building owners should do instead

Rather than asking whether RAAC is present in a flat roof, a more useful starting point is:

• Where is the RAAC located within the structure?
• What role does it play structurally?
• How is it supported and restrained?
• What exposure and loading conditions apply?

A RAAC survey and a structural assessment by a qualified, experienced engineer will answer these questions and avoid assumptions based on building typology alone.

Seeing the whole RAAC picture

RAAC is not confined to flat roofs, and neither should RAAC assessment be. Structural form matters, and understanding how RAAC behaves in different configurations is essential to making proportionate, evidence-led decisions.

For a wider overview, read our guide to the five misconceptions about RAAC.

What guidance has been issued?

Numerous bodies have issued guidance on the subject of RAAC:

• Health and Safety Executive (HSE) Guidance on RAAC: Comprehensive guidelines are available on the HSE website.
• Confidential Reporting on Structural Safety (CROSS): Refer to CROSS reports for documented incidents and expert recommendations on the Cross Safety website.
• The Institution of Structural Engineers: information on Reinforced Autoclaved Aerated Concrete (RAAC) on istructe.org.
• From the Local Government Association Office of Government Property: RAAC Safety Briefing Notice and Information on Reinforced Autoclaved Aerated Concrete (RAAC) on local.gov.uk
• For England: Department for Education collection on reinforced autoclaved aerated concrete in education settings including Reinforced autoclaved aerated concrete: guidance for responsible bodies and education settings with confirmed RAAC and Reinforced autoclaved aerated concrete: identification guidance.
• For Scotland: Local authorities publish RAAC data and Reinforced Autoclaved Aerated Concrete (RAAC): Parliamentary statement
• For Wales: Written Statement: Update on Reinforced Autoclaved Aerated Concrete in Welsh public estate (8 September 2023)

RAAC assessment shouldn’t start with assumptions

If RAAC may be present in your building, understanding where it is and how it behaves structurally is critical. A science-led RAAC assessment provides clarity on risk, options and next steps, based on evidence, not assumptions.

Speak to RCS about a proportionate RAAC assessment.

FAQs

1: Is RAAC only found in flat roofs?

No. While RAAC is commonly associated with flat roofs, site investigations have identified it in pitched roofs, sloping roof decks, wall panels, façades and hybrid structural systems. Risk depends on where RAAC is located and how it is structurally designed, not simply on roof type.

2: Can pitched roofs contain RAAC?

Yes. RAAC has been found in pitched and sloping roof structures, where it behaves differently from flat roof systems. Load paths, restraint conditions and moisture exposure can all vary, which is why pitched RAAC roofs require a form-specific assessment.

3: Why can flat roof guidance be misleading?

Much published RAAC guidance is based on flat roof construction. Applying this guidance directly to other structural forms can lead to incorrect conclusions, either overstating or understating risk. Accurate assessment requires understanding how RAAC works within the wider structure.

4: Does RAAC in walls or façades present different risks?

Yes. RAAC used in walls, or façade elements, may be subject to different loading, restraint, and exposure conditions than roofs. These differences can significantly affect performance and must be assessed on a case-by-case basis.

5: How should RAAC be assessed if it is not in a flat roof?

RAAC should be assessed based on its location, structural role, support conditions, loading and exposure to moisture. A science-led RAAC survey and structural assessment provide the evidence needed to manage risk proportionately and avoid assumptions based on building type alone.