External chimney breast air spacing
40 mm
for the presence of a minimum 40mm air space or equivalent non-combustible separation within external chimney breasts where thickness is less than 200mm.
External chimney breasts, particularly those with a masonry inner leaf, require specific air gaps or non-combustible material separation. A minimum 40mm air space is necessary where the chimney is less than 200mm thick to prevent thermal bridging and ensure fire safety.
Basis in regulation
How to check it on site
Check for the presence of a minimum 40mm air space or equivalent non-combustible separation within external chimney breasts where thickness is less than 200mm.
Common failure
Failure to provide adequate air spacing within external chimney breasts, leading to potential heat transfer issues or dampness.
Last updated 28 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.