The short answer
It depends on the crack. A hairline crack in a mortar joint is a repair to book, not an emergency. A crack that runs through the bricks, steps across several courses, opens wide or comes with a lean means the stack may be unstable. Keep people away from below it and have it looked at straight away.
Two cracks that look alike from the garden can mean very different things, so the width, the direction and any movement all count.
Hairline cracks in the mortar
Mortar is the soft layer between the bricks, and it is meant to absorb small movements in the stack as it warms, cools and dries. A fine crack that follows a joint line is common as a stack ages and weathers.
Is a hairline crack worth worrying about?
It deserves a note in your diary, and a date for a proper look before the next winter. Such a crack lets rain in, which feeds frost damage over the following winters, and it often goes with other signs of tired pointing: sand in the gutter, a recessed joint, green bloom on the face. Left alone, the gap widens, and each frost prises it open a little further.
The usual cure is repointing, where the old mortar is cut back and new mortar is pushed into the joint. MyJobQuote gives a typical UK range of £450 to £1,200 for chimney repointing (MyJobQuote, Oct 2026). The page on chimney repointing shows how it works.
Take a photograph of the crack from the same spot each month, with a coin held beside it for scale where you can safely do that from a window. If it looks the same, it is not moving. If it grows, the cause is more than weather, and the stack needs a closer look.
Cracks that mean movement
A crack that cuts through the bricks themselves, not only the mortar, belongs to a different order of problem. So does a stepped crack, one that climbs the stack in a diagonal following the joints, and a gap that has opened between the stack and the roof.
What signs say the stack is unstable?
A visible lean, a bulge in one face, a pot that has shifted, bricks out of line, and cracks wider at one end. These point to movement, and movement means the top of the stack is no longer held firmly to the bottom. Long-term water damage, a failed damp-proof tray inside the stack, heavy fixings such as an aerial and old structural movement can all be behind it, and a survey is how the likeliest cause gets found.
Treat a leaning stack as urgent; a leaning chimney has its own page. If a gale caused the damage, the first step is the same: make it safe, then plan the repair.
Until a roofer has looked, keep people, pets and parked cars away from the ground beneath the stack, and do not let anyone use a fire or boiler that vents through it.
Dangerous structures and who pays
Somerset Building Control gives "unstable chimneys" as an example of a dangerous structure, next to loose or falling roof tiles. Under that page, the owner of a dangerous building meets the bill for making it safe.
Who do I call about an unstable stack?
If a stack looks as though it could fall onto a neighbour's land or a public place, tell Somerset Building Control on 0300 303 7790. For your own house, a roofer can make the stack safe by taking the top courses down or by securing them, and the proper repair is planned afterwards.
The lesson from that page is simple. A cracked stack that falls becomes the owner's problem, and acting early costs less than acting after the damage is done.
This is general guidance. Somerset Building Control can advise on a specific building, and an engineer or surveyor can assess structural movement where the cause is unclear.
If you have older photos of the same stack, dig them out. A crack that was not in last year's picture tells its own story.
How the repair gets decided
The choice lies between three routes. A fine crack in the joints can be repointed and the stack kept in service. A small patch of broken brick can be chiselled out and fresh brick set in. A stack that has moved or split through its bricks needs to come down and be rebuilt.
When does repair give way to rebuilding?
When the bricks and the bed beneath them can no longer be trusted to hold. A rebuild costs more at the start, but patching a stack that keeps cracking costs more over the years. The page on chimney rebuilds sets out how that work is done.
We study each crack from a safe position, photograph it so its width and direction are plain, then say which of the three routes fits. Send us any photos you have taken over time, with the dates if you know them. A run of pictures showing a crack that grows makes the decision easier. A written price follows the survey. The full range of stack work is under chimney repairs.
More on cracked stacks
Can I fill a crack in my chimney myself?
Squeezing filler along a crack from the ground does not reach the joint and does not stop the cause. Work on the stack itself means working at height. Ask a roofer to look first, particularly if the crack is wide, stepped or on a stack that leans.
Does a cracked chimney always need rebuilding?
No. A fine crack in the mortar is usually repointed, and a few damaged bricks can be swapped for sound ones. Rebuilding is for a stack that has moved, leans, or has split through its bricks, because no patch will hold bricks that are no longer bonded. The survey photos show which of these applies to yours.
The full picture: Chimney repairs in Taunton · More short answers