Maximum
England
Maximum unbroken cavity before a barrier is needed
20 m
Concealed cavity in a wall or roof of a dwelling
Greatest dimension of a concealed cavity in a dwelling before it must be sub-divided by a cavity barrier.
Basis in regulation
How to check it on site
Check barriers at the head and edges of cavities, around openings, and where a cavity crosses a compartment line — the run length is the easiest one to miss.
Common failure
Barriers fitted around openings but the long elevation run left continuous.
Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.