Fire Stopping: Penetration Seals, Cavity Barriers & Intumescent Products

Quick Answer: Fire stopping is the sealing of gaps around service penetrations through fire-resisting elements (walls, floors, ceilings) to prevent spread of fire and smoke. Building Regulations Part B requires all penetrations through fire-resisting compartment walls and floors to be appropriately sealed. Intumescent materials expand when heated to fill gaps created when plastic pipes melt. Cavity barriers must be installed at specific intervals and at the edges of concealed cavities per Approved Document B.

Summary

Fire stopping is one of the most frequently overlooked and incorrectly installed elements of building work. Many tradespeople — particularly plumbers, electricians, and HVAC engineers — cut holes through walls and floors for services and leave them unsealed, or seal them with expanding foam (which is not a fire-stopping product). The consequence of inadequate fire stopping can be devastating: a fire that should have been contained to one room or compartment spreads rapidly through service penetrations, reducing escape time significantly.

Building Regs Approved Document B (Volume 1: Dwellings) sets out the fire-stopping requirements for domestic properties, including which elements are fire-resisting, what performance standard is required (measured in minutes of fire resistance), and what products are acceptable. For commercial and larger residential projects, Approved Document B Volume 2 applies.

The third-party certification of fire-stopping products is critical. Products should be tested to BS EN 1366-3 (linear seals) or BS EN 13501-2 (system classification) and carry CERTIFIRE or similar third-party approval. Installers should keep records of all fire-stopping work done — increasingly, fire-stopping surveys are part of building safety compliance for high-rise residential.

Key Facts

Quick Reference Table

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Penetration Type Appropriate Fire-Stopping Method Notes
Small cable bundles (<100mm) Intumescent sealant + mineral wool backing Both sides if accessible
Single cable ≤ 20mm Intumescent putty pad or sealant
Plastic pipe ≤ 110mm Intumescent collar (e.g., Quelfire, Hilti, Rockwool Rockcollar) Collar each side of wall
Plastic pipe 110–160mm Intumescent collar + mineral wool wrap
Steel pipe (any size) Intumescent sealant to annular gap Sealant both sides
Ductwork Fire-resisting damper OR intumescent wrap Damper activated by fusible link or actuator
Cable tray through wall Intumescent pillows/blocks + sealant perimeter
Service void/riser Compartment floor/wall at each level Treat as penetration

Detailed Guidance

Which Elements Require Fire Stopping

Not every wall and floor requires fire stopping. The requirement applies to fire-resisting elements — those that are part of the compartmentation strategy:

In dwellings (Approved Document B, Volume 1):

In commercial/residential (Volume 2):

Where you're working through a non-fire-resisting partition (e.g., an internal stud wall within a single dwelling), standard sealing with acoustic/fire sealant is good practice but not technically required by Part B.

Intumescent Products — How to Specify

Always specify products by their certification, not just generic type. Key certifications:

Key product families used on UK sites:

Always read the system datasheet — the fire rating applies to a specific combination of products installed in a specific way (pipe size, wall thickness, sealant depth). You cannot mix components from different systems and retain the certification.

Cavity Barriers in Roof Spaces

Approved Document B requires cavity barriers in roof spaces at:

In a simple semi-detached house, the party wall must continue through the roof space to the underside of the roof covering (or close the cavity there). This is a commonly missed requirement on extensions and loft conversions.

For timber-framed buildings, cavity barriers must be installed at all edges of cavities within the timber frame construction, at every floor level, and at roof/wall junctions.

Cavity barrier materials: must achieve 30 minutes integrity. Common options:

Fire-Stopping on Older Properties

Pre-2000 construction rarely has correctly installed fire stopping at penetrations — pipes were commonly run through walls with no sealing at all, or with standard foam. When working on an older property where penetrations are exposed (e.g., during a rewire or replumbing), fire stopping should be retroactively installed.

This is also a client-care issue — if you're a plumber who discovers that the boiler flue passes through an unsealed party wall, advise the client in writing. You have a duty of care to flag significant fire safety deficiencies, even if it's outside your immediate scope of work.

Fire Dampers in Ductwork

Where ventilation ductwork passes through a fire-resisting compartment wall or floor, either:

Fire dampers must be accessible for maintenance and testing. They must be tested annually (BS 9999 recommends; HVCA guidance). Omitting fire dampers in ductwork through compartment walls is a significant life-safety deficiency.

Frequently Asked Questions

Is expanding foam acceptable for fire stopping?

Standard polyurethane expanding foam (the type in a can from a builders' merchant) is not classified as a fire-stopping product. It has very low fire resistance and will burn. Specific fire-rated intumescent foams exist (e.g., Hilti CF 912, 3M Fire Barrier 1000 NS) but these are much more expensive and have specific application requirements. Always use the correct certified product.

Do I need to fire-stop plastic pipe in a single-dwelling house?

Where the pipe passes through a compartment element (e.g., party wall, floor between separate flats), yes — intumescent collars are required. Within a single-dwelling (e.g., pipes running between floors of the same house where neither floor is a compartment boundary), fire stopping is not strictly required by Part B, though fire sealant to close the annular gap is good practice.

How do I access the back of a wall to install fire stopping?

For intumescent collars on plastic pipes, access to both sides is not needed — the collar is installed on one face and the wall penetration is sealed on the other with sealant only (or from the accessible side with an approved single-sided system). Always follow the product's certification sheet for single-sided vs double-sided installation — not all products are rated for single-sided use.

Who is responsible for fire stopping — the plumber, electrician, or builder?

Each trade is responsible for the penetrations they make. The plumber fire-stops pipe penetrations; the electrician fire-stops cable penetrations. The main contractor (if there is one) has an overarching responsibility to ensure all penetrations are closed. A building control inspection at the right stage should check this, but in practice many penetrations are hidden before inspection.

Regulations & Standards