Minimum masonry thickness around a flue

100 mm around the flue; 200 mm between flues serving different dwellings

Masonry chimneys built with a suitable liner.

The masonry around a flue, excluding the liner, should be at least 100 mm thick. Where a flue passes beside a flue serving another dwelling, at least 200 mm of solid masonry should separate the two.

Basis in regulation

Part J
Approved Document J
Read the source

How to check it on site

Measure the wall thickness at the chimney breast before plastering, and check the liners are the right way up with sockets uppermost.

Common failure

Chimney cheeks built in half brick with a liner squeezed in, leaving less than 100 mm of masonry around the flue.

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

Minimum

Minimum eaves ventilation for pitched roofs

7,000 mm

For specific pitched roof constructions, a minimum continuous eaves ventilation opening of 7000 mm²/m is a common requirement

Minimum

Minimum strap dimensions for holding down

30 mm

A strap used for holding down purposes, particularly in roof structures, must have a minimum cross-section of 30mm x 2.5mm and be at least 1000mm long to effectively resist uplift forces.

Minimum

Equivalent ventilation areas for a cold roof

10,000 mm²/m at pitched eaves; 25,000 mm²/m at low level on flat or shallow roofs; 5,000 mm²/m at the ridge

Cold roof construction with the insulation at ceiling or deck level.

Minimum

Clear air gap above insulation in a cold flat or shallow roof

50 mm

Cold flat roofs and cold roofs pitched below 15 degrees.

Minimum

Ventilated batten space beneath a roof underlay

25 mm

Roofs designed with a ventilated space beneath the underlay, including counter-battened details.

Minimum

Minimum tile or slate width at a verge

150 mm

All verges on plain tiled, interlocking tiled and slated roofs.