Most leaks blamed on the roof near a chimney are the chimney itself. The usual culprits are perished flaunching on top, eroded pointing letting water into the brickwork, failed stepped flashing and soakers where the stack passes through the roof, or a missing chimney tray. Each needs a different repair, so the first job is working out which.
- A chimney is a wall exposed on all four sides and the top. It weathers faster than anything else.
- Flaunching is the mortar cap around the pots, and it is the most commonly failed element.
- Lead flashing fails from thermal movement — over-long pieces split. Length matters.
- A redundant stack still needs capping and ventilating, not just sealing.
Why chimneys fail first
A chimney stack is the most exposed masonry on the building. It projects above the roof, so it takes weather on four faces plus the top, with none of the shelter the rest of the walls get from the roof overhang.
It also goes through more thermal cycling than the rest of the structure, particularly if the flue is still in use, and historically it was built with lime mortar that was never intended to last indefinitely without maintenance.
Add that water entering a stack can travel down inside the brickwork and emerge well below roof level, and you have the reason chimneys generate so many of the damp problems we are called to — including ones that present as a wall problem in a bedroom with no obvious connection to the roof.
The parts, and what goes wrong with each
| Element | What it is | How it fails |
|---|---|---|
| Pots | The clay or terracotta tops | Cracked, loose, or displaced in wind |
| Flaunching | The mortar cap bedding the pots | Cracks, loses its fall, lets water into the stack |
| Pointing | Mortar between the bricks | Erodes; water enters the brickwork directly |
| Brickwork | The stack itself | Spalling faces from frost on saturated brick |
| Chimney tray / DPC | A damp barrier within the stack | Often absent entirely on older stacks |
| Soakers | Lead pieces under each tile course | Corrode or were never fitted properly |
| Stepped flashing | Lead let into the mortar joints | Pulls out as the joint erodes; splits |
| Front apron / back gutter | Lead at the lower and upper faces | Splits; back gutter blocks with debris |
The back gutter — the lead channel on the upslope side of the stack — is the detail most often neglected. It is hidden from the ground, it collects debris, and when it blocks, water backs up under the tiles.
Flaunching: the small job that prevents the big one
Flaunching is the sloping mortar cap on top of the stack that beds the pots and sheds water off the crown.
It is a relatively thin layer of mortar fully exposed to the weather, so it cracks with thermal movement and frost over the years. Once cracked, water runs into the top of the stack rather than off it, and from there it soaks down through the brickwork.
Re-flaunching is a contained job — cut out the old cap, rebed the pots and form a new cap with proper falls. It is substantially cheaper than dealing with the consequences of leaving it, which is a saturated stack, spalling brickwork and internal damp.
It is also the item we most often find has never been looked at. Nobody can see the top of a chimney from the ground, so it gets attention only once something appears inside.
Leadwork: why length matters
Lead is the traditional and still the best material for chimney flashings, and the way it fails is specific and avoidable.
Lead expands and contracts significantly with temperature. If a piece is too long, it cannot accommodate that movement at its fixings, and the stress concentrates until it splits — usually along a fold. This is why flashings are formed in limited lengths with laps between, rather than as one continuous strip.
The second variable is thickness, specified as a code number. Too thin for the span and exposure and it fatigues early.
A proper chimney flashing has several components working together: soakers interleaved with each tile course, stepped cover flashing let into the mortar joints and wedged, an apron at the front, and a back gutter behind. Each is dressed to shape and secured.
What you do not want to see is mastic. Sealant smeared along a flashing line is a sign the lead has failed and been patched rather than replaced, and it buys a season or two while making the eventual repair harder.
Redundant chimneys: cap, but ventilate
Where a chimney is no longer used, the decision is between removing it and capping it, and both have a requirement people miss.
Capping means fitting a cowl or cap to keep rain out while allowing air movement. The ventilation part is not optional. A completely sealed flue traps moisture, and that moisture condenses inside the stack and shows up as damp on the chimney breast indoors — often diagnosed as rising damp or a roof leak.
The flue should also be ventilated at the bottom, with an airbrick or vent in the fireplace opening, so there is a path for air to move through. Capped top, sealed bottom is the combination that causes problems.
Removal is more work than it looks. A stack removed above roof level needs the roof making good properly, and removing the breast inside involves supporting whatever is above it — which is structural work needing calculations and Building Control.
When a stack needs rebuilding
- Brickwork spalling across multiple courses, so there is no sound face left to point.
- The stack is visibly leaning, or mortar joints are open through its thickness.
- Repointing has already been done and failed again, indicating the brick is saturated.
- There is no chimney tray and persistent damp is tracking down the stack.
- The stack is tall and unrestrained, with no evidence of adequate support.
- Several elements have failed together, so a partial repair means returning.
Rebuilding above the roof line is the usual scope — the stack is taken down to a sound course and rebuilt, with a tray, new flaunching and new leadwork. It is substantially more than a repair, which is why catching the flaunching early matters.
What it costs
These are the roofing figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Chimney repointing / flashing | £300–£900 | Access dependent |
| Minor roof repair | from £150–£400 | Slipped or broken tiles, usually one day |
| Full re-roof (typical semi) | £5,000–£9,000 | Strip, membrane, batten, re-tile |
| Larger / detached re-roof | £9,000–£12,000+ | Scaffolding included |
Access drives much of it. A chimney needs scaffold, which is why doing the stack at the same time as any other roof or roofline work is far better value than a separate visit.
It is usually the stack rather than the roof. We inspect the flaunching, pointing, tray and leadwork and tell you which element has gone.
Frequently asked questions
Why is water coming in around my chimney?
Almost always the chimney itself rather than the roof around it. The usual causes are cracked flaunching on top letting water into the stack, eroded pointing letting it into the brickwork, failed stepped flashing or soakers where the stack passes through the roof, or a missing chimney tray. Each needs a different repair, so diagnosis comes first.
What is flaunching and why does it matter?
It is the sloping mortar cap on top of the stack that beds the pots and sheds water off the crown. Being thin and fully exposed, it cracks over the years, and once it does, water runs into the top of the stack instead of off it. Re-flaunching is a contained job and far cheaper than dealing with a saturated stack later.
Can I just seal up an unused chimney?
Cap it, but do not seal it. A completely sealed flue traps moisture, which condenses inside the stack and appears as damp on the chimney breast — frequently misdiagnosed as rising damp. Fit a cowl that keeps rain out while allowing airflow, and provide a vent at the bottom so air can move through.
Why does lead flashing split?
Thermal movement. Lead expands and contracts significantly, and a piece that is too long cannot accommodate that at its fixings, so stress concentrates until it splits, usually along a fold. That is why flashings are formed in limited lengths with laps. Lead that is too thin for its span fatigues early for the same reason.
Is mastic an acceptable chimney repair?
As a genuine temporary measure before a proper repair, occasionally. As a repair in itself, no. Sealant along a flashing line indicates the lead has failed and been patched rather than replaced. It buys a season or two, and it has to be cleaned off before the correct repair can be made, so it adds cost later.
Should I remove my chimney or repair it?
Repair or cap is usually cheaper and less disruptive. Removal above roof level means making the roof good properly, and removing the breast inside is structural work needing calculations and Building Control. Removal makes sense where the stack is badly deteriorated, unused, and you have no interest in ever reinstating a fire.
Find out which element has failed
Flaunching, pointing, tray or leadwork — they fail differently and need different repairs. We inspect the stack properly rather than assuming. Free inspection, fixed written quote.