Fire Stopping In Cladding

Understanding fire stopping in cladding is fundamental to the safety and compliance of any multi-storey building in the UK. In simple terms, fire stopping refers to the physical barriers and materials installed within the hidden cavities of a building’s external wall system to prevent the spread of fire and smoke. These barriers are designed to seal the gaps between different components, such as the insulation, the cladding panels, and the building’s structural frame.

For building owners, managing agents, and property professionals, ensuring that the external wall fire stopping detail is correctly specified and installed is a legal and moral necessity. Without functional fire stops, a small fire originating within a flat or office can quickly enter the wall cavity. Once inside this "chimney-like" space, flames can bypass internal fire compartments, spreading rapidly across the entire height and width of the facade.

Key Takeaways#

• Cavity Barriers: These are the primary components of fire stopping, designed to close the gap between the building and the cladding in the event of a fire.
• Regulatory Compliance: Post-Grenfell, the cladding fire stopping requirements under Building Regulations (specifically Approved Document B) have become significantly more stringent.
• Intumescent Materials: Many modern fire stops use materials that expand when heated, allowing for necessary ventilation during normal conditions while providing a seal during a fire.
• Installation Quality: Even the highest-rated fire stop will fail if there are gaps, incorrect fixings, or if it is installed in the wrong orientation.
• Remediation: Defective fire stopping does not always require a full reclad; targeted remediation is often possible following a detailed cladding testing and inspection process.

What is fire stopping in cladding?#

What is fire stopping in cladding? It is a system of fire-resistant components installed within the internal cavities of a facade to restrict the passage of fire. These systems usually consist of horizontal and vertical cavity barriers. While horizontal barriers often feature an "open-state" design to allow moisture drainage and airflow, they contain intumescent strips that react to heat, expanding to seal the cavity completely if a fire occurs.

• Horizontal Barriers: Prevent the vertical spread of fire up the building.
• Vertical Barriers: Prevent the lateral (sideways) spread of fire around the building perimeter.
• Closures: Fire stopping installed around openings such as windows and doors to prevent fire from jumping from the interior to the external wall cavity.

The Mechanics of Fire Spread in Facades#

To appreciate the importance of a facade fire stopping detail, one must understand how fire behaves within an external wall system. When a cladding system is installed, a cavity is usually created between the insulation (attached to the building) and the external rainscreen panels. This cavity is essential for moisture management, preventing damp and rot within the structure.

However, this same cavity can act as a flue. If fire enters this space, the "stack effect" pulls hot gases and flames upward at high velocity. Without robust fire stopping in cladding, the fire can bypass the fire-rated floors of the building, reaching upper storeys in a matter of minutes. This renders the internal "stay put" policy ineffective, as the fire is effectively attacking the building from the outside.

The Role of Cavity Barriers vs. Fire Stops#

While the terms are often used interchangeably, there is a technical distinction. Cavity barriers are intended to close a hidden space to prevent fire and smoke spread within that space. Fire stops are typically used to seal penetrations (like pipes or cables) or joints between fire-separating elements. In the context of cladding, we are primarily discussing cavity barriers that perform a fire-stopping function.

Modern external wall fire stopping detail usually relies on "open-state" horizontal barriers. These allow the building to "breathe" under normal circumstances. The intumescent strip is the critical technology here; it must react quickly enough to close the air gap before the fire can pass through. If the gap is too wide for the specific barrier used, the seal will fail.

Cladding Fire Stopping Requirements and Regulations#

In the United Kingdom, the regulatory landscape for fire safety has undergone a total transformation. The cladding fire stopping requirements are primarily governed by the Building Regulations 2010, specifically Approved Document B (Fire Safety). For buildings over 11 metres, the requirements are exceptionally strict, often necessitating non-combustible materials (Euroclass A1 or A2-s1, d0).

Section 9 of Approved Document B, Volume 2, outlines that cavity barriers must be provided at specific locations:

• At the junction between the external wall and every compartment floor and compartment wall.
• Around the edges of openings through the wall, such as windows and doors.
• At the top of the wall (e.g., at the eaves or parapet).

The Importance of BS 8414 Testing#

Static material ratings are often not enough to prove safety. Large-scale fire testing, such as BS 8414, looks at how the entire system performs together. This includes the panels, the insulation, the rails, and the facade fire stopping detail. A failure in the fire stopping during a BS 8414 test usually leads to a failure of the entire system, as it allows the fire to reach the top of the test rig too quickly.

Building owners should ensure that their current facade system has a valid BR 135 classification, which is the certificate issued following a successful BS 8414 test. If this documentation is missing, a professional cladding testing exercise is required to verify what has actually been installed behind the panels.

Common Defects in External Wall Fire Stopping#

Even when high-quality fire-stopping materials are purchased, poor installation can render them useless. At CladdingTesting.com, our inspections frequently uncover systemic failures in how fire stops have been integrated into the facade. These defects are often hidden from view, requiring invasive inspections or "opening up" works to identify.

Incorrect Positioning#

A common issue is the misalignment of the cavity barrier with the floor slab. To be effective, the horizontal fire stop must be tightly fitted against the structural floor slab to maintain the compartmentation of the building. If the barrier is installed too high or too low, fire can bypass the floor level through the gap behind the barrier.

Gaps and Incomplete Seals#

Fire stopping must be continuous. We often find vertical barriers that stop short of the horizontal ones, or barriers that have been cut to fit around brackets but not sealed correctly. Even a small 10mm gap can allow enough heat and smoke through to ignite combustible insulation or panels on the level above.

Compression Issues#

Horizontal "open-state" barriers require a specific "air gap" to function (usually 25mm to 50mm). If the cladding panels are installed too close to the barrier, the intumescent strip may not have room to expand. Conversely, if the gap is too wide, the expanded material will not reach the back of the cladding panel, failing to create a seal.

• Mechanical Fixings: Fire stops must be mechanically fixed to the inner leaf of the wall. Relying on "friction fits" is often insufficient as materials can warp or shrink under fire conditions.
• Corrosion: In older buildings, the steel fixings for fire stops may have corroded, meaning the barrier could fall away during a fire.
• Compatibility: Mixing different manufacturers' products without tested evidence can lead to chemical reactions or performance failures.

Investigating Fire Stopping Compliance#

Because fire stopping in cladding is concealed, a desktop study of architectural drawings is rarely sufficient. As-built drawings often differ significantly from what was actually constructed on-site. Therefore, a physical investigation is the only way to gain certainty regarding building safety.

Intrusive Surveys (FRAEW)#

A Fire Risk Appraisal of External Walls (FRAEW), conducted to the PAS 9980 standard, usually involves removing specific cladding panels at various heights and locations. This allows a specialist to inspect the external wall fire stopping detail firsthand. The inspector will check for the presence, type, and quality of installation of all cavity barriers.

Use of Borescopes#

In cases where removing panels is difficult or would cause significant aesthetic damage, borescopes can be used. By drilling a small hole and inserting a camera, we can see inside the cavity. While useful, this is often less definitive than a full panel removal, as it can be difficult to assess the quality of the fixings behind the barrier.

Documentation Review#

Parallel to physical inspections, we review the Operation and Maintenance (O&M) manuals. We look for delivery notes, photographic evidence of installation, and manufacturer data sheets. A lack of evidence is often treated with the same level of concern as a physical defect, as the "Golden Thread" of information is a requirement of the Building Safety Act.

Remediation Options for Defective Fire Stopping#

Discovering that your facade fire stopping detail is non-compliant does not automatically mean the building needs a multi-million-pound recladding project. Depending on the type of cladding and insulation present, targeted remediation may be a viable and cost-effective solution.

Targeted Remediation#

If the cladding panels and insulation are non-combustible (e.g., A1 rated), but the fire stopping is missing or poorly installed, we can often perform "spot repairs." This involves removing rows of panels at floor levels, installing compliant cavity barriers, and then replacing the panels. This significantly reduces costs and resident disruption compared to a full replacement.

Partial Replacement#

In some scenarios, the fire stopping is adequate, but the insulation in specific areas (like around window reveals) is combustible. Replacing these specific sections while upgrading the fire stopping in cladding can bring the building up to a safe standard. This approach is common in buildings between 11m and 18m where a risk-based approach is permitted under PAS 9980.

Full Recladding#

When the cladding panels themselves are highly combustible (such as ACM PE) and the fire stopping is also defective, a full reclad is usually the only path to safety and insurance compliance. In these projects, the entire external wall system is stripped back to the substrate and rebuilt using modern, non-combustible materials and a precision-engineered external wall fire stopping detail.