Cavity subdivision in a roof void

20 m

Maximum distance between cavity barriers in a concealed roof space

Every figure for this topic is on one page: Roof void ventilation, access and fire compartmentation

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Large concealed roof spaces are divided so a fire in the void cannot spread across the whole terrace or block.

Basis in regulation

Part B
Approved Document B
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How to check it on site

Check the barrier line matches the drawing before the loft is closed up.

Common failure

Barriers omitted over corridors and shared roof spaces where the void runs on unbroken.

Effective from 22 August 2026. Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.

Related dimensions at this stage

Minimum

Fire barrier at the party wall and roof junction

Continuous to the underside of the roof covering

Gaps at truss positions and at the tile battens closed with a fire-resisting material

Maximum

Maximum unbroken cavity before a barrier is needed

20 m

Concealed cavity in a wall or roof of a dwelling

Minimum

Fire resistance of a wall separating dwellings

60 minutes

Applies to the wall and to its junctions with floors, roofs and external walls

Maximum

Maximum pipe diameter through a compartment wall or floor

40 mm

Nominal internal diameter for a non-fire-rated pipe; larger pipes need a tested sleeve, collar or non-combustible material

Minimum

Doors onto a protected stairway in a three-storey house

Fire doors to every habitable room opening onto the stair

Applies where any floor level is more than 4.5 m above ground level

Minimum

Fire resistance of a compartment floor between flats

60 minutes

Includes the ceiling line, service voids and any penetrations through it