A distinct leaning of the chimney stack – often away from the prevailing wind direction – is a tell-tale sign, so too is blown mortar joints.
A small degree of lean is quite common in old stacks. Many have leaned considerably more than the official limit of 1 in 100mm, but in most cases are still stable. Where there is pronounced leaning, a structural engineer will need to confirm whether it is too extreme to be made safe without rebuilding.
Implications and Causes:
When loose bricks get consistently very wet, any sulphates in the masonry or mortar can react, causing horizontal expansion cracks along mortar joints, indicative of sulphate attack. The good news is that traditional lime mortar used in older buildings is not as vulnerable to sulphate attack as more modern cement mortar.
Constant wetting can lead to expansion of mortar joints too, effectively jacking up the stack and causing it to lean away from the wind. The type most at risk are tall, thin single flue stacks — common on 1930s houses. These are highly exposed and operate at colder temperatures compared to thicker main stacks containing clusters of flues. The stack may be colder on its windward side and hence suffer more erosion from condensing acidic gases and/or expansion from sulphate attack.
Finally, the most alarming cause of leaning chimneys is from botched structural alterations, such as the removal of chimney breasts without sufficient support to the remaining stack masonry above. Thankfully, this is relatively rare.
Older chimneys were generally built with weaker lime mortars than the cement-based types used on modern houses. So, where a chimney is only leaning a bit, it may not take much for it to tilt further, so the problem should be nipped in the bud before it gets worse.
What’s more an old mortar joint may have become loose or eroded with age, so that Repointing is now overdue. Joints should be raked out to a depth of 15–25mm and repointed with a sulphate-resistant mortar.
Interior erosion can normally be kept at bay by installing the correct type of flue liner.
Bear in mind also that where a chimney stack is leaning, the flashings may have suffered movement and may need re-fixing or even replacing.
Where instability is due to the removal of a chimney breast to a room below, the usual solution is to support the remaining masonry within the loft with a steel beam (which will require Building Regulations consent).
Where the degree of lean to a chimney is quite pronounced, a structural engineer may consider the option of stabilising it with a stainless steel tie bar (if there’s a handy adjoining roof slope in which to anchor it).
The remedy of last resort in severe cases is to take down and rebuild the stack, or remove it entirely. When rebuilding, the new stack should look the same (in height, style, etc.) as the original. So It is a good idea to take photos in advance as a guide to help recreate it. It is normally possible to reuse most of the original bricks.
Where the chimney is being taken down and rebuilt, It is important to agree with the builder where the demolition will stop — usually a couple of courses above the flashings. If the decision is taken to completely remove the chimney, the old opening will need to be covered over with a couple of square metres of replacement tiles or slates.
Gleaming new tiles or slates on an old roof can stick out like a sore thumb, so it might be possible to borrow some existing tiles or slates from a less visible rear roof slope. Otherwise, source matching second-hand ones from a reclamation yard.
Damaged Chimney Pots and Flaunching:
Signs include broken, missing or crooked pots sagging into the chimney stack, and frost damage in the form of delamination (flaking). Cracked and loose mortar flaunching – this is the large expanse of mortar that holds pots in place – are also signs. Problems with downdraught to the fireplace and rain penetration are also symptoms.
- If there’s any risk of collapse or chunks of chimney raining down and causing injury, urgent attention will obviously be needed.
- Long-term exposure to extremes of weather can ultimately cause erosion to the supporting masonry at the head of the stack.
- Cracks to the flaunching can develop over time with consequent water penetration leading to frost expansion and loosening.
- Internally, dampness from condensing flue gases can be a contributing factor that can eventually erode the masonry.
Damaged chimney pots need to be replaced with matching reclaimed or reproduction versions. Unstable pots can be taken out and re-bedded (e.g. on new corner tiles fixed into the upper courses of Brickwork).
Loose flaunching needs to be completely hacked off and replaced with a suitable mortar mix, waterproofed and carefully moulded so it slopes outwards to disperse rainwater — it would be a relatively simple job if it wasn't for the heights involved!
Lining flues should resolve internal problems, combined with fitting an all-purpose chimney cowl to each chimney pot to reduce the risk of rain entering, and to stop birds flying in. Where there’s a redundant, disused flue, it should be capped off and ventilated both to the top and to any boarded up fireplaces in the rooms below to prevent a build up of condensation.
The cost of hacking off defective cement fillet and forming a new flaunching in cement mortar for a single pot will be around £54.
Removing a damaged clay chimney pot and flaunching, and replacing it with a new 300x900mm-high roll-top pot and new flaunching will be about £89.