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Property Surveying Blog

Written by Tughan Musa

Winter is here and as some of us are probably using our fireplaces, I thought it would be a good time to write a blog about a particular defect ‘leaning chimney’.

Although smog and chimney sweep boys are now a thing of the past, chimneys can contain defects, such as dislodged pots, leaks and significant leanings, which is the main focus of this blog. The biggest hazard for a leaning chimney stack is the risk of falling masonry, an unstable chimney stack is effectively an accident waiting to happen.

Chimneys being the most exposed part of the roof system bear the full force of any adverse weather. The combinations of high winds, heavy rain, snow, frost and internal chemical corrosion from intense heating and cooling will eventually weaken the structure.

Is the lean to the Chimney Stack safe?

When you look at your roof to view your chimney stack it should be standing upright. However, if you see signs that it is leaning distinctively, often away from wind direction and with eroded mortar joints, then there are few underlying issues.

While it may look worrisome, a small degree of lean is not uncommon. The BRE Good Repair Guide says that any chimney leaning more than 1mm in 100mm is unsafe. However, many have leaned more than the official limit and in most cases are still stable. If the lean is too distinctive, than a Structural Engineer will be required to analyse its safety.

What causes the Lean?

The main causes for leaning chimneys are the expansion of eroded mortar joints, acid attack and structural alterations.

Expansion of Eroded Mortar Joints

One common cause for a leaning chimney is from the expansion of eroded mortar joints.

If the surface of the chimney stack is exposed to the prevailing wind, constant wetting can result to the expansion of mortar joints, thereby causing the stack to lean away from the wind. This is prominent on the cold windward side of the stack as it is wetter from the rain and internal condensation.

Constant wetting can also cause the sulphates in the masonry to attack the mortar; thereby causing horizontal expansion cracks through the mortar joints, as a result of ‘sulphate attack’.

Victorian properties, constructed mainly in the 19th century, fortunately had lime mortar joints, which are not as vulnerable to sulphate attack in comparison to modern cement mortar.

The chimney stacks that are most at risk are the tall, slender flue stacks, which were built predominantly in the 1930s. These stacks are highly exposed to the elements as opposed to the thicker stacks, which consist of multiple flues.

Expansion of Eroded Mortar Joints Solution

Where the mortar joints have become friable and reached the end of its life, the solution will be to rake out to a depth of approx. 20-35mm and repoint with sulphate resistant mortar. New mortar joints should be always be weaker than the bricks as it effectively acts as lungs doing most of the breathing and allowing moisture to evaporate through the mortar joints rather than the brick or stone.

If the leaning is severe, the worst-case scenario is to re-build or partially rebuild the chimney stack using salvaged material as possible or remove it completely. To comply with The Building Regulations, the rebuilt section will require an internal diameter of 200mm or 185mm for a relined flue. Alternatively, the chimney stack can be tied with a steel rod to prevent it leaning further and collapsing.

Acid Attack within the Flue

Eroded mortar joints can also occur within the chimney flue. As previously mentioned, the chimney stack may be colder on its windward side and suffer more erosions there from condensing acidic gasses, which can result in weakening the structure.

Burning fuels can develop hydroscopic salts, which makes the brickwork vulnerable if its not been lined. These salts can attract moisture from condensation or externally through the brickwork. When the salts dry out they crystallise within the mortar and expand.

Solution for Acid Attack within the Flue

Flues over a century old would not be adequately lined to protect the mortar and brickwork. Old flues must be lined and those with existing steel liners should be examined as they eventually suffer from corrosion and may need renewal every 10-15 years or so.

Structural Alterations

Although it’s uncommon, the most alarming cause of leaning chimneys is from botched alterations. Removing a chimney breast without providing sufficient support to the remaining masonry above will cause movement and instability to the stack.

Structural Alterations Solution

If a chimney breast is missing from any of your rooms or you intend to remove a chimney breast, the requirement is to insert a gallow bracket or a steel beam parallel to the party wall. This type of works triggers the Party Wall Act and requires a Building Control inspection to confirm the adequacy of the chimney breast support.


When inspecting chimneys from ground level, it is desirable to bring a good pair of binoculars for assessing all and noticeable defects. Due to the substantial cost of erecting the scaffolding, I would suggest the chimney stack works to be carried out in conjunction with other major roof works, such as a loft conversion.