What Is Fire Compartmentation and How Should It Be Maintained?

Quick Answer: Fire compartmentation is the division of a building into enclosed sections using fire-resisting elements — walls, floors, and protected shafts — to limit the spread of fire and smoke, allowing occupants time to escape and firefighters time to operate. Requirements are set out in Approved Document B (ADB) of the Building Regulations and BS 9999:2017, with compartment sizes and fire resistance periods varying by building type and use.

Summary

Fire compartmentation is one of the foundational principles of passive fire protection. By constructing physical barriers from fire-resisting materials, a building is divided into compartments that contain a fire for a defined period — typically 30, 60, 90, or 120 minutes. This containment limits fire and smoke spread, protects means of escape, and gives the fire and rescue service a stable structure to work in.

Every opening or penetration in a compartment wall or floor — for pipes, cables, ducts, structural beams, or doors — is a potential breach point. The integrity of the compartmentation system depends entirely on every penetration being properly sealed with appropriate fire-stopping products. A single unsealed cable penetration can undermine the protection of an entire floor in minutes.

Compartmentation surveys have become an important part of the fire safety landscape since the Grenfell Tower fire in 2017. Many buildings that were considered compliant have subsequently been found to have significant hidden breaches — often introduced during maintenance, refurbishment, or M&E installation work by contractors who either did not know or did not care about the fire-stopping implications of their work.

Key Facts

Quick Reference Table

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Building Purpose Group Max Compartment Floor Area (ADB) Min. Fire Resistance
Residential (flats, over 2 storeys) No limit (each flat = 1 compartment) 60 min
Residential institutional (care homes, hospitals) 2,000 m² 60 min
Office 2,000 m² 60 min (sprinklered: 4,000 m²)
Shop / commercial 2,000 m² 60 min (sprinklered: 4,000 m²)
Assembly and recreation 2,000 m² 60 min
Industrial (normal) 7,000 m² 60 min
Industrial (sprinklered) 14,000 m² 60 min
Storage / general purpose (normal) 4,000 m² 60 min
Buildings over 30m height Reduced compartment sizes apply 90–120 min

Floor areas are for a single storey compartment. Multi-storey buildings have different volume limits. Always verify against current ADB and structural engineer's specification.

Detailed Guidance

Fire-Resisting Elements

Compartment walls separate adjacent compartments on the same floor. They must run full height from floor slab to the underside of the floor or roof slab above — not just to the suspended ceiling. A common (and critical) error is fire-rated walls that stop at the ceiling tile level, leaving the ceiling void open above. The void above a suspended ceiling is part of the compartment and must also be protected.

Compartment floors separate compartments on different storeys. In residential buildings, each flat is typically treated as its own fire compartment, with the floor and ceiling construction providing the separation. This is why the ceiling of a flat below must be maintained with the same fire resistance as the compartment floor — it is part of the system.

Protected shafts enclose services, stairs, and lifts that pass through compartment floors. The enclosing walls of a protected shaft must match or exceed the fire resistance of the floors they penetrate, and every opening into the shaft (doorways, access panels, service entries) must be fire-resisting and self-closing.

Cavity Barriers

Concealed spaces — the voids above suspended ceilings, within wall cavities, between the inner lining and the external cladding — provide channels through which fire and smoke can bypass compartment walls. Cavity barriers are used to subdivide these spaces and prevent this bypass.

ADB requires cavity barriers:

Cavity barriers must achieve at least 30 minutes' fire resistance (integrity) and must be mechanically fixed and abutted tightly to all adjacent construction elements. In practice, cavity barriers are frequently found to be unsealed at junctions, compressed out of shape, or missing entirely where ducts and pipes pass through them.

Sealing Service Penetrations

Every penetration through a compartment wall or floor — a water pipe, gas pipe, electrical cable bundle, mechanical duct, or structural element — must be sealed with an appropriate fire-stopping product. The choice of product depends on:

Common fire-stopping products:

Penetration Type Typical Product Notes
Steel pipe (up to 160mm) Intumescent pipe collar Clips around pipe; expands on heat to crush pipe and seal gap
Plastic pipe (soil, waste) Intumescent pipe collar Critical — plastic melts, collar seals the void
Electrical cable bundles Intumescent mastic + fire batt Pack tightly; cables cannot be added later without re-sealing
Large duct (fire-rated) Fire damper + frame Damper closes automatically; requires annual testing
Large duct (unrated) Ablative coating or enclosure Duct must be made fire-resisting
Structural beam through wall Intumescent mastic + mineral wool Must achieve full thickness seal
General gaps (small) Intumescent mastic BS EN 1366-4 tested products
General gaps (large) Fire batt / composite sheet Product selection depends on test evidence

All products must be used in accordance with their third-party test evidence. Using a product outside its tested application (e.g., a collar rated for 100mm pipes on a 160mm pipe) renders the installation non-compliant regardless of how well it is fitted.

Third-Party Certification

Fire-stopping products must have credible evidence of performance. The most robust route is third-party certification — products and installation systems tested and assessed by independent bodies to:

Using FIRAS or IFC-certified contractors provides documented evidence of competency and is increasingly required by building owners, insurers, and enforcement authorities.

Common Breaches Found During Surveys

Compartmentation surveys — particularly intrusive surveys that open up ceiling voids and wall cavities — routinely find:

  1. Unsealed cable penetrations — the single most common finding; IT and electrical contractors drilling through compartment walls without fire-stopping
  2. Missing or displaced cavity barriers — particularly at compartment wall junctions with ceiling voids
  3. Compartment walls that stop at suspended ceiling level — leaving the ceiling void open
  4. Incorrect pipe collars — wrong size, wrong product, or not clipped correctly
  5. Redundant penetrations — old pipe or cable runs abandoned but gaps not sealed
  6. Mechanical duct penetrations without fire dampers — particularly in older buildings
  7. Fire-stopping applied over previous fire-stopping — successive remediation attempts without clearing the original failed products
  8. Fire-resisting walls and floors with unrated access panels — access hatches and inspection covers that are not fire-rated

Carrying Out a Compartmentation Survey

A visual compartmentation survey (walking the building) will identify obvious issues — propped fire doors, missing intumescent strips, visible gaps around pipes. However, the most serious breaches are typically hidden within ceiling voids and wall cavities. A thorough compartmentation survey requires:

For higher-risk buildings, a Type 4 Housing Health and Safety Rating System (HHSRS) inspection or a fully intrusive compartmentation survey (involving some destructive investigation) may be required to give confidence in the findings.

Frequently Asked Questions

Does compartmentation apply to existing buildings or only new ones?

Building Regulations apply at the point of construction or material change of use. However, the RRO places an ongoing duty on the responsible person to maintain fire precautions in an effective state — and compartmentation is a fire precaution. Where a fire risk assessment identifies breaches in compartmentation, those breaches must be remediated regardless of when the building was built. There is no grandfather clause for legacy defects once they have been identified.

Who is responsible for maintaining compartmentation in a block of flats?

The responsible person for the common parts (typically the freeholder or managing agent) is responsible for the compartmentation of common areas and the structure. Individual flat owners or tenants are responsible for not breaching compartmentation within or from their flat — for example, by drilling holes in walls for cables without fire-stopping. In practice, this means managing agents must educate residents and conduct periodic surveys.

Can I use any intumescent mastic to seal a pipe penetration?

No. Intumescent mastic products are tested for specific applications — particular gap sizes, substrate types, and penetration types. A product tested for sealing 32mm plastic pipe penetrations in a 100mm block wall may not be valid for a 110mm soil pipe in a drywall partition. Always check the product's third-party test evidence or BBA certificate against the specific application. If in doubt, contact the manufacturer's technical team.

How do I know if a fire batt is correctly installed?

Fire batts (mineral wool or intumescent batts) must completely fill the annular gap around a penetration, be cut to the correct dimensions (no more than 25mm compression as a general guide — check manufacturer guidance), and be secured in position. The batt must sit flush with both faces of the wall or floor element, with any intumescent mastic applied to the exposed face. The surrounding structure must be intact — batts installed into crumbling plasterboard will not perform.

Regulations & Standards