Fire Stopping: Penetration Seals, Collars and Compartmentation

Summary

Fire stopping is the single most-failed element in UK construction. Government Building Safety reviews following Grenfell consistently identify breached compartmentation as the most common defect in service voids, risers and ceiling plenums. Anyone running cables, soil pipes or ductwork through a fire-resisting wall or floor is responsible for restoring the fire resistance of that element — and the only legally defensible way to do that is with a tested, certified system installed exactly as the test report specifies.

The principle behind compartmentation (Approved Document B Volume 2, Section 8) is to divide a building into fire-resisting cells so a fire in one compartment cannot spread to another within a defined time period — typically 30, 60, 90 or 120 minutes. Every service that crosses a compartment line is, by definition, a hole in that fire wall. The hole must be filled by something rated to at least the same fire resistance as the wall itself.

A common misconception is that any fire-rated sealant or foam will do. In fact, fire-stopping systems are tested as complete assemblies — pipe, substrate, sealant, depth of fill, annular gap, support method — and the certification is only valid if the installation matches the test report. Expanding builder's foam (B&Q yellow can) is not fire-rated and must never be used in fire-stopping. Even fire-rated polyurethane foam is only valid for the configurations explicitly tested.

This article covers compartmentation principles, the tested-system requirement, intumescent pipe collars, batt-and-sealant cable systems, mortar collars for metallic pipes, fire dampers in ductwork, the fire-stopping register, and the Building Safety Act 2022 implications for Higher-Risk Buildings.

Key Facts

• Compartmentation requirement — Approved Document B Volume 2 Section 8 sets compartment sizes and fire resistance periods by building use and height
• Domestic compartment floors — generally 30 minutes (REI 30) for two-storey houses; 60 minutes (REI 60) for three-storey or more
• Flat compartment walls/floors — typically 60 minutes (REI 60); 90 minutes (REI 90) for buildings over 30m
• Service penetration rule — Approved Document B Volume 2, Paragraph 10.17–10.19: openings for pipes, ducts and cables must be fire-stopped to maintain the fire resistance of the element
• Test standards — BS 476-20:1987 (UK national, still widely cited) or BS EN 1366-3:2021 (European, mandatory under CPR for CE/UKCA-marked products)
• Third-party certification — required for all fire-stopping systems; UK schemes include LPCB (Loss Prevention Certification Board), IFC Certification, Warrington Certification, Certifire
• Intumescent pipe collar — required for combustible (plastic) pipes ≥40mm passing through fire-resisting elements; the collar contains an intumescent compound that expands when heated to ~150°C, crushing the melting pipe
• Plastic pipe ≤40mm exemption — small-diameter plastic waste pipes (≤40mm) in domestic use may be sealed with intumescent sealant to a tested configuration (check certification)
• Metal pipe seal — copper, steel and ductile iron pipes seal with intumescent sealant, fire-rated mortar, or a graphite-based wrap; collar usually not needed below ~110mm
• Cable penetration — single cables sealed with intumescent sealant; cable bunches and trays require a fire-rated batt (Promat Promastop, Hilti CFS-CT, Rockwool) with sealant overcoat
• Ductwork penetrations — fire damper (intumescent or mechanical-spring) installed in the duct at the compartment line; certified to BS EN 1366-2
• Annular gap — the gap between pipe/cable and the surrounding substrate; the tested system specifies a maximum gap (typically 10–25mm)
• Substrate — walls and floors must be tested with the same substrate (masonry, plasterboard, concrete); a seal tested on blockwork is not automatically valid on plasterboard
• Fire-stopping register — mandatory record of every fire-stop installed, including product, location, installer, date, photographs and test certificate; required under Regulatory Reform (Fire Safety) Order 2005 Article 17 (record-keeping) and Building Safety Act 2022 for HRBs
• Higher-Risk Building (HRB) — defined under Building Safety Act 2022 as residential buildings ≥18m or 7+ storeys with two or more dwellings; subject to Gateway approvals and the Golden Thread of information

Quick Reference Table

Detailed Guidance

The Tested-System Principle

The single most-misunderstood concept in fire stopping is that a tested-and-certified product is only valid for the configurations it was tested in. A Hilti intumescent collar tested on a 150mm blockwork wall with a 110mm PVC-U pipe and a 15mm annular gap is certified for exactly that — not for a 100mm plasterboard wall, not for a 160mm pipe, not for a 30mm gap. The test report (Classification Report from a notified body) lists the permitted variations.

When specifying a fire-stop:

1. Identify the substrate — masonry, concrete, plasterboard, concrete floor slab
2. Identify the service — pipe material, diameter, wall thickness (Schedule 40 vs 80, B vs U)
3. Measure the opening and annular gap — must fall within tested range
4. Find a certified system that matches all three — manufacturer's online configurator (Hilti CFS, Promat, Rockwool) is the quickest route
5. Install exactly as the test report specifies — depth of fill, anchoring, both faces or one face

If you cannot find a certified system that matches, you must either change the configuration (move the penetration, change the pipe material, increase the substrate fire rating) or commission a project-specific Engineering Judgement from a third-party body. This is expensive and slow.

Intumescent Pipe Collars for Plastic Pipes

Plastic SVPs and waste pipes melt at relatively low temperatures (PVC-U softens around 80°C, flows at 200°C). Without protection, a melting pipe leaves a clear hole through which fire and smoke pass freely. The intumescent collar contains a graphite-based compound that expands rapidly above ~150°C, crushing the softening pipe and sealing the void.

Installation rules:

• Collar size must match pipe OD exactly — a 110mm collar for a 110mm SVP
• Mount on the fire-exposed side of the wall for one-sided installation, or both sides for two-sided protection (test report specifies which)
• Anchor to the substrate with M6 expansion bolts or the manufacturer's specified fixings — 4 fixings minimum on a 110mm collar
• Annular gap around pipe packed with mineral wool to the depth specified in the test report (typically 50mm)
• Concrete-floor penetrations often use a cast-in collar (e.g. Hilti CP 642) installed before the slab is poured
• Photograph and record every collar with location, product code and installer details for the fire-stopping register

Batt and Sealant Systems for Cable Penetrations

Where cable trays or bunches pass through fire walls and floors, individual sealants are impractical. The standard solution is a fire-rated batt system:

• Open up the penetration to the full width of the cable bunch plus 50mm clearance each side
• Friction-fit fire batts (typically 60mm Rockwool RockClose, Promat PROMASTOP-CC or Hilti CFS-BL) cut to fill the opening, around and between the cables
• Coat both faces of the batt with the matching intumescent ablative coating (typically 1mm dry-film thickness)
• Seal cable-to-batt interface with the matching intumescent sealant
• Tag the assembly with the manufacturer's identification label visible at both faces

Cable derating may be required where the cables pass through the fire-stop — the batt insulates the cables, reducing their current-carrying capacity. BS 7671 Appendix 4 covers grouping factors.

Mortar Collars and Sealant for Metal Pipes

Metal pipes (copper, steel, ductile iron) do not melt at fire temperatures, so the failure mode is different: hot gas conducts along the pipe and the annular gap, not through the pipe itself. The seal is therefore designed to fill the annular gap and prevent gas migration.

• Small bore (≤28mm) — intumescent sealant in the annular gap, 25mm minimum depth
• Medium bore (32–54mm) — intumescent sealant 25–50mm depth, or fire-rated mortar
• Large bore (>54mm) — fire-rated mortar (Promat PROMASTOP, Hilti CFS-CT mortar) packed to full wall thickness, or graphite wrap for high-rated walls
• Insulated pipes — the insulation must either continue through the wall (with appropriate fire-rated lagging) or stop short of the wall with the seal applied directly to the pipe

Fire Dampers in Ductwork

Where heating, ventilation or extract ductwork crosses a compartment line, the ductwork itself becomes a fire path. Two solutions:

• Fire damper — a steel blade held open by a fusible link or magnetic latch, which drops closed at ~70°C (mechanical-spring) or at smoke detection (motorised). Tested to BS EN 1366-2. Installed within the compartment wall thickness, with the damper frame fixed directly to the substrate.
• Fire-rated ductwork — the duct itself is wrapped or constructed to the same fire rating as the compartment, eliminating the need for a damper. Tested to BS EN 1366-1.

Common defect: damper installed in the duct but with the duct passing through the wall without the frame being grouted/sealed to the substrate. The damper then closes correctly but fire passes around the frame.

Common Defects (and Why They Fail)

The Hackitt Review and subsequent Building Safety Regulator inspections of HRBs have catalogued recurring fire-stopping defects:

• Expanding builder's foam (PU foam, non-fire-rated) — burns rapidly, leaves a clear hole. Never acceptable.
• Fire-rated foam used outside its tested configuration — e.g. tested for 25mm gap, installed in a 50mm gap. Fails.
• Insufficient depth of sealant — sealant smeared 5mm deep when the test required 25mm. Fails almost immediately.
• Wrong substrate — collar tested on masonry, installed in plasterboard, with no additional reinforcement. Fails.
• Cables added later — a fire-stop installed correctly, then services pulled through it months later, leaving holes around the new cables. Fire-stop must be reinstated and re-recorded.
• Multiple services through one opening — every combination must be in the test report; mixing pipe materials and cable trays through one penetration usually requires a custom Engineering Judgement.

The Fire-Stopping Register and Golden Thread

Under Article 17 of the Regulatory Reform (Fire Safety) Order 2005, the Responsible Person for a building must maintain records of fire safety measures, including passive fire protection. For Higher-Risk Buildings under the Building Safety Act 2022, this becomes part of the Golden Thread of information held by the Accountable Person.

A fire-stopping register should include, for every penetration:

• Location (room, level, grid reference, photograph)
• Service type (pipe material, diameter; cable bunch size)
• Substrate (wall/floor construction and fire rating)
• Product used (manufacturer, product code, test certificate reference)
• Installer (name, qualification — FIRAS, IFC, BM TRADA installer scheme)
• Date of installation
• Inspection records (date, inspector, defects, remediation)

For HRBs, the Building Safety Regulator can require the register at any time. Inadequate records are themselves a Building Safety Act breach.

Frequently Asked Questions

Can I use ordinary expanding foam from the builders' merchant for fire-stopping?

No. Standard PU expanding foam (B&Q yellow can, Soudal Soudafoam, etc.) is not fire-rated and will burn rapidly when exposed to fire, leaving the penetration open. Even fire-rated PU foams (e.g. Soudal Soudafoam FR or Hilti CF-FE) are only valid for the specific tested configurations — never assume one product covers everything. The shortest path is to use a manufacturer's online configurator (Hilti CFS, Rockwool FirePro, Promat Tunnel) which lists the certified system for your exact substrate, pipe and gap.

Who is responsible for fire-stopping on a typical extension?

The principal contractor under CDM 2015 has overall responsibility for compliance with Building Regulations, including Part B. In practice, the trade installing the service through the compartment line is responsible for restoring the fire resistance — the plumber installs the seal for the SVP, the electrician for the cable, the M&E contractor for the duct. The building inspector or approved inspector will check at completion. On HRBs, the Accountable Person under the Building Safety Act 2022 carries the long-term responsibility for the integrity of the compartmentation throughout the building's life.

What's the difference between BS 476-20 and BS EN 1366-3?

BS 476-20 (1987) is the historic UK national standard for fire resistance testing of building elements; many existing fire-stopping certificates reference it. BS EN 1366-3 (2021) is the harmonised European standard specifically for fire resistance of service penetrations, and is the standard required under the Construction Products Regulation for CE/UKCA marking. New products are tested to BS EN 1366-3; older certificates to BS 476-20 remain valid. The two are not directly equivalent — BS EN 1366-3 includes additional smoke leakage criteria (Sa rating) and integrity/insulation rating in different time bands.

Do I need to fire-stop every cable that passes through an internal wall?

Only if the wall is a fire-resisting compartment wall. Internal partitions that are not compartment walls (e.g. ordinary stud walls between bedrooms within a single dwelling) do not require fire-stopping for service penetrations. The compartment lines are shown on the building's fire strategy drawings. In a typical house: the wall between an integral garage and the habitable space is a compartment wall (REI 30); the floor between a dwelling and the flat above is a compartment floor (REI 60). Service penetrations through these need fire-stopping; bedroom-to-bedroom partitions do not.

How long does an intumescent collar last? Do I need to replace it periodically?

Intumescent collars have an indefinite design life provided they are kept dry and undisturbed. They do not have an expiry date in the way that, say, a fire extinguisher does. However, the Responsible Person under the Fire Safety Order must verify their condition during the periodic fire risk assessment (typically annually). Inspection items: collar still securely fixed; pipe still original (not replaced with different material); no impact damage; no water staining indicating leakage that may have washed out the intumescent compound. Any damaged or disturbed collar must be replaced with a new certified unit.

Regulations & Standards

• Approved Document B Volume 2 (Buildings other than dwellinghouses) — compartmentation requirements, fire-stopping principle, Section 8

• Approved Document B Volume 1 (Dwellinghouses) — domestic compartment floors and walls; protected stair fire-stopping

• BS 476-20:1987 — Methods for determination of the fire resistance of elements of construction (UK national)

• BS EN 1366-3:2021 — Fire resistance tests for service installations — penetration seals

• BS EN 1366-2:2015 — Fire resistance tests for service installations — fire dampers

• BS EN 1366-1:2014+A1:2020 — Fire resistance tests for service installations — ducts

• BS 9999:2017 — Fire safety in the design, management and use of buildings — Code of practice (referenced for non-dwelling fire strategy)

• BS 9991:2015 — Fire safety in the design, management and use of residential buildings — Code of practice

• Regulatory Reform (Fire Safety) Order 2005 — Article 17 record-keeping requirement

• Building Safety Act 2022 — Higher-Risk Building regime, Gateway approvals, Golden Thread, Accountable Person duties

• Fire Safety (England) Regulations 2022 — additional duties on Responsible Persons of multi-occupied residential buildings

• Approved Document B (Fire Safety) Volume 2 — MHCLG official guidance

• HSE: Building Safety Act 2022 — Building Safety Regulator and HRB regime

• Association for Specialist Fire Protection (ASFP) — industry technical guidance on passive fire protection

• LPCB Red Book — Certified Products — directory of LPCB-certified fire-stopping systems

• IFC Certification — third-party certification for fire-stopping installers and products

• Building Safety Regulator Guidance — HRB Gateway and Golden Thread requirements

• compartmentation — compartment sizes, fire resistance periods and where compartmentation is required

• intumescent seals — intumescent products for fire doors, pipes and electrical penetrations

• passive fire protection — overview of passive fire protection systems and design principles

• part b fire — full Approved Document B requirements summary