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

The separating wall has to carry on up to the underside of the roof covering, otherwise the loft is a shared space in a fire.

Basis in regulation

Part B
Approved Document B
Read the source

How to check it on site

Inspect from inside the loft with a light before the plot is handed over.

Common failure

Wall built to ceiling level only, with a gap left along the truss line.

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

Maximum

Maximum unbroken cavity before a barrier is needed

20 m

Concealed cavity in a wall or roof of a dwelling

Maximum

Cavity subdivision in a roof void

20 m

Maximum distance between cavity barriers in a concealed roof space

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