Last edited 24 Jul 2018

Cavity barrier

Approved document B of the building regulations, Fire Safety, defines a cavity barrier as:

A construction, other than a smoke curtain, provided to close a concealed space against penetration of smoke or flame, or provided to restrict the movement of smoke or flame within such a space.

Where a concealed space is:

A space enclosed by elements of a building (including a suspended ceiling) or contained within an element, but not a room, cupboard, circulation space, protected shaft or space within a flue, chute, duct, pipe or conduit.

As most buildings contain a multitude of concealed cavities and voids within walls, floors, ceilings and roofs, effective cavity barriers are essential to restrict the spread of smoke or flames. If there is a fire, the intumescent material which makes up the barrier will expand, sealing off the gaps.

Cavity barriers are typically pieces of fire-stopping material which are fitted within building cavities - horizontally at each floor, and vertically at each party wall. This assists with fire compartmentation.

The location of each cavity will determine the appropriate maximum distance between the barriers, as well as the class of surface which is exposed within the cavity. The performance of cavity barriers much be maintained throughout the lifecycle of a building, with consideration given to any building movement due to subsidence, shrinkage, and so on.

Cavity barriers must be fitted tightly to rigid construction, or suitably fire-stopped at a junction between itself and construction to which it cannot be fitted so tightly (e.g. slates, tiles, sheeting, etc.). The likelihood of the fixings failing in the event of a fire should be carefully considered. Services that may run through a barrier should be fire-stopped using appropriate materials.

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