Why are my chimney bricks crumbling?

A chimney stands higher and more exposed than any other part of the house. Rain reaches it from every direction, sun and wind dry one face faster than another, and nothing shelters the top. A sound stack copes with all that. A stack with open joints or a broken cap does not, because water soaks into the bricks and lingers.

Wet brick that freezes can lose its outer face in flakes, and over a run of winters the damage goes deeper. You notice orange dust on the roof, soft bricks that rub away under a thumb, and fragments in the gutters. Sea weather and driving rain keep stacks near the coast wet, so a look from the roof every so often is worthwhile. Where the flakes come off the brick face and the mortar between is still firm, the fix is more likely to be swapping individual bricks than rebuilding. It helps to photograph the stack through binoculars from the road and compare it with a picture from a year or two earlier, if you have one, because slow change is easy to miss.

Crumbling brick is usually a symptom. Water gets in through pointing, the mortar joints, once they have eroded back from the face. It gets in through a cracked flaunching at the top, where the sloping mortar crown meets the pot. It gets in where the lead flashing has lifted at the base. Replace bricks without closing the water route and the new ones start to fail in turn, often within a few winters, so the cause has to be treated first.

That is why we survey the whole stack and the roof junction, with photographs, before proposing anything. Often the most economical repair is the one that stops the water: fresh pointing, a mended crown, a renewed flashing. Our pages on repointing and flaunching describe those repairs, and on an older house the choice of mortar matters, as covered in lime mortar repointing.

Our rule is to recommend the minimum job that genuinely cures the fault. Where most of the brickwork is sound and the joints are simply worn, repointing with the odd badly decayed brick cut out will do. Where the top courses have lost their faces, or the brick is soft right through, the stack is better taken down to good brickwork and rebuilt in matching brick. A stack that has moved or leans is a different problem again, covered on our leaning chimney page.

You receive a plain verdict backed by evidence. We show where the brick has failed, say whether patching it is likely to last, and name the route we would choose. If rebuilding is the only option that will last, that is the recommendation and the photographs give the reasons. That work is explained on our chimney stack rebuild page, and every repair we do sits under chimney repairs. A stack rebuilt to match the old one, in brick that suits the house, should sit on the roofline as if it had never been touched.

A crumbling stack is a hazard to whoever stands beneath it. Loose brick and pot fragments can land on a footpath, on a parked vehicle or over the fence next door. If pieces are already coming down, keep people clear, avoid parking below, and arrange a survey soon instead of waiting for the next gale.

Resist the urge to knock loose material off from a ladder. We make dangerous areas safe first where we can, then repair once weather and access allow. A stack of this height normally needs scaffold, and where it overhangs the highway a scaffold licence from the highway authority is required. Checkatrade's scaffolding cost guide puts chimney scaffold at around £660 per week, a national figure and not a quote. Our page on scaffolding for chimney repairs covers the rest. Check that any quote you receive says whether scaffold and VAT are included.

Repairing a stack like for like changes nothing about how the house appears from outside, so it is rarely a planning matter. Listed buildings differ: works affecting their character need listed building consent, and a conservation area can bring extra controls. Portsmouth's Article 4 directions in parts of Southsea, for example, reach as far as taking down or changing a chimney stack.

On a Victorian or Regency terrace, the chimneys belong to the street as well as the house. Arun's guidance on The Steyne calls chimneys important features of the skyline, and replacement bricks and mortar should match what stands there now. Mention at the outset if the house carries a listing or lies within a conservation area, and ask the council's planning team which rules apply to your street. Our page on chimneys in conservation areas goes into more detail.