Fire Compartmentation in Residential Buildings

What This Requirement Covers

Fire compartmentation is the division of a building into fire-resistant compartments to prevent the spread of fire and smoke. Approved Document B, Requirement B3 addresses internal fire spread (structure) and sets out the standards for compartment walls and floors.

Effective compartmentation buys time for occupants to escape, limits property damage, and enables the fire service to carry out search and rescue operations. It is particularly critical in blocks of flats, where the 'stay put' strategy depends on each flat being a separate fire compartment.

Key Requirements

Houses

1. A separating wall between semi-detached or terraced houses must provide a minimum of 60 minutes fire resistance (from both sides) and extend to the roof covering
2. Wall and floor construction between a house and an integral garage must provide at least 30 minutes fire resistance
3. Any door between the house and garage must be FD30 and fitted with a self-closing device
4. The garage floor must be at least 100 mm below the floor level of the house, or the connecting door threshold must be raised to prevent fuel spillage entering the house

Flats

1. Every flat must be a separate fire compartment, separated from every other flat and from common areas by compartment walls and floors
2. Buildings up to 11 metres: Compartment walls and floors between flats require 30 minutes fire resistance minimum
3. Buildings 11-18 metres: Minimum 60 minutes fire resistance
4. Buildings over 18 metres: Minimum 120 minutes fire resistance for compartment walls and 60 minutes for compartment floors
5. Compartment walls must extend the full height of the building and be carried through any roof void

Fire Stopping

1. All openings and penetrations in compartment walls and floors must be fire-stopped to maintain the fire resistance of the element
2. Service penetrations (pipes, cables, ducts) must be sealed with proprietary fire-stopping products tested to BS EN 1366
3. Fire-stopping must be installed by competent persons and inspected before being concealed
4. Cavity barriers must be provided at compartment wall junctions with cavities in external walls, floor voids, and roof voids

Cavity Barriers

1. Required at the junction of a compartment wall with an external cavity wall
2. Required at maximum 20 metre intervals in concealed roof spaces
3. Required at the edges of cavities, including around openings such as windows and doors in external walls with cavities
4. Must provide at least 30 minutes fire resistance (or be of non-combustible material at least 38 mm thick)

Residential vs Commercial

Commercial buildings have maximum compartment sizes based on building use and height (Table 12.1, Approved Document B Volume 2). For example, an office building over 18 metres is limited to 4000 m² compartment floor area. Residential buildings do not have floor area limits but each dwelling unit must be a separate compartment.

Exceptions and Exemptions

1. In houses of up to two storeys, no compartmentation is required between storeys within the same dwelling (apart from to any integral garage)
2. An alternative fire-engineered approach using BS 7974 may be used to demonstrate equivalent safety with different compartmentation arrangements
3. Where sprinklers complying with BS 9251 (residential) or BS EN 12845 (commercial) are installed, some compensatory relaxations in compartmentation may be agreed with Building Control

Practical Compliance Tips

1. Ensure fire-stopping is installed by operatives holding third-party certification (e.g., FIRAS, IFC Certification)
2. Photograph all fire-stopping installations before they are concealed by finishes
3. Use cavity closers at all window and door openings in compartment walls or walls with fire-rated cavities
4. Check that plumbers and electricians have not breached compartment walls or floors without proper fire-stopping
5. Pay particular attention to service risers in flats; these are common locations for fire-stopping failures
6. Verify that compartment walls are carried through into roof voids to the underside of the roof covering
7. In conversions and refurbishments, survey the existing structure to identify gaps in historic compartmentation