Leaning and cracked chimneys
Chimneys take more weather than any other part of a house and are often the least maintained. They also hide one of the most common unsafe alterations in British housing, the chimney breast removed without support.
Why is my chimney leaning?
Most commonly sulphate attack. Sulphates react with cement in the mortar in wet conditions and the mortar expands, so the weather side of the stack grows slightly and the stack curves away from it.
Why stacks lean one way
A stack that leans consistently in one direction, usually away from the prevailing wet weather, is often showing sulphate attack. Sulphates in the flue gases and in the brickwork react with the cement in the mortar in the presence of moisture, and the mortar expands. The wet side of the stack grows slightly taller than the dry side and the stack curves.
The effect is slow and cumulative, and it shows in the mortar joints rather than the bricks. Joints on the exposed face look thickened and crumbling, and the horizontal joints on that side open up. Once it is established, repointing alone does not arrest it because the reaction is in the existing material.
An unused flue makes it worse rather than better. Without warm gases drying the stack it stays damp for longer, and a chimney that was serviceable while in use can deteriorate faster after the fire is taken out of commission.
The chimney breast that was removed
Removing a chimney breast in a ground floor or first floor room, while leaving the stack above and the breast in the loft, is extremely common and very often done without any structural support. The masonry above is then bearing on nothing, or on something never designed to carry it.
Sometimes the remaining breast is corbelled out, with courses of brick stepped to carry the load sideways onto the wall. Corbelling was traditional practice but it is not adequate for the loads a full stack imposes, and it is not something a modern assessment would accept.
The signs are cracking on the wall where the breast used to be, sagging of the floor or ceiling in that area, and sometimes nothing at all until someone opens up. It is one of the standard items a purchaser's surveyor flags, and one of the more common reasons a sale stalls pending a structural inspection report.
What the remedies look like
For a leaning stack, the usual answer is to take it down to a sound level and rebuild, in materials appropriate to the exposure and with a liner where the flue is still in use. Rebuilding to the same detail that failed is not a repair, so the exposure and the flue arrangement matter as much as the masonry.
For an unsupported breast, the remedy is to install proper support. Gallows brackets are commonly proposed and are appropriate only in limited circumstances, when the wall they fix into is sound solid masonry of adequate thickness and the load is modest. They are frequently specified where they are not suitable.
The more dependable solution is a beam carrying the retained masonry down to supports that themselves carry down to something adequate. That requires the load to be calculated rather than assumed, and it is the calculation, rather than the bracket, that determines what is appropriate.
What we check.
The points our calculations resolve for a project like this.
- Direction of lean relative to the prevailing weather, which suggests sulphate attack
- Condition of mortar joints on the exposed face compared with the sheltered face
- Whether the flue is disused, which keeps the stack damp for longer
- Whether a chimney breast has been removed at any level, and what supports what remains
- Whether gallows brackets are actually appropriate for the wall and the load, or simply convenient
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
Why is my chimney leaning?
Most commonly sulphate attack. Sulphates react with cement in the mortar in wet conditions and the mortar expands, so the weather side of the stack grows slightly and the stack curves away from it.
Is a removed chimney breast dangerous?
It can be. Removing a breast while leaving the masonry above it means that masonry needs support. Where none was installed, or where it relies on corbelling, the arrangement would not be accepted by an engineer today.
Are gallows brackets acceptable?
Only in specific circumstances, where the supporting wall is sound solid masonry of adequate thickness and the retained load is modest. They are often specified where the conditions do not justify them, so the load should be calculated rather than assumed.
My surveyor flagged the chimney and the sale has stalled. What is needed?
Usually a structural appraisal setting out what supports the remaining masonry, whether it is adequate, and what work is needed if it is not. That report is normally what allows the transaction to proceed.
Related in structural problems & symptoms.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
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