Why Victorian stacks need regular attention

A Victorian chimney stack is one of the most exposed parts of a house and, all too often, the last thing anyone thinks to look at. A typical stack on a terraced or semi-detached property rises a metre or more above the roofline, frequently only one brick thick, with three or four flues separated by thin dividing walls. Everything above the roof covering is battered by driving rain, frost, wind and summer sun from every direction.

Because the stack sits above the waterproof layer of the roof, a small failure high up can quietly admit water into a flue, and from there into ceilings, chimney breasts and plaster well below. The first sign is usually a damp patch, a brown tide mark, blown plaster or a scattering of white salts on an upstairs wall — long after the actual defect appeared. Checking the stack twice a year is one of the cheapest pieces of maintenance a period property owner can carry out.

Pointing: keep it lime and keep it sound

Original Victorian stacks were pointed in lime mortar, which is soft, slightly flexible and breathable. It allows the stack to move gently with temperature changes and lets trapped moisture escape. Cement pointing does the opposite: it is harder than the bricks it surrounds, traps water behind it and forces that moisture out through the brick faces, which then spall and crumble.

• Look first at the most weather-beaten face — usually the south-western side in most of the UK. Receding joints, crumbling mortar and hollow-sounding bricks are the warning signs.
• Flakes of brick face lying on the roof or collecting in the gutter below the stack are a clear indication that repointing is overdue.
• Good repointing means raking out to roughly twice the width of the joint, typically 15–20mm, brushing out the dust, damping the brickwork and packing in a lime mortar matched to the original — usually a natural hydraulic lime or lime putty mix with sharp sand.
• Joints should finish flush or very slightly recessed. Proud, feathered edges simply break away in the first hard frost.

Never allow anyone to point a Victorian stack in cement, and be wary of any suggestion to seal the whole stack with a waterproof coating. Both trap moisture and accelerate the damage they claim to prevent.

Flashings and leadwork where the stack meets the roof

The junction between stack and roof covering is the most common source of leaks in older properties. Look for stepped flashing chased into the brickwork on the sloping sides, an apron flashing at the front, and — on a pitched roof — a back gutter behind the stack. All of it should be lead.

Typical failures include cracks in the mortar chase, splits in the lead from years of thermal movement, and gaps at corners where the metal is dressed over the tiles. The back gutter is the classic culprit: it collects everything the roof sends down, and a small hole there will show up as a damp patch some distance away. Repairs mean recutting the chase to a decent depth, fitting new lead of an appropriate thickness, securing it with lead wedges and clips, and pointing the chase neatly. Tiles immediately around the stack should be checked at the same time, since bedding and mortar fillets also perish.

Pots, cowls and ventilation

Clay chimney pots are usually bedded in mortar and can work loose over decades. A cracked pot, or one that has shifted, lets rainwater straight down the flue. Where a pot is broken, replace it rather than patching around it.

• Fitting a cowl or vented cap keeps rain and nesting birds out. Jackdaws and starlings are particularly fond of open flues.
• Any flue still serving a gas appliance must have the correct terminal and must never be blocked — this is a carbon monoxide risk, not a draught problem.
• Disused flues should be capped in a way that still allows air to circulate, so the stack can dry out and internal damp is kept at bay.

Crowns, haunching and blocked flues

The crown is the mortar haunching around the base of the pots, and it is the stack's own little roof. Once it cracks, water runs directly down the flues and appears internally as damp staining, crumbling plaster and that distinctive musty smell in an upstairs room.

A sound crown should slope gently away from the pots and finish with a drip so water cannot track back underneath. Where the existing mortar is perished, it is usually worth rebuilding the crown properly rather than smearing on a patch. If the flues are no longer used, a ventilated cap over each one is a sensible investment.

While you are looking up there, note any ivy or valerian growing out of joints, and any birds' nests. Both hold moisture against the brickwork and should be cleared with care, outside the nesting season.

A practical inspection routine

Twice a year is enough: once in spring, once in early autumn before the winter storms arrive. Stand well back in the garden with binoculars, and use the zoom on a phone camera to record what you see so you can compare year on year. Check gutters and downpipes at the same time, since overflowing rainwater often does more harm to a stack than direct rainfall.

• Have solid fuel flues swept annually — a good sweep will spot problems inside as well as out.
• If scaffolding is needed, do everything in one visit: pointing, flashings, crown, pots and any roof repairs. Access is the biggest single cost.
• On a shared stack, speak to your neighbour early, as work may fall under the Party Wall etc. Act 1996.
• For listed buildings or properties in a conservation area, check with the local planning authority before altering the appearance of a stack.
• Never work from a ladder above gutter level. Use a proper tower or scaffold, or bring in a competent roofer.

Look after the stack and it will quietly do its job for another century. Ignore it, and the repair bill will arrive indoors, in the middle of a wet January.