Most chimney stacks stand perfectly upright for generations, so a lean is noticeable once you see it. It may be a slight tilt against the sky, a stack that appears to bow in the middle or a top section that has shifted sideways. Whatever the degree, the cause is that something has changed: the mortar, the bricks, the support beneath the stack or the house itself.
It is worth knowing straight away that a leaning stack is a safety matter. A stack is a tall, narrow mass of brick, and if it falls, it comes down through the roof or onto the ground beside the house. Stopping use of the rooms beneath, and keeping the area outside clear, costs nothing and is sensible until the stack has been assessed.
This guide explains the main causes in general terms, including sulphate attack and mortar erosion, what to do today, when you need a structural engineer, and how the choice between rebuilding the stack, bracing it or taking it down is made. A survey of the stack from the roof shows what we find, and where the cause lies in the house rather than the stack, the report says so and names the right specialist.
Why a chimney starts to lean
A stack stands upright because its weight bears straight down through a solid base. A lean means that something has shifted that balance, and the explanations fall into a few groups.
The mortar has changed
If the joints on one side of the stack have eroded or expanded more than the other, the stack tilts towards the weaker side. This is the most common cause of a lean in the part above the roof, and it can happen slowly over decades.
The bricks have decayed
Bricks on the exposed face may have spalled or softened, so the stack is thinner on one side. The effect is similar: the load no longer sits centrally.
The support has moved
A stack that sits on a chimney breast inside the house depends on that breast being sound. If the breast has been altered, removed or damaged lower down, the stack above may be unsupported or carried badly. A lean above the roof can be the first sign.
The house has moved
Subsidence, roof spread or movement in the walls can tilt a stack along with the part of the house it stands on. Cracks inside, at the ceiling or across the breast, in the same area suggest this.
Wind and age
A tall, thin stack that has weakened over time can be pushed by a gust beyond what it can take. Often the wind is the trigger rather than the cause.
Why it matters which
A mortar problem is a stack repair, and a structural problem needs an engineer. We help you tell which by looking closely, and we do not guess.
Sulphate attack and mortar erosion
Two processes are often behind a lean in an older stack. Both concern the mortar, and both are slow.
Sulphate attack
Fuel burned in a fireplace produces gases that contain sulphur compounds. Over years, those can condense on the flue walls and combine with moisture to form sulphates. Sulphates can react with certain ingredients in the mortar and cause it to expand. In general terms, the joints on the flue side swell, while those on the outer face do not, so the stack bows or leans away from the flue.
Why it affects the whole stack
Because it happens inside the structure, it may not be visible until the movement is well advanced. The stack may show horizontal cracks in the joints, a curve along its length or a lean in one direction. The condition is more of a risk where the flue has been damp for a long time.
Frost and rain erosion
Separate from sulphates, the weather wears mortar away. Water enters the joints, freezes and pushes them apart. The Met Office's South Farnborough station shows annual rainfall of about 715 mm over 118 days of 1 mm or more and around 53 days of air frost on 1991 to 2020 averages, which describes a stack top that is wetted often and sometimes frozen. Where one face gets more weather than another, it erodes faster.
The result
Both processes leave the stack thinner or looser on one side. Our guide to chimney repointing covers repair where the bricks are sound, but a stack that has already bowed is normally a rebuild.
Other causes: movement, load and tie-in
Not every lean starts with the mortar. A roofer looks for evidence of other causes, and sometimes the answer lies in the structure of the house.
Loss of support below
When a chimney breast has been removed in a lower room, the stack above needs to be carried by something else. If that support was inadequate, or the work was done without proper approval, the stack can settle or tilt. Building Regulations approval always applies when removing a chimney breast, so records should exist. Our guide to chimney breast removal covers the process.
Wall movement
A stack on a gable wall that is bowing, or on a party wall that has moved, follows the wall. Cracks in the brickwork inside and outside the house, doors that stick and gaps opening at skirtings may point to this. It is a structural question, and an engineer is the right person.
Roof timbers
A stack that passes through the roof is held in position by its connection with the timbers around it. If rafters or purlins have rotted, spread or been cut, the stack may lose some of its restraint.
Weight above
Heavy pots, a TV aerial or a satellite dish add load and leverage at the very top of a stack that is already weak. Our short answer on a TV aerial on a chimney stack looks at this.
The point
The cause decides the cure, and a repair aimed at the wrong cause only delays the day the stack has to be taken down. Re-pointing a stack that is leaning because the house is moving will not stop the movement.
What to do today if your chimney is leaning
Until a professional has looked at it, treat the stack with caution. The steps are simple, and some are free.
Keep clear
Stay out of the area beneath and beside the stack, and keep children and pets away, since a fall of brick can reach well beyond the footprint of the chimney itself. If the stack could fall onto a path, a drive or a neighbour's property, tell them. If part of it comes down, it can break through the roof and into the room below.
Stop using the flue
Do not light a fire or run an appliance on the flue until it has been checked. A stack that is moving can be cracked internally, and a damaged flue can let smoke and carbon monoxide into the house, which is why an alarm in the room is worth having wherever a flue is in use.
Avoid the rooms below
If the stack is large, or the lean is obvious, avoid sleeping in the rooms directly under it until it is assessed.
Record it
Photograph the stack from several angles, from the ground, in daylight. Note the date. If you suspect storm damage, tell your insurer, as our answer on a cracked chimney stack explains. A comparison with older photographs shows how far it has moved.
Do not climb
Do not climb on the roof to look. A leaning stack and a wet roof are a bad combination. A roofer with a scaffold or tower can reach it safely.
If it is urgent
If the stack is visibly about to collapse, or has come down in part, call a roofer and, if people are at risk, the emergency services. We make a roof safe first, and agree the rest once the danger has passed.
Structural advice and the survey
Our role is the stack and the roof. Where the problem might be structural, we tell you plainly and recommend that you ask a structural engineer.
What we check
On the roof, we examine each face of the stack, the joints, the brick condition, the pots, the flaunching and the flashings, and we photograph them. We measure the lean where we can and look for cracks that run through the stack. In the loft, we look at how the stack connects with the roof timbers and whether there is any sign of movement.
When an engineer is needed
We recommend a structural engineer where the lean is large, the cracks run into the house, the supports below have been altered or the movement seems to be continuing. An engineer can calculate what the structure can carry and design a fix, which a roofer cannot do from a photograph.
Building control
Rebuilding a stack is not always a Building Regulations matter, but taking out part of a breast is. Ask your building control body if you are unsure. For a house in Farnborough that body is the Hart and Rushmoor Building Control Partnership, which also serves Hart.
Neighbours
Where the stack is shared with next door, tell the neighbour early. A party wall surveyor can tell you whether the Party Wall etc. Act 1996 requires a notice before work starts.
Your photographs
We send you what we photographed, and the written explanation of what it shows. That record is also useful if you claim on insurance.
Rebuild, brace or take it down
Once the cause is clear, there are three realistic choices. The right one depends on the condition of the stack, the cause of the lean and what you want from the chimney.
Rebuild above the roof
If the stack has bowed because of mortar or brick decay, taking it down to a sound level and rebuilding is the standard repair. It resets the clock and lets the flashings, pots and flaunching be renewed at the same time. Our guide to rebuilding a chimney above the roofline covers the work, the matching and the consents.
Brace temporarily
In some cases, a stack can be held in place for a short period while a plan is made. This is a stop-gap and not a cure, and it is the structural engineer's decision.
Take the stack down below the roof
If the chimney is no longer needed, taking the stack down to below the roofline and making the roof good is a permanent answer. It removes the risk and the future maintenance. Our answer on removing a stack below the roof line sets out what is involved.
Check the rules first
In a conservation area, in a place with an Article 4 direction or on a listed house, consent may be needed to take a stack down or alter it. Rushmoor's Cargate direction in Aldershot, for example, covers chimneys visible from a road or open space. Ask the council before work starts.
The decision
We give you the evidence, explain the options and write one price for the one you choose. Our chimney repairs page lists the rest of our stack work.
