Research and analysis

Fire safety: Trigger thresholds

Research on the trigger thresholds used in Approved Document B, including the height threshold for the ban on combustible materials in or on external walls.

• From: Health and Safety Executive
• Published: 22 December 2025

Applies to England

Documents

Fire safety: Trigger thresholds (excluding appendices)

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Fire safety: Trigger thresholds (full report)

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Details

The research assesses the current provisions related to trigger and threshold levels in Approved Document B (fire safety) and regulation 7(4) of the Building Regulations 2010.  

The researchers reviewed whether the thresholds in the document are based on relevant and up-to-date evidence and provide designers with adequate guidance. 

The project comprised two workstreams: 

• workstream A reviewed all the trigger and threshold levels 

• workstream B examined the height above which buildings should not include combustible external walls or cladding systems

Background

The government commissioned the research as part of the technical review of Approved Document B (fire safety).  

BRE Global, Design Fire Consultants, the University of Edinburgh, and Anamil Ltd carried out the work, and the findings and recommendations do not represent government policy.

Published 22 December 2025