Separation of flues from combustible materials

25 mm

Flues must be adequately separated from combustible materials to prevent fire hazards, typically requiring a minimum clearance of 25mm

Flues must be adequately separated from combustible materials to prevent fire hazards, typically requiring a minimum clearance of 25mm. When flues pass through combustible elements like walls, floors, or roofs, a non-combustible sleeve is necessary, maintaining a 25mm air gap or using non-combustible insulation within that gap. This separation is measured from the outer surface of the flue wall or the inner wall of multi-walled products.

Basis in regulation

Part J
Approved Document J
Read the source

How to check it on site

Confirm minimum 25mm clearance between flue pipe outer surface and any combustible materials, and verify non-combustible sleeving where flues pass through combustible structures.

Common failure

Insufficient clearance between flue pipes and combustible timbers or plasterboard, posing a fire risk.

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

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