The short answer
Chimneys crack because of frost and water in the masonry, movement in the wall below, a flue that has heated and cooled over many years, or a stack whose mortar has perished until it started to lean. Hairline mortar cracks are repointed. Cracks through the bricks, or ones that are widening, need a closer look and may mean rebuilding part of the stack.
Why chimneys crack in the first place
Think of a stack as a narrow brick tower standing alone above the roof, with weather hitting every face. Few parts of a house are more exposed. Rain soaks into the mortar, frost expands that water on a cold night, and across many winters the joints open and the bricks begin to part company.
Heat adds to it. Flue gases warm the inside of the masonry while the outside stays cold, and the uneven movement stresses old mortar. Sulphates carried in soot can attack lime and cement from within, which is why some stacks that were once upright start to bow away from the prevailing wind.
Movement lower down matters as well. If the wall beneath settles, or a chimney breast has been altered in the house, the crack can begin indoors and appear on the roof. So when damage looks out of proportion to the weather, the roof inspection includes a look at the room beneath, since the cause may sit there.
Reading the crack from the garden
You can learn a good deal without leaving the ground. A fine line that follows the mortar joints is usually a weathering crack and the mildest kind. A stepped crack climbing diagonally through several courses suggests movement. A crack running straight through the bricks, or one wide enough to slide a coin into, deserves an early visit.
Notice the neighbouring signs too. A stack leaning one way, a gap opening where the chimney meets the roof, pots that have shifted, or brown marks on the wall below the stack indoors all suggest water is entering. Our page on a blocked chimney explains why a flue that is neither used nor ventilated can make the moisture problem worse.
Photograph the crack with your phone and repeat the picture a month later against the same background. If it has grown between the two dates, send both to us. That tells us whether the movement is still active, and an active crack changes the repair we recommend.
The repair options, smallest first
The lightest repair is raking out the cracked joints and repointing them using a mortar chosen to match the brick. A cement-rich mix on soft old brick is a frequent mistake, because the brick then takes the weather and erodes. A more breathable mortar lets the joint take the wear instead. Protection at the top is part of the answer, and our page on capping a chimney covers it.
Where individual bricks have split or crumbled, they may be cut out one at a time and replaced with bricks of a similar colour and texture. If the crack runs through a longer length, the usual route is taking down the affected courses and rebuilding them, keeping the sound brick below. A stack that leans, or has failed at its base, is a rebuild from the roofline upwards.
Stainless ties or a flue liner can reinforce a stack, but neither cures one that is damp or still moving. We keep the repair to what the fault needs, and the survey photographs show the reasoning. Often that means pointing and a handful of bricks, leaving the rest of a sound stack alone.
What a cracked chimney repair costs
National guides give ranges rather than single prices. MyBuilder's chimney repair guide, updated in August 2026, puts repointing at around £750, within a range of £500 to £2,000 depending on the stack and the house, and a full stack rebuild at £2,000 to £10,000 or more. Those numbers describe the market generally and say nothing firm about your own chimney.
Access moves the total more than most people expect. MyBuilder's scaffolding guide gives £400 to £900 for chimney-height scaffolding, and a steep roof or tall stack can make a simple ladder arrangement unsafe. Our notes on chimney scaffolding explain when it is needed.
After the survey you receive a written quote that gives one price for everything. If you hold it up against another firm's, ask both whether VAT and scaffolding are in the figure, since those two explain most differences between similar-looking quotes.
When a crack should not wait
Most cracks develop slowly, but a few justify a prompt call. Loose bricks at the top, a stack that has visibly shifted after a storm, or any piece of masonry that could drop onto a path, a neighbour's garden or a conservatory roof is a safety matter. We prioritise it, make the stack safe first and then agree the permanent repair with you.
Leave the fireplace unlit beneath a stack with a wide crack until a roofer or sweep has checked it. A cracked flue can let smoke or fumes into the walls or loft, and a sweep can tell you whether the liner is intact. Our guide to chimney repointing shows how small cracks are tackled before they grow.
After the repair
A repaired stack still needs an occasional glance. Check it after the first hard frost and after any big storm, and compare against the photographs from your survey. New pointing should look uniform and slightly recessed, with no dark gaps along the bricks. If a new crack appears in the same place, the cause was movement rather than weather, and that is the moment to ring us rather than wait.
Repairs are covered by our 10-year workmanship guarantee, which you receive in writing, and nothing is left behind on the site afterwards. The chimney repairs hub lists the other faults we deal with, from flashing to a full rebuild, if the survey turns up something beyond the crack itself.