Chimney Stacks & Flues

Chimney stacks and flues carry combustion gases from appliances out of the building. The element covers masonry stacks with their pots, cappings, flaunching and flashings; flue liners (clay, concrete, pumice, stainless steel); prefabricated metal flues; boiler and generator flues on commercial buildings; kitchen exhaust ducts; and industrial stacks. It includes decommissioned stacks, which still stand above the roof and still need to be stable and weathertight.

Commercial and institutional buildings carry flues of their own: boiler flues rising through risers, generator exhausts, kitchen exhaust ducts, laboratory fume exhausts and, on older industrial and hospital sites, tall free-standing stacks. These follow appliance and fire codes as much as building codes, and their failures show up as fire, fume and condensation problems as well as fabric problems.

What shortens life:

• Exposure on all faces: stacks get wetter than walls and dry less.
• Flue gases: sulfates from coal and oil, acid condensate from modern efficient appliances, tar from smouldering wood.
• Cement repairs on lime-built stacks.
• Missing damp-proof trays where the stack passes through the roof.
• Wrong liner for the fuel.

Lean from sulfate attack

Sulfates from flue gases condense in the flue lining and mortar, more on the wetter side of the stack. Mortar expands on that side, and the stack bends away from the prevailing weather. A leaning stack can be stable for decades, but a large lean, cracked joints and loose upper courses mean it may fall in a storm.

Weathering

Frost and salts spall bricks; pointing erodes; flaunching around the pots cracks and lets water into the flue; pots loosen.

Flashing failure

Apron, step and back gutter flashings lift and pull from their chases; back gutters fill with debris. Water runs down inside the stack and into the building, often appearing on chimney breasts below.

Liner failure and gas leakage

Cracked or open-jointed liners, and unlined flues with decayed parging, let gases and smoke into rooms, roof spaces and adjoining dwellings. Carbon monoxide leakage is the life-safety failure.

Tar and condensate staining

Wet tar from wood, and acid condensate from efficient gas boilers in unlined flues, soak through to the inside and stain chimney breasts brown; the staining is a sign of an unsuitable flue.

Removed chimney breasts

Chimney breasts removed from lower floors without supporting what remains above leave the stack and upper breasts hanging on the masonry. Gallows brackets or steel beams are needed; without them the stack can drop.

Kitchen exhaust fires

Grease builds up in commercial kitchen exhaust ducts and ignites; ducts without access for cleaning are the problem.

Tall stacks

Tall masonry stacks are held by steel bands that corrode and loosen; their tops weather fastest; lightning protection conductors corrode and detach. Steel stacks corrode internally from acid condensate. These are structural items needing periodic engineering inspection.

Chimney fires

Creosote and soot build up in wood and coal flues and ignite. A chimney fire cracks liners and masonry and can spread to timbers built into or against the stack.

From the street and from aerial imagery

• Lean visible by sighting against vertical lines; cracks in stack joints.
• Spalled bricks, eroded pointing, cracked flaunching, missing or loose pots.
• Lifted flashings; vegetation on stacks.
• Pots open on decommissioned stacks; cowls or caps present.

In the roof space

• Damp, efflorescence and staining on the stack where it passes through.
• Mortar fallen from joints; holes in the masonry.
• Chimney breast removed below with no support.

Inside the building

• Brown staining and damp on chimney breasts; smoke smell; soot.
• Carbon monoxide alarm activations.
• Missing breasts on lower floors.

In the records

• Chimney sweep and CCTV inspection records.
• Gas safety certificates; appliance register.
• Liner installation certificates.
• Decommissioning records.
• Building approvals for breast removal.

• Timbers in the roof space built into or touching the stack, which are a fire risk in working flues.
• Scorching or cracking on breasts after a chimney fire.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A building surveyor inspects the stack; a structural engineer assesses lean; a licensed gas fitter, chimney sweep or flue specialist inspects the flue.

• External inspection from the roof or a platform.
• Plumb measurement of the stack.
• CCTV inspection of the flue.
• Smoke test for leakage.
• Appliance and flue compatibility check against the appliance's requirements.

The report should state stack plumb and condition, flashing condition, flue lining condition from CCTV, whether each flue suits the appliance connected to it, and the status of disused flues.

• Moisture and salts survey of chimney breasts, to separate rain entering at the top, condensate from appliances, and hygroscopic salts left by years of coal and wood burning, which draw moisture from the air and keep plaster damp after the source has stopped.
• Support check wherever breasts have been removed: look in the roof space and the floor below for gallows brackets, steel beams or corbelling carrying the remaining masonry, and for approvals.
• Carbon monoxide testing in rooms served and in adjoining dwellings with appliances running, since leaking flues often affect neighbours first.
• Kitchen exhaust inspection of the duct from hood to discharge through access panels, measuring grease deposits against the cleaning standard.

• Repoint and reflaunch in lime-based mortar for lime-built stacks.
• Rebuild the upper courses or the stack from the roof line.
• Renew flashings with a damp-proof tray.
• Reline with a liner suited to the appliance.
• Cap and ventilate decommissioned flues.
• Remove redundant stacks above the roof with approvals.
• Support stacks over removed breasts with gallows brackets or steel beams.

• Liners matched to the fuel: suitable stainless grades for wood and oil, condensate-resistant liners for condensing gas appliances, and cast-in-place pumice or ceramic liners where an old stack needs structural support as well.
• Cowls and bird guards on working flues; capping with ventilation on disused flues, and a vent at the base of disused breasts so the flue can dry.
• Damp-proof trays built in where stacks are rebuilt at roof level.
• Bands, ladders and lightning protection repaired on tall industrial stacks.
• Grease duct programme: access panels at changes of direction and a cleaning frequency set by the cooking volume.

Repairs are lump sums per stack; rebuilds by height; relining per flue. Scaffold dominates on roofs. Flue inspections and sweeping recur.

Rebuild is lifecycle; repairs are repairs and maintenance; flue compliance is code compliance.

Where a stack is redundant, removal above the roof can be cheaper than repair over the long term, subject to heritage and planning controls.

A stack rebuild carries scaffold, demolition to below the defect, rebuild, new pots and flaunching, flashings and tray, and often relining in the same visit. Capping a disused flue is a small item per flue but needs the same access. Support for stacks over removed breasts is a structural lump sum and a safety item. Kitchen exhaust cleaning recurs at the frequency the standard sets for the cooking type.

• Australia. Solid fuel appliances follow AS/NZS 2918 and gas appliances and flues AS/NZS 5601.1; commercial kitchen exhaust follows AS 1668.1.
• United Kingdom. Approved Document J covers combustion appliances and fuel storage; the Gas Safety (Installation and Use) Regulations 1998 require annual landlord gas safety checks; BS 5440 covers gas flues.
• United States. NFPA 211 governs chimneys and venting systems, with levels of inspection; NFPA 54 covers gas, NFPA 96 commercial kitchen exhaust; International Building Code Chapter 21 covers masonry chimneys.

Removing a chimney breast or stack is structural work needing building approval in all three jurisdictions. In Britain, work to a stack shared with a neighbour usually needs a party wall notice under the Party Wall etc. Act 1996. Heritage controls in Australia, Britain and the United States frequently protect chimneys as part of a building's character, which can rule out removal and set the materials for repair.

• Sweep and CCTV inspection records.
• Gas safety certificates and the appliance register.
• Liner installation certificates.
• Decommissioning records.
• Approvals and engineering for chimney breast removal.

• Approvals and engineering for any breast or stack removal.
• Carbon monoxide alarm records and any incidents.
• Kitchen exhaust cleaning certificates.
• Heritage listing where chimneys are protected.