A chimney stack that leans is easy to notice and hard to judge. From the pavement a slight tilt can look alarming and turn out to be a stack that settled long ago and has not moved since. Another stack, which looks only a little off, may be shifting a few millimetres a year as its mortar breaks down from the inside. The difference matters because a stack is a tall, slender mass of masonry sitting on the roof, and when it fails it comes down on slates, gutters and sometimes the street. This guide covers the causes we look for, above all sulphate attack in old mortar and the long action of weather on joints and flaunching, and then the repair options in the order we would reach for them. It deals with the lean itself. If the stack is badly decayed all the way through, our guide to rebuilding is the next stop, and the cost, flashing and damp questions are covered on their own pages.
Reading the lean from the ground
You can learn a good deal without leaving the garden. Stand square to each face of the stack in turn and look along the vertical lines of the shaft against the edge of a door frame or the corner of a neighbouring wall. A true stack lines up with those verticals. A leaning one drifts off them as your eye climbs. Binoculars help you see the joints and the pots, and your phone camera gives you a record.
Look for these signs alongside the lean.
- A horizontal crack or open joint across the shaft, often a little above the roof line, where the stack is bending.
- Joints that have opened on one face and closed on the opposite face.
- Pots that tip with the stack or have slid off their bed.
- Lead at the base that has pulled away from the masonry, leaving a gap on the low side.
- Staining or damp on the chimney breast inside, which suggests water is entering at the stack.
Repeating the check is what helps most. Take a photograph from the same spot, with the same lens, once a season. A lean that looks identical year to year is a different problem from one that has visibly grown. We would rather see a dated set of pictures than a single alarmed phone call, and we use them when we inspect.
A word on safety. Do not climb to look, and do not lean a ladder against the stack, because the stack may not take the load. If the shaft looks as though it could drop pieces onto a path, a driveway or a neighbour's roof, keep people away from below it and tell us. We prioritise stacks like that and make them safe first, which can mean taking the top section down, before we discuss anything longer-term.
Sulphate attack: how old mortar pushes a stack over
The best-known cause of a leaning stack is sulphate attack, and it is a slow chemical process rather than a sudden failure. When fuel burns, the flue gases carry sulphur compounds. As the gases cool in the flue they form condensation, and that moisture is mildly acidic. Over many years it soaks into the inner part of the stack and reacts with the cement in the mortar, producing new compounds that take up more space than the original. The mortar swells.
The swelling is rarely even. The face of the stack that stays wetter, because it takes more of the rain or gets less sun, swells more than the drier face. Each bed joint grows slightly taller on the wet side, and across many courses those small differences add up to a tilt. This is why a stack suffering from sulphate attack often leans in a clear direction, and why the horizontal joints on one side look fat and cracked while the others look sound.
Sulphate attack is a particular worry on stacks that were repointed or built in a hard cement mix. Lime mortar is more forgiving, and it holds moisture differently, so the damage is usually less severe. The older and heavier the use of cement, the more likely that the mortar behind the visible face is the problem and not the pointing you can see.
It is worth knowing that sulphate attack is not cured by repointing. Fresh mortar on the face leaves the swollen, contaminated mortar behind it where it is. If sulphates are the cause, a lasting repair means taking the affected section down and rebuilding it, with the flue lined so the same thing does not start again. We explain that process in our guide to rebuilding a chimney stack.
Weathering and the other causes
Sulphate attack is not the only way a stack ends up out of true. Weather does a great deal on its own, and the two often work together.
Eroded joints and frost
Rain, wind and sun wear mortar from the joints of an exposed stack. A stack stands above the roof with every face open to the weather, so it is wetter and colder than the walls below it. Water enters a worn joint, freezes and expands, and the joint opens further. Once the joints have lost depth, the units above them are carried by little more than their own weight. A stack in that condition can shift under gusts of wind long before the lean is obvious.
Failed flaunching and lead
When the flaunching, the mortar slope around the base of the pots, cracks, water runs straight into the head of the stack. If the lead at the base has also lifted, the same water reaches the masonry at both ends. A stack kept wet from top to bottom decays faster, and the wettest side is the side that tends to move. Our guide to chimney flaunching explains how that top layer fails and is renewed.
Movement in the structure below
Sometimes the stack is not the cause at all. A gable wall that has spread, a roof timber that has dropped, or a stack that was built on a shallow base with weak courses can carry the shaft out of line. In that case no amount of repair to the masonry will hold, and the support has to be dealt with first. This is one reason we inspect the roof space as well as the stack.
Height and exposure
Tall, thin stacks catch more wind and are more prone to bending at their weakest joint. A stack that has been shortened or lost its pots can behave differently from the one the builder intended. We take the proportions into account before suggesting a fix.
How serious is the lean?
Not every lean calls for the same response. We sort leaning stacks into three broad groups after looking at them.
The first group is old and stable. The lean has been there for as long as anyone remembers, the joints are sound, the stack does not rock when tested, and there is no new cracking. These stacks are watched, repointed and flaunched in the normal way, and the lean is left as part of the building. Straightening a stable stack would be risky and unnecessary.
The second group has a modest lean with joints in poor condition. The mortar has eroded to a point where the stack is no longer well held, but the units are sound. Here a repair can work: raking out the joints to a proper depth, repointing in a mix chosen for the masonry, renewing the flaunching and checking the flashings. This restores strength and slows further movement, though it does not pull the stack back to upright.
The third group is the stack that is moving, cracked across the shaft, or tilted far enough that its weight sits outside its base. These are not repair jobs. The stack is made safe, then taken down to sound masonry and rebuilt. Where the stack serves a neighbour as well, the neighbour must be brought in, and our note on shared chimney stacks explains how that tends to run.
Rocking, new cracks, falling mortar or a visible increase in the lean are all reasons to call sooner, not at the next convenient moment.
We will tell you which group we think your stack belongs to, and show you the evidence on your own photographs. If it can safely be left and watched, you will hear that.
Repair options in order
Repairs follow a ladder. We go only as far up it as the stack requires.
Watch and record
For a stable lean, the answer is a photographic record and a check after storms. Cost is nil, and it makes any later change obvious.
Repoint and renew the top
Where the joints are worn but the stack is plumb enough, the repair is lime repointing, new flaunching and, where it has failed, fresh lead at the base. The mortar is chosen to be softer than the stone or brick, so the joint, not the masonry, takes the weather. How soon this recurs depends on exposure and workmanship, and our guide to how often a chimney needs repointing goes through it. It is condition-led rather than a fixed schedule.
Rebuild the upper section
Where the damage is confined to the top of the stack, the section above the roof line can be taken down and rebuilt on the sound masonry below. This is often the repair for a stack with sulphate damage in its upper courses and a sound base.
Full rebuild
If the shaft is unsound down to the roof, the whole stack is rebuilt using the original units where possible, with new lead and a lined flue. This is the repair that removes the cause, not just the symptom.
Removal
In some cases a stack that is no longer in use can be taken down below the ridge and the roof made good. That is a larger decision, with knock-on effects inside the house, and it has its own page on chimney stack removal.
MyBuilder's chimney repair guide (2026) shows how far apart the rungs sit: a typical UK repair is priced well below a rebuild, with a full rebuild reaching into five figures on a large stack. Your own figure comes after the survey.
Surveying a leaning stack in Bath
A leaning stack on a Bath terrace is rarely only a masonry problem. The stacks are tall, many serve more than one house, and they rise behind a parapet where lead gutters, flashings and stone copings all meet. A lean can start in any of those details and spread to the others.
The survey begins from the roof, not the ground. We check each face for open joints, test the pots, lift the flashing where we need to see the base and look at the roof space below for damp staining, tide marks and movement in the timbers. Pictures of your own stack come back to you, and we explain what they show. We then write down what we recommend, which part of the ladder it sits on and why.
Permission is worth settling early. A listed house needs consent from the B&NES conservation team before its stack is altered or rebuilt, so contact the council while the work is still being planned. Matching repairs to a stack in the conservation area, on an unlisted house, seldom cause difficulty, though any change of height or look should go past the planners first. If a rebuild might be notifiable under the Building Regulations, we check that with the building control route that applies to your address.
After the visit, a fixed price reaches you in writing. It names the scaffold, the mortar, the lead, any stone or brick to be replaced and the clearing away, and it says whether the stack is to be rebuilt from the roofline or only taken down to sound courses. If the right answer is to leave the lean alone and keep an eye on it, you will hear that from us too. The rest of our chimney repairs work is set out on the service page.