Why Concealed Voids Are A Fire Risk

Concealed voids are hidden spaces within a building. They are often found above suspended ceilings, within roof voids, behind wall linings, around service risers or below raised floors.

These areas can create hidden pathways for fire and smoke. If a fire reaches a ceiling void or roof space, it may spread beyond the room of origin before occupants or building managers realise how far it has travelled.

Common risk areas include:

• Suspended ceiling voids
• Roof voids and loft spaces
• Service risers
• Plant areas
• Wall and floor junctions
• Atriums
• Mezzanine areas
• Lift shafts
• Stairwell openings
• Corridor ceiling voids

Without suitable fire barriers, these hidden areas can undermine the fire resistance of the surrounding walls, floors and compartments.

What Are Flexible Fire Barriers?

Flexible fire barriers are fire-resisting systems used to help contain fire and smoke in concealed or open spaces. Unlike rigid fire-stopping boards or masonry construction, flexible barrier systems are lightweight and can often be fitted around complex building details.

They are commonly used where a fire-resisting line needs to continue through a void, roof space or ceiling zone.

A flexible fire barrier system may include:

• Fire-resistant barrier curtain material
• Fixing angles or support brackets
• Staples, clips or mechanical fixings
• Foil-faced or mineral-based fire-resisting layers
• Intumescent or fire-rated ancillary products
• Overlap joints and sealed edges
• Supporting framework where required

The exact components depend on the tested system and the required fire rating.

How Flexible Fire Barriers Support Fire Compartmentation

Fire compartmentation divides a building into separate fire-resisting zones. The purpose is to slow the spread of fire and smoke, protect escape routes, support firefighting access and reduce the risk of total building involvement.

Flexible fire barriers help by continuing this compartment line through areas that are not always visible, such as roof spaces and suspended ceilings.

They can help:

• Maintain compartmentation above ceilings
• Reduce fire and smoke spread through voids
• Protect escape routes and corridors
• Support the building’s passive fire protection strategy
• Separate high-risk and lower-risk areas
• Improve fire safety in open-plan or multi-level spaces
• Protect structural and service zones

Where Are Flexible Fire Barriers Used?

Flexible fire barriers are used in areas where a fire-resisting barrier is needed, but a rigid system may not be suitable.

Suspended Ceilings

Ceiling voids can allow fire and smoke to bypass walls below. A flexible fire barrier can continue the compartment line above the ceiling, helping prevent hidden fire spread.

Roof Voids And Loft Spaces

Roof spaces are a common route for rapid fire spread, especially in large buildings or connected units. Flexible barriers can help divide roof voids into controlled fire compartments.

Atriums And Open-Plan Buildings

Atriums and open internal spaces can create vertical routes for fire and smoke. Horizontal or vertical fire barriers may be used as part of a wider fire-engineered solution.

Service Risers And Plant Areas

Mechanical, electrical and plumbing services often pass through compartments. Flexible barriers can help protect larger service zones where tested systems are suitable.

Warehouses And Industrial Buildings

Large-volume buildings often need carefully designed compartmentation to manage fire spread. Flexible fire barriers can be useful where roof structures, steelwork or services make rigid construction difficult.

Residential And Commercial Buildings

Flexible barriers may be used in blocks of flats, offices, schools, hospitals, retail buildings, leisure facilities and other buildings where concealed voids need fire-resisting separation.

Vertical Fire Barriers Vs Horizontal Fire Barriers

Flexible fire barriers are usually specified as either vertical or horizontal systems. The correct option depends on the fire strategy, building layout and direction of potential fire spread.

Vertical Fire Barriers

Vertical flexible fire barriers are used to form a fire-resisting line from one level or surface to another. They are commonly installed in roof spaces, ceiling voids and concealed areas where a compartment wall needs to continue upwards.

They are often used to help stop fire and smoke from travelling sideways through voids.

Horizontal Fire Barriers

Horizontal flexible fire barriers are used where fire resistance is required across a horizontal opening or void. They may be used in atriums, service zones, stairwell openings, mezzanine spaces or other multi-level areas.

They are often used to help control vertical fire spread.

Integrity And Insulation Ratings Explained

When specifying flexible fire barriers, two key ratings are commonly used: integrity and insulation.

Integrity, often shown as E, refers to the barrier’s ability to prevent flames and hot gases passing through.

Insulation, often shown as I, refers to the barrier’s ability to limit heat transfer to the protected side.

For example, a product rated EI60 is designed to provide 60 minutes of integrity and 60 minutes of insulation, based on the relevant test conditions.

This is important because a barrier that stops flames may not always provide the same level of protection against heat transfer. The required performance should be confirmed from the fire strategy, specification, test evidence or manufacturer’s documentation.

Choosing The Right Flexible Fire Barrier

The correct fire barrier should be selected based on the building design, fire strategy and required performance.

Before choosing a product, consider:

• Required fire resistance period
• Integrity and insulation rating
• Vertical or horizontal application
• Size and shape of the void
• Substrate and fixing method
• Movement requirements
• Services passing through the barrier
• Compatibility with surrounding fire-stopping products
• Manufacturer test evidence
• Installation requirements
• Access for future inspection and maintenance

A barrier should not be selected purely because it looks similar to an existing product. Fire barriers work as tested systems, so the complete installation detail matters.

Flexible Fire Barriers Vs Cavity Barriers

Flexible fire barriers and cavity barriers are sometimes confused, but they are not always the same thing.

Cavity barriers are usually used to close or subdivide cavities, helping restrict fire and smoke spread within concealed spaces.

Flexible fire barriers are often used where a larger or more specialist fire-resisting barrier is required, particularly across roof voids, ceiling voids, atriums or open spaces.

The right product depends on the application, required rating and building design. In some cases, both cavity barriers and flexible fire barriers may be needed as part of the same passive fire protection strategy.

Common Specification Mistakes To Avoid

Flexible fire barriers need to be correctly specified and installed to perform as intended. Common mistakes include:

• Using a barrier without confirming the required fire rating
• Confusing integrity-only performance with integrity and insulation performance
• Not checking whether the system is suitable for vertical or horizontal use
• Leaving gaps at edges, overlaps or junctions
• Installing around services without suitable fire-stopping details
• Using non-approved fixings
• Failing to maintain the compartment line above a ceiling
• Mixing components from different systems without evidence
• Ignoring future access and inspection requirements
• Treating the barrier as a standalone product rather than a complete tested system

These issues can reduce performance and may create problems during inspection, handover or later remedial works.

Installation Considerations

Flexible fire barriers should be installed by competent installers in line with the manufacturer’s instructions and project specification.

Key installation points include:

• Correct orientation of the barrier material
• Correct overlaps and sealed joints
• Suitable fixing centres
• Compatible supporting substrates
• Fire-stopped service penetrations
• Correct treatment of edges and abutments
• Protection from damage during follow-on trades
• Clear records of what has been installed
• Access for future inspection where required

Poor installation can compromise even a high-quality product, so workmanship and documentation are just as important as product selection.

Why Documentation Matters

For commercial buildings, residential developments and higher-risk buildings, product records and installation evidence are increasingly important.

Useful documentation may include:

• Product datasheets
• Installation instructions
• Fire test evidence
• Technical approvals
• Photos of installed barriers
• Installer details
• Location plans
• Inspection records
• Maintenance information

Keeping accurate records helps building owners, contractors, designers and responsible persons demonstrate what has been installed and where.

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