Climb a ladder to the top of an old stack and you will find the pots do not sit on bare brick. Each one is bedded in a dome of mortar, sloped down towards the edges of the stack. That mortar is the flaunching. It holds the pots upright, closes the gap between the pots and the brickwork, and gives rain somewhere to run off, away from the flues and the joints below.
Of all the parts of a stack, it tends to give way early. It sits at the very top, with nothing above it to take the weather, and it is made of a thin layer of mortar. This guide covers the job of the flaunching, the reasons it cracks, how to tell that it has, and how a roofer takes it out and renews it. If your question is about the pots themselves, our answer on replacing a cracked chimney pot deals with that. The wider service sits on our chimney repairs page.
The job the mortar cap does around the pots
A chimney pot is a simple clay or concrete tube, and without something to hold it, it would not stay where it is put. Flaunching does three things at once.
It holds the pots
The base of each pot is pressed into a bed of mortar, and the mortar is built up around it. That bond is what keeps the pot upright in a gale. When it breaks, the pot can work loose, lean or, in a bad case, come off the stack.
It seals the top of the stack
The top of a chimney is a ring of brick or stone around one or more flue openings. Without a cap, rain would run straight down into the gap between the flue lining and the masonry. The flaunching fills that gap and closes the top, so that water falling on the stack stays outside it.
It sheds rain
The profile matters. The mortar is built up around the pot and then sloped away to the outer edge of the stack, with a slight overhang at the rim, called a drip, so that water falls clear of the brickwork instead of running down the face. A flat or hollowed cap would hold a pool of water, and standing water on top of a stack is a quick route to cracking and frost damage.
What it is not
It is not a waterproof membrane, and nobody expects it to last for ever. It is a sacrificial layer of mortar that takes the weather so the stack does not have to. Like pointing, it is renewed from time to time as a normal part of keeping a chimney sound.
Why flaunching cracks and crumbles
No single cause explains every failed cap. In practice a few things work together.
Weather
The flaunching is the highest and most exposed mortar on the house. Rain soaks into it, frost freezes the water and expands, and the surface gradually breaks up. Heat from the sun, and from a flue in use, adds movement of its own. Over many seasons the mortar goes from hard and dense to sandy and cracked.
A weak or poorly made mix
Mortar that has too much sand or too little binder is weak from the start, and a cap laid in poor conditions, on a wet day or in a frost, never cures properly. A thin cap, feathered out at the edges to nothing, breaks away at exactly the point where it needs to be thickest.
Movement of the pots
The pots expand and contract in the heat of a working flue, while the mortar around them is rigid. Hairline cracks open at the junction and let water in. Once water is trapped under the surface, each frost lifts the cap a little more.
Cement against soft materials
On an old stack, a hard cement flaunching bonds firmly to the pot but can pull away from soft brick or stone at the edges, because the two materials move differently. The crack follows the edge of the stack and lets water into the top courses. Our answer on the best mortar for chimney repairs covers why the choice matters.
Age and earlier repairs
Flaunching is often patched over rather than renewed. A thin skim over a failing base does not stick, and a few years later both layers come away together.
Spotting failed flaunching from the ground and the loft
Because the cap is at the very top, you will not see its surface without a close look. But there are signs to watch for.
From outside
With binoculars, look for a pale, crumbling ring around each pot. Look for dark cracks between the mortar and the pot, or between the mortar and the brick. A hollow underneath, where the cap has lifted, shows as a shadow. Fragments of mortar lying on the roof below or in a gutter are the cap breaking away. A pot that is not quite upright, or that seems to wobble in the wind, has lost its bed.
Inside the house
Water passing a failed cap runs down the outside of the flue liner and into the brickwork below. In the loft, the stack may show damp patches, white salts or darker brick. In the room below, brown stains on the chimney breast, a musty smell or wet soot in an unused fireplace all point to water coming down the stack. Our answer on damp patches on a chimney breast looks at those signs from indoors.
One cause among several
Failed flaunching is a leak path, not the only one. Flashing, mortar joints, porous masonry and open flues all let water in, and the stain below does not say which. Our guide to why Stroud chimneys leak works through each of them, because replacing a sound cap while the flashing is split solves nothing. We check them all during a survey.
If the pot is loose or leaning, treat it as a safety matter. A pot that can fall can hurt someone below, so keep people away from that side of the house until it is made safe.
How a roofer rebuilds flaunching
Where the cap is sound but cracked, a repair can sometimes be made. Where the mortar is hollow, crumbling or loose, there is little point in patching, and the whole cap is renewed.
Taking off the old cap
Working from a safe platform, the old mortar is broken away from around each pot with a hammer and bolster, down to firm material. We try not to loosen the bricks beneath or chip the pot. If the brick top is broken or the pots have to come off to get at it, they are lifted and set to one side. Rubble is bagged and lowered, not dropped.
Cleaning and wetting
The surface is brushed free of dust and loose grit, and dampened so that the dry masonry does not pull the water out of the new mortar. A clean, damp, sound surface is what a good bond depends on.
Bedding the pots
The pots are set upright and level on a bed of fresh mortar, and checked from two directions. Each pot should stand at the same height as its neighbours, unless the original arrangement was different.
Building the new cap
Mortar is packed around each pot, pressed down into the gap and built up in the centre of the stack. It is then sloped outwards to the edges, with a modest overhang past the brickwork to shed water. The slope is firm and smooth, without hollows, and the mortar is built thick enough to have body at the edges, not feathered to a thin line.
Curing
New mortar has to firm up slowly, safe from rain washing it out or the sun drying it too fast. We cover it in poor weather, and avoid doing the job when frost is likely.
Choosing the mortar for a new cap
The binder in the mortar decides how the cap behaves for the next decades, and the right one depends on what the stack is made of.
Modern brick and concrete pots
A stack of hard modern brick, with clay or concrete pots, takes a sand and cement mortar well. It is dense, sheds water and bonds firmly to a hard base. A sensible mix is a workable one, not so dry that it crumbles in the hand and not so wet that it slumps. Some roofers add a plasticiser to make the mortar easier to work and to improve its resistance to frost.
Old brick and stone
On a stone chimney or a stack of soft old brick, the cap often goes on in lime-based mortar, matched to the joints below it. Lime moves with the stack, lets the masonry breathe and does not tear away from soft edges as it ages. Our guide to repairing stone chimneys with lime explains the principle. It is the same reasoning as for the joints in the rest of the stack.
Colour and finish
A new cap that is pale and shiny against a dark old stack stands out from the road. We choose a sand that suits the colour of the mortar around it. A clean, tidy, sloped finish, with no visible trowel marks that trap water, looks right and sheds rain better.
Listed and conservation homes
If the house is listed, any change to a chimney that affects its character may need consent from the council, and a change of mortar or pot can count. Stroud District Council says listed buildings are legally protected throughout, so ask its conservation team first. We handle questions of this kind before any work is quoted.
When the stack needs more than a new cap
Flaunching is the top layer. What lies below determines whether a new cap will last.
Repointing at the same time
If the joints in the top courses have weathered too, renewing the cap alone leaves a gap in the defence. The two jobs are normally done together, from the same platform, and our guide to chimney repointing describes the other half.
Loose bricks and broken tops
The top courses of a stack are the most exposed to frost. Where bricks have worked loose or the stack leans, a fresh cap sits on a poor foundation. Where the damage reaches that far, rebuilding the top few courses, or the whole stack, can be the only lasting fix. Our guide to when a stack needs rebuilding shows how that call is made.
Pots, cowls and open flues
A broken pot, or a flue left open to the weather, sends water down the inside of the stack while the cap looks fine. A cowl or cap suited to the flue reduces that, and we look at the pots whenever we are on the stack.
Cost and records
No separate national price is published for flaunching, so it is priced on survey. Typical UK ranges for related work sit in our chimney repair cost guide. From the survey you receive photographs of your own stack and a written quote, and afterwards the rubble is cleared. The workmanship carries a 10-year guarantee, in writing.