Look up at almost any chimney stack and you will notice a rounded mortar slope hugging the foot of each pot. That is the flaunching. It is easy to overlook and it carries a lot of responsibility: it holds the pots in place, closes the top of the stack, and sheds rain outwards and away from the joints below. When it cracks, rain drains into the top courses, and the damage that follows can show up a long way down, as damp on a chimney breast two floors below or as mortar washing out of the shaft. The repair is small compared with what it prevents, which makes flaunching one of the most cost-effective jobs on a roof. This guide explains what flaunching is made of, why it fails, how to recognise a failing one from the ground, and how we cut it out and replace it. It stays on the crown of the stack and does not cover the shaft or the lead.
What flaunching is and what it has to do
Flaunching is the mortar laid on top of the stack, around the foot of the pots, and shaped to fall away from them on every side. Strictly it is the sloped bed, not the pot and not the stack below. It is usually formed with a slight overhang at the edge so that drips fall beyond the masonry instead of tracking down its face.
It does three jobs at once. First, it fixes the pots. A clay pot is heavy, it stands high in the wind, and the bed it sits in has to hold it upright through every gale. Second, it covers the flue wall tops, the thin edge of the masonry around each flue, which would otherwise be exposed to rain. Third, it throws water off. A good profile gives water a short route to the edge and no flat spot where it can sit.
On a stack with several flues the flaunching is a single mass that spans all the pots. On a stack with a cap or a slab, as is common on stacks of stone, the flaunching may be a smaller bed beneath the pots, with the capping carrying the main weather protection. In both cases the principle is the same: a continuous, sound, sloping surface with no gaps at the pot or at the edge.
Because it sits at the highest and most exposed point of the building, flaunching works in difficult conditions. It is hit by rain from every direction, baked in summer, frozen in winter and shaken by wind through the pots. Few other mortar elements on a house face that sort of treatment, and that is why it is normally the earliest part of a stack to give up.
Why flaunching cracks and breaks away
Cracking is the most common sign, and it has several causes that often combine.
Shrinkage and age
Mortar shrinks a little as it dries, and a hard mix can pull away from the clay of the pot, leaving a hairline gap around its base. Rain finds that gap, and every frost widens it. Over time the flaunching loosens around each pot and eventually breaks in pieces.
Frost and saturation
Flaunching that absorbs water and then freezes will crumble from the surface inwards. Mixes that are too strong tend to crack instead of absorbing water, while mixes that are too weak erode. A mortar chosen to suit the stack and its exposure is the answer to both.
Movement
A pot can shift if the stack beneath it is moving, and pots loosened by wind rock in their bed and break the surrounding mortar. If the flaunching is cracking again soon after it has been renewed, the cause is probably below it, and our guide to leaning chimney stacks looks at what to check.
Poor original workmanship
Flaunching that is built too thin at the edges, left flat, or laid over a dirty and dusty surface does not bond, and it fails early. The edge is the weak point, because a thin feather of mortar has nothing to hold it.
Where cement has been used on an old stone stack, it can also create problems below. A hard cement bed over soft limestone holds water in the stone below and hastens its breakdown, a problem we cover in cement on Bath stone.
Signs that yours needs attention
Most homeowners first notice a problem inside the house, as a tide mark on the chimney breast or a damp patch near the top of a bedroom wall. The flaunching is worth checking when that happens, since it is among the possible sources of water, and we cover the full tracing order in our guide to damp chimney breasts.
From outside, a pair of binoculars or the zoom on a camera will show most of the signs. Look for these.
- Visible cracks running across the flaunching or radiating from a pot.
- A gap around the base of a pot where the mortar has pulled away.
- Pieces of mortar lying on the roof, in a gutter or on the ground below.
- A flaunching that has gone soft, crumbly or sandy on the surface.
- Green growth or a dark stain trailing down from the top of the stack, which means water is sitting there.
- A pot that leans or has a visible gap between it and the bed.
You do not need to climb up to confirm any of these, and you should not. A ladder against a stack is a poor idea at the best of times. A photograph taken from the garden or the street is enough for us to see whether a survey is needed.
Fallen pieces of mortar are worth keeping a note of. If more appear after each storm, the flaunching is breaking up and the pots are loosening.
Flaunching is rarely an emergency on its own, but a cracked one lets in more water with each rainfall, and the cost of renewing it rises if the stack below becomes saturated. We prioritise anything that has loose pots or pieces that could fall, and make that safe first.
How flaunching is renewed, step by step
Renewing flaunching is a short job in the scheme of things, but it goes wrong when it is rushed. These are the stages we follow.
Access
Safe access comes first. On a low or simple stack that may be a roof ladder and edge protection, and on a taller or more exposed one it is scaffold. The stack is never reached by leaning on the pots.
Taking off the old bed
The pots are checked and, where they are loose, lifted off and set aside. The old flaunching is cut out by hand with a bolster and hammer, or with careful use of a power tool, down to firm, clean masonry. Any loose or crumbling mortar below it is removed as well, because new flaunching bedded on weak material fails with it.
Checking the stack head
With the top clear, we look at the upper courses. If the head of the stack is cracked, open at the joints or loose, it is repaired or rebuilt before the new bed goes on. The pots are checked for cracks too, and any that are damaged are replaced.
Laying the new bed
The surface is brushed clean and dampened, which stops the dry masonry drawing water out of the fresh mortar too quickly. The pots are re-bedded plumb, and the new mortar is built up around each, packed tight at the base, and shaped to fall evenly on every side with a slight overhang at the edge. It is trowelled firm but not overworked, since too much trowelling brings fine cement to the surface and weakens it.
Curing
New flaunching should not dry too fast in sun or wind, and it needs protection from frost. We choose the day for the job with that in mind and cover fresh work if the weather turns.
Choosing the mortar: lime or cement
The mix is the most important decision in the job. For many years flaunching was done in a strong sand and cement mortar because it was quick and hard. On a modern brick stack that can be perfectly adequate. On an older stack in stone or soft brick, a strong cement mix is a poor partner for the masonry underneath.
The reason is simple. Cement is hard and relatively impermeable. Old stone and old brick are softer and are meant to breathe, letting moisture pass out through the mortar. Put a rigid, sealing crown on top and the water trapped below has only the stone to escape through, so the stone takes the weathering. The Bath Preservation Trust made the point about a Walcot Buildings property in Bath, where it advised lime for the flaunching work proposed. Lime is more flexible, it is more permeable, and it is kinder to the stone around it.
A lime mortar for flaunching is mixed with an aggregate that gives it body and sometimes with a hydraulic lime, which sets better in wet and exposed positions. It needs more care in cold or wet weather because it cures more slowly than cement, which is why timing matters.
On a modern house, a suitable sand and cement mix may be right, and we will say which is appropriate for yours. What we avoid is applying the same mortar to every stack regardless of age and material. The right mix is the one that matches the masonry below, as with the pointing on the shaft. Our chimney repairs page covers how those jobs fit together.
Cost, consent and what the survey confirms
Flaunching is a modest repair, and one of the least expensive jobs that a stack needs. None of the cost guides we draw on lists flaunching by itself, so no figure is offered for it here. As a rough frame, MyBuilder's chimney repair guide (2026) puts typical UK chimney repair costs across the whole category at £500 to £2,000, with scaffold a large part of the bill on a small job. Where the flaunching is renewed at the same time as other work, such as repointing or fresh lead, the scaffold is shared and the combined price is better value than separate visits.
Consent is a question for listed buildings and some sensitive locations. Renewing flaunching like for like normally does not alter the appearance of a stack, but if the house is listed we advise speaking to the B&NES conservation team, because the council decides what counts as a change to a listed building. An unlisted house inside the conservation area can usually have matching repairs without fuss.
The survey is where we confirm the condition of the flaunching and everything it protects. We look at the cracks, test the pots, check the upper courses and the lead below, and take photographs of your own stack so you can see the evidence. A written fixed price follows, naming the mortar we propose and the scaffold. If the flaunching is the only fault, we say that, and the repair is priced as a small job.
Flaunching is best renewed in dry, mild weather, because mortar set in frost or heavy rain can crack again within a season, and we clear away the old mortar and take it with us.