At the very top of most chimney stacks, the pots do not sit on bare brick. They are bedded in a mound of mortar that slopes down and outwards from each pot to the edge of the stack. That mound is the flaunching. It looks like an afterthought from the ground, a grey lump of cement, yet it does two jobs at once: it grips the pots so that wind cannot rock them loose, and it covers the top course of bricks so rain runs off instead of soaking in.
When it cracks or shrinks, the stack loses its roof. Water gets into the flue, into the brickwork below and, over a few winters, into the ceilings of the rooms beneath. This guide explains how flaunching is made, why it fails, what a proper repair involves and when it stops being a repair job. For the full list of ways a stack lets water in, see why chimneys leak.
What flaunching is and what it is doing up there
A chimney stack is a column of brick with an open top. Left like that, every drop of rain landing on the final course of bricks would sink into the joints and pass down the stack. Flaunching closes that top. Mortar is built up around the base of each pot, shaped into a smooth sloping mound and brought out to the edge of the stack, often with a small overhang so that water drips clear of the brickwork instead of running down the face.
The pots themselves are set into the mortar, and the mortar holds them upright. A pot with a sound flaunching round it is held on every side. A pot with a cracked or hollow base can rock when the wind blows, and a pot that moves breaks its own seal and lets in more water.
The mortar also protects the flue, in one respect. The pot is the top of the flue lining, and flaunching seals the junction between that lining and the brickwork around it. If that seal goes, water runs down between the pot and the wall of the stack, and a damp patch shows on the chimney breast after rain.
Is flaunching the same thing as a chimney cap?
No, although people mix the terms. A cap, in the usual sense, is a slab or a cover laid over a stack where the flue is closed off, and our guide to capping a chimney covers that. Flaunching is the mortar slope round a pot. Both can be present on one stack, and both can fail separately.
How and why flaunching cracks
Mortar is hard and rigid, and it sits at the highest, windiest point of the building with no shelter at all. It takes driven rain, bakes in summer sun and, in winter, soaks up water, freezes and thaws. A few causes do most of the damage.
- Shrinkage at the first cure. A mix with too much cement or too much water dries with fine cracks that open further each winter.
- Frost. Water gets into those cracks, freezes, expands and breaks off the surface in flakes.
- Movement of the pots. A pot that sways in a gale works at the mortar around it, breaking the bond between the clay and the mix.
- Thin edges. Where the mortar tapers to nothing at the stack edge, it chips away and leaves the top course of brickwork exposed.
- Soot and flue gases. Mortar next to an active flue is exposed to acidic condensate, which slowly attacks it from the inside.
Old flaunching is often sand and lime, which is how stacks were built before cement became the default. Lime is softer and more forgiving, but it does not sit well beside hard cement repairs, as the section on mortar choice explains.
Spotting trouble from the ground and from indoors
Most flaunching faults show through binoculars. Look for hairline cracks running round the base of a pot, a gap between the clay and the mortar, mortar that has flaked away to expose bare brick, or green and black growth where moisture sits. Pots that lean slightly or look different from their neighbours deserve a closer look.
Indoors, the clues are subtler. A damp patch on the chimney breast, usually near the ceiling of the room below the stack, can start in the flaunching. So can a brown stain that grows after rain and fades after dry weather. If the room has an open fire or a stove, a gritty fall of mortar crumbs in the grate is a sign that something is breaking up high in the flue.
Is a crack in the flaunching urgent?
A fine hairline crack is not an emergency, but it worsens every winter. A wide crack, a gap round a pot or a pot that moves needs attention soon, because a loose pot can slide off in wind and land on the roof or in the garden. Leaks and storm damage are prioritised here, and a loose pot is treated as a safety job: made safe first, repaired properly afterwards.
Why not just fill the cracks with sealant?
Sealant and bitumen paint do not bond to cold, damp mortar. They peel, trap moisture underneath and make the proper repair harder. A crack too wide to ignore needs the loose material taken out and new mortar put in.
What a proper flaunching repair involves
The repair starts by tapping the old flaunching to find how much has lost its bond. Mortar that sounds hollow is removed back to a firm edge, and the exposed brick is brushed clean and dampened so the new mix does not dry too fast.
If the pots are loose or damaged, they are lifted off, checked and set aside. Cracked pots are replaced, not re-bedded. The tops of the flue liners are cleaned, and the pots are set level on a bed of fresh mortar, then packed round so that no void is left between clay and brick.
The new flaunching goes on in stages. The mix is firm rather than sloppy, and it is shaped to fall away from each pot towards the edges of the stack, with a drip edge at the perimeter so water sheds clear of the face.
Does the weather matter when the mortar goes on?
It does. Fresh mortar has to cure without being washed out by rain or frozen in the first day or two, so we plan for a dry forecast, cover the work if showers threaten and hold off in a hard frost. A rushed job in poor conditions is how many cracked flaunchings begin.
How long does it take?
A single-pot stack is a short job once access is in place. Larger stacks take longer because each pot is bedded in turn, and setting up safe access is often the biggest part of the visit.
Which mortar: cement, lime or something in between
The right mix depends on the age of the stack. A Victorian terrace stack of soft brick in lime mortar will be happiest with a lime-based flaunching, which moves a little with the brickwork and lets moisture out. A 1980s estate stack of hard modern brick is better matched by a sand and cement mix.
The mistake is putting a hard cement cap on a soft old stack. Cement lets moisture out less freely than lime, so water in the brickwork is driven into the softer bricks and joints below, which flake and dissolve. Our page on lime mortar chimneys sets out how to tell which type of stack you have and why the choice matters for repointing as well as flaunching.
Additives help a little: a water-repellent one for cement mixes, fibre against shrinkage. Nothing replaces good preparation and a mix of the right strength.
The mortar should be no harder than the brick it protects. A flaunching stronger than the stack beneath it can tear the top courses apart.
A listed stack, or one in a conservation area, may need mortar that matches the original in colour and texture. If your house falls under either, check with Swindon Borough Council's conservation team first, and see chimney repointing for the rest of the stack.
When flaunching repair is not enough
Sometimes the flaunching is a symptom of something deeper. If the top courses of brickwork are loose, spalled or frost-damaged, new mortar on top does not last. The sound repair is then to take down the top few courses, rebuild them and flaunch afresh. A lean or wide cracks running down the stack turn it into a rebuild, and our guide to chimney rebuild explains when that point is reached.
A flue that has collapsed inside or filled with debris holds water and keeps the pot and flaunching permanently damp. Our guide on a blocked chimney explains what a blockage means and how it is cleared.
The choice between repair and rebuild is made on site. If the flaunching and a pot or two are the whole problem, that is all the quote covers. If the brickwork below is the real fault, the photographs show it and the quote reflects it.
Should I replace the pots at the same time?
If they are cracked, loose or crazed, yes, since they come off for the flaunching anyway. Sound pots are re-bedded and kept, because replacing good ones adds cost and no benefit.
Costs, access and what to send us
Flaunching on its own has no usual national price, so it is priced on survey. National guides do describe the surrounding work. MyBuilder's chimney repair guide, updated 21 August 2026, puts repointing at around £750, within a range of £500 to £2,000, and a full stack rebuild at £2,000 to £10,000 or more. Flaunching often comes alongside repointing, so the total depends on what the survey finds.
Access moves the total most. MyBuilder's scaffolding guide, updated the same day, puts chimney-height scaffold at £400 to £900, a national third-party range and not our price. The number of pots, the state of the top brickwork and any pot replacements add to the work.
To help us scope the job, send two or three photographs: the whole stack from the garden, a close view of the pots and any staining indoors. The survey then photographs your stack from up close, a written quote follows, and the work carries a 10-year workmanship guarantee in writing.
If a pot has shifted or you can hear mortar falling in the grate, ring 01793 626096 or message us on WhatsApp. When the job finishes, we leave no debris behind. Other stack work is listed on our chimney repairs page.