A chimney stack is one of the most exposed parts of any house. It takes rain and wind from every side, and on many older homes it hasn’t been used for a fire in decades. Because it’s high up, it’s easy to ignore until water appears on a bedroom wall or a piece of mortar lands in the garden.
This guide covers the defects that let water in or make a stack unsafe, the signs you can check from the ground, and when a closer look makes sense.
The parts of a chimney stack
A stack has several parts, and each one can fail:
- Pots: the clay pots on top. They can crack or work loose.
- Flaunching: the sloped mortar bed around the pots. It throws rain off the top of the stack.
- Brickwork and pointing: the mortar joints between bricks weather away over time.
- Flashings: usually lead, sealing the joint between the stack and the roof slope.
- Cowls and caps: fitted to keep rain or birds out of the flue.
Flaunching and pots
Flaunching cracks with frost and age. Once water gets into the cracks, it soaks into the top of the stack and can loosen the pots. A loose pot is a real hazard in high winds. Redundant flues with no cap or cowl also let rain fall straight down inside the chimney. That can show as damp patches on chimney breasts inside, often in the loft or top-floor rooms.
Pointing, brickwork and leaning stacks
Weathered pointing lets water into the brickwork. Over many years, this can lead to spalling (the face of the brick breaking away) and loose bricks near the top.
Some stacks lean. A lean can develop over a long time, sometimes because of chemical attack on the mortar from flue gases or because one side gets wetter than the other. A small lean that hasn’t changed in years may not be urgent. A lean that is new, getting worse, or paired with cracked or loose brickwork needs proper advice. Our survey records what we see. It isn’t a structural engineer’s report, and we’ll say if one is needed.
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Flashings where the stack meets the roof
Lead flashings seal the gap between the stack and the slates or tiles. Faults include lead that has lifted, slipped out of the mortar joint, cracked, or been patched with sealant. A back gutter behind the stack can also collect debris and overflow. Our guide to lead flashing problems has more on this.
Signs you can check from the ground
With binoculars from the garden, the street or a window, look for:
- pots that look out of line or are missing
- plants growing from the top of the stack
- gaps in the pointing or bricks that look darker than the rest
- a stack that doesn’t look straight against the skyline
- lead that is lifted, torn or obviously patched.
Don’t climb up to check. Chimneys are one of the hardest parts of a roof to reach safely.
When to get a closer look
If you see any of the signs above, or have damp on a chimney breast, a chimney survey looks at the stack, pots, flaunching, pointing, flashings and any cowls. Where a flight is possible, a drone gets close-up photos of the top of the stack without anyone going up. A chimney survey isn’t a flue, gas or sweep inspection. For those, use a sweep or a suitable engineer.
Quick answers
Why is my chimney breast damp when I never light a fire?
Common causes are rain entering an uncapped flue, failed flaunching or flashings, or porous brickwork. Condensation inside a sealed flue can also play a part.
Is a leaning chimney dangerous?
Not always, but it should be looked at. A survey records the lean and the condition of the stack and says whether a structural engineer should assess it.
Do you sweep or test flues?
No. A chimney survey looks at the outside of the stack and related damp. Flue and gas checks need a sweep or a suitable engineer.
Related services
More advice
- Lead Flashing Problems on Older Roofs
- Victorian Terrace Roofs in London: Common Defects
- Winter Roof Checklist for Homeowners
- Condensation or a Roof Leak? How to Tell
- All advice
This guide is general information about roofs and roof surveys. It isn’t legal, insurance or structural engineering advice; for those, speak to a suitably qualified professional.