2.4 Cavities

Mandatory Standard

Standard 2.4

Every building must be designed and constructed in such a way that in the event of an outbreak of fire within the building, the unseen spread of fire and smoke within concealed spaces in its structure and fabric is inhibited.

2.4.0 Introduction

Fire and smoke spread in concealed spaces is particularly hazardous because fire can spread quickly throughout a building and remain undetected by the occupants of the building or by fire and rescue service personnel. Ventilated cavities generally promote more rapid fire spread around the building than unventilated cavities due to the plentiful supply of replacement air. Buildings containing sleeping accommodation pose an even greater risk to life safety and demand a higher level of fire precautions. For these reasons, it is important to control the size of cavities and the type of material in the cavity.

The guidance for protection to cavities should not be assessed in isolation and reference should be made to the guidance to Standard 2.6 for spread to adjoining buildings and the guidance to Standard 2.7 for fire spread on external walls.

A cavity is a concealed space enclosed by elements of a building (including a suspended ceiling) or contained within a building element, but not a room, cupboard, circulation space, stair enclosure, lift well, flue or a space within a chute, duct, pipe or conduit. For the purposes of this guidance, a cavity includes a roof space, a service riser or any other space used to run services around the building.

Reference to surfaces in a cavity is intended to include the surface of the enclosing envelope of the cavity (including insulation material) but excludes timber roof trusses or lintols, joist ends, pipes, conduits or cables.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).

A cavity barrier means any construction provided to seal a cavity against the penetration of fire and smoke or to restrict its movement within the cavity.

In order to inhibit fire spread in a cavity, every cavity within a building should have cavity barriers with at least a short fire resistance duration (see annex 2.A) installed around the edges of the cavity. This includes for example, around the head, jambs and sill of an external door or window opening. A cavity barrier should also be installed between a roof space and any other roof space or between a cavity and any other cavity such as at the wall-head between a wall cavity and a roof space cavity.

However cavity barriers are not necessary at a junction between two cavity walls each comprising two leaves of masonry or concrete at least 75mm thick.

Sealing cavities can sometimes create difficulties, especially where construction techniques rely on through ventilation of the cavity (see Section 3 Environment) or where the detailing should take into account the effect of thermal bridging (see Section 6 Energy).

2.4.2 Dividing up cavities

Cavities should be measured either horizontally or vertically, as the case maybe, along the centre-line of the cavity and not diagonally.

Every cavity should be divided by cavity barriers so that the maximum distance between cavity barriers is not more than 20m where the cavity has surfaces which are non-combustible or low risk materials, or 10m where the cavity has surfaces which are medium, high or very high risk materials.

Exclusions - cavity barriers are not necessary to divide a cavity:

  1. formed by two leaves of masonry or concrete at least 75mm thick, or

  2. in a ceiling void between a floor and a ceiling constructed in accordance with the guidance in clause 2.4.3, or

  3. between a roof and a ceiling constructed in accordance with the guidance in clause 2.4.3, or

  4. below a floor next to the ground where the cavity is either inaccessible or is not more than 1m high, or

  5. formed by external wall or roof cladding, where the inner, outer or other exposed surfaces of the cladding are low risk materials or non-combustible attached to a masonry or concrete external wall or a concrete roof, and where the cavity contains only non-combustible material (see also the guidance to Standard 2.7).

2.4.3 Fire resisting ceilings as an alternative to cavity barriers

Where a ceiling is provided as an alternative to cavity barriers as in clauses 2.4.2b and 2.4.2c, the ceiling should have a short fire resistance duration, and be constructed in accordance with the following recommendations:

  • the ceiling should not be easily demountable

  • openings and service penetrations in the ceiling should be protected in accordance with clause 2.2.9

  • the ceiling lining should be constructed in accordance with the guidance to Standard 2.5

  • the ceiling may contain an access hatch which, when closed, will maintain the fire resistance duration of the ceiling.

Fire-fighters may not be able to apply a water jet from a fire-fighting hose directly onto a fire that has spread within an external wall or onto an external wall. This is because the external wall is either inaccessible or is too high above the ground to be within the reach capability of fire-fighting equipment such as hydraulic platforms or turntable ladders. Therefore, the construction of external walls should not contribute to the development of fire or contribute to fire spread within cavities or vertical fire spread up the facade of the building.

In order to satisfy Standards 2.4, 2.6 and 2.7, the external wall, including any associated cavity barrier or cladding system, should be designed and constructed from:

  1. products that achieve a non-combustible reaction to fire classification in accordance with annex 2.B, or

  2. achieve the performance levels in BR 135, ‘Fire Performance of external thermal insulation for walls of multi-storey buildings’ when read in conjunction with the test methodology in BS 8414: Part 1: 2002 or BS 8414: Part 2: 2005.

However an insulation product need not achieve a non-combustible classification in (a) above where:

  • the insulation product is located between two leaves of masonry or concrete at least 75mm thick, and

  • the external wall is provided with cavity barriers around all openings and at the top of the wall-head.

2.4.5 Supporting structure

A cavity barrier should be fixed so that its performance is not affected by:

  • movement of the building due to subsidence, shrinkage or thermal collapse in a fire of any services penetrating it

  • failure in a fire of its fixings, or

  • failure in a fire of any material or element of structure which it abuts.

However where a cavity barrier is installed in a roof space, there is no need to protect roof members that support the cavity barrier.

2.4.6 Openings and service penetrations

A cavity barrier and a ceiling provided as an alternative to a cavity barrier may contain a self-closing fire door (or a hatch in the case of a ceiling), or a service opening constructed in accordance with the guidance in clause 2.2.9.

2.4.7 Junctions

All cavity barriers should be tightly fitted to rigid construction. Where this is not possible as in the case of a junction with slates, tiles, corrugated sheeting or similar materials, the junction should be fire stopped. See clause 2.2.10 for additional guidance on junctions and clause 2.2.9 for additional guidance on fire stopping materials.

Where a wall, floor or other part of a building which has a fire resistance duration abuts a structure containing a cavity, a cavity barrier should be installed so as to extend the line of the structure. However this is not necessary where the cavity is:

  • formed by two leaves of masonry or concrete at least 75mm thick

  • formed by external wall or roof cladding, where the inner, outer or other exposed surfaces of the cladding are non-combustible or low risk materials and attached to a masonry or concrete external wall or a concrete roof, and where the cavity contains only non-combustible or low risk material (see also the guidance to Standard 2.7)

  • in a wall which has a fire resistance duration for load-bearing capacity only.