The short answer
A cracked chimney stack is not always dangerous, but it should be looked at. A hairline crack in a mortar joint is usually weathering. A wide crack, a lean, shifted bricks or stones, a rocking pot or a crack that has grown since the last storm can mean the stack is unstable and could shed masonry. Treat any sign of movement as urgent and keep people away from below.
Cracks that are cosmetic and cracks that are not
A stack is a tall, thin column standing in the weather, so it is bound to show some age. Fine cracks in the mortar joints, crazing in an old render coat and a little loss of pointing are common on stacks that have stood for a long time. They need watching and in time a repair, but they do not mean the stack is about to fall.
The cracks to take seriously look different. A stepped crack that follows the joints and runs down several courses suggests the masonry is separating. A crack running through bricks or stones themselves, rather than around them, points to something that has been loaded or pushed. A gap you can fit a coin into, a face that bulges, a stack that leans away from the ridge or a pot that moves when the wind blows all mean the structure is no longer acting as one piece.
One useful test is whether the crack is new. Photograph it with a coin or ruler beside it, and look again after the next stormy week. A crack that has not changed is a maintenance item. A crack that has opened is a warning.
What makes a chimney stack crack in the first place
Water is behind most stack problems. Rain gets into a stack where the pointing has eroded, where the flaunching around the pots has cracked, or where lead flashing has lifted at the base. Once the masonry is saturated, frost expands the water in the joints and pushes the faces apart. Repeated over winters, that opens the cracks you see from below.
The flue plays its part too. Gases from a boiler or a solid fuel fire are damp and slightly acidic, and where a flue has no liner they can attack the mortar on the inside of the stack. The side facing the prevailing weather then dries more slowly than the sheltered side, and the stack can bend away. That is one reason a stack can lean even when the outside joints look reasonable.
Poor earlier repairs cause cracks as well. A hard cement mortar used on soft old brick or Bath stone does not flex with the masonry, so the stone around it flakes while the cement stays put. In its comments on a listed Walcot stack, the Bath Preservation Trust noted that cement might have made the stone damage worse. Movement in the roof structure below, ivy rooting in the joints and wind load on a tall stack add to the list.
How to check a stack safely from the ground
Climbing is not needed to learn something. From the garden or street, in good light, study the stack through binoculars or a long phone zoom. Run your eye up each face. Look for gaps in the mortar, a change in the angle of the courses, stains below the pots and missing pieces at the top.
Then check the usual places where fallen material turns up: the gutter beneath, the flat roof or lean-to below the stack, the path and the garden bed. Small pieces of mortar and the odd brick fragment in those places mean the stack is losing material.
Indoors, look at the chimney breast on the upper floor and in the loft. Brown patches, damp or peeling paper beside the breast tell you water is reaching the masonry from above. Do not light fires in a flue you suspect is damaged, and do not climb onto the roof or a ladder to look closer. The top of a chimney is the worst place to find out how loose it is. A close inspection is best done from a platform, which is why chimney scaffolding comes up so often in stack work.
When a cracked stack needs attention straight away
Some cracks cannot wait for a convenient month. If the stack leans, if masonry has fallen, if a pot rocks or the top courses have shifted, treat it as a hazard. Work out where a falling brick would land and stop anyone standing or parking there. A stack above a pavement, a neighbour's yard or a bedroom roof deserves extra caution.
We prioritise this kind of fault. Step one is making the stack safe. That usually means taking down the loose upper courses by hand, or fencing off the area below until a proper repair can be arranged. It does not mean rushing a permanent repair onto unstable masonry.
If the stack is shared with next door, tell your neighbour at once, since the same instability affects them. Where the crack runs close to the roof covering, the flashing may also have been disturbed. The lead is explained on our page on how long chimney flashing lasts, and a stack that has moved often needs it renewed after the repair.
What a repair involves and what it costs
The remedy depends on how deep the problem goes. Where cracks are confined to the joints, the old mortar is raked out and the stack repointed with a mortar chosen to suit the masonry, which means lime for older stone and brick. The full range of stack work is on our chimney repairs page. Where the top courses are loose, they are taken down and rebuilt, with a new flaunching and, if the flue is out of use, a ventilated cap. Our guide to capping a chimney explains that option. A stack that has gone out of line or is badly decayed may be rebuilt to a lower level.
On cost, MyBuilder's 2026 chimney repair guide reports typical UK prices of £500 to £2,000 for repair, with a full stack rebuild at £2,000 to £10,000 or more. The range is wide because of stack height, the stone or brick, and the scaffold needed. That spread is wide and comes from third-party data.
Our free survey, with photographs of your own stack, shows where you sit in that spread. A hairline crack in the render is a different job from a stack that leans or has open joints right through, and the photographs make that difference clear.
