Look up at most chimney stacks and you will see a pale, sloping band of mortar around the foot of each pot. That band is the flaunching. It looks like a small detail, yet it does two jobs at once: it grips the pot so the wind cannot shift it, and it throws rainwater outwards, away from the flue and the top of the brickwork. When it cracks or crumbles, rain runs down beside the pot and into the stack. This guide explains how flaunching is built, why it fails, what a repair involves and how we decide between patching and starting again.
What flaunching is made of and how it is shaped
The stack top is a flat bed of brick, finished with a layer of mortar. The pots stand on that bed and are surrounded by a mass of mortar that rises up their sides and slopes down to the edges of the stack. That sloped mass is the flaunching. A good one is dense, has a smooth fall to every edge and stands slightly proud of the pot so that nothing can pool against it. The slope is the point: water that lands on it keeps travelling until it drips clear.
On older stacks the mortar was lime-based, which stays slightly flexible and lets the brick behind it dry out. On many later stacks it was cement, harder and more brittle. Both do the same job when sound. When they are not sound, the pot is no longer held and the top of the stack is no longer shed.
Why flaunching fails
Flaunching sits in the harshest spot on the house. It is the highest mortar on the roof, with wind on every side and nothing to shelter it. The usual causes of failure are these.
- Frost. Water gets into hairline cracks, freezes, expands and levers the crack wider. Each winter repeats the process until pieces fall away.
- Shrinkage and movement. A mortar that was mixed too wet or dried too fast shrinks away from the pot. Clay pots also expand and contract at a different rate from the mortar around them, so a gap opens at the join.
- Weak mix. A sandy, under-cemented mix wears away and crumbles to powder under a finger.
- Flat or hollow slopes. If the collar was built without enough fall, water sits on top and soaks in rather than draining off.
- Age. Even a good bed is not permanent, and stacks that have never been touched are the most likely to need it.
Signs that the flaunching has gone
You can see a good deal from the ground, especially with binoculars on a clear day. Look for dark gaps around the base of a pot, mortar that has turned from pale grey to green or black, pieces of rubble in the gutter or on the path, or a pot that looks to be leaning. Inside the house, the clues are slower: a damp patch on the chimney breast, wallpaper that lifts, white salts on the plaster, or a brown stain appearing after heavy rain. Those can also come from a failed flashing where the stack meets the roof, so it is worth reading our guide on why a chimney leaks to separate the possible causes. If you are not sure whether the problem is the top of the stack or the junction below it, our short answer on what chimney flashing is explains where the lead sits.
Patch repair or full replacement
Not every cracked collar needs to come off. The decision depends on how much of the mortar is sound.
When a patch is enough
If the flaunching is firm and the cracks are fine and shallow, we may be able to rake out the loose material, clean the joint and fill it. This keeps the original bed, which suits an old lime collar that is mostly healthy. The condition is that nothing sounds hollow or moves when pressed.
When it should come off
If the mortar sounds hollow when tapped, crumbles, has separated from the pot, or has large missing areas, patching only hides the fault. The collar has to be removed to the brickwork and rebuilt. The same applies where a pot is loose, since a pot cannot be rebedded without taking out the mortar around it. Rebuilding is not much slower than patching, and it gives a result that should last far longer.
| What we find | Usual answer | Why |
|---|---|---|
| Fine surface cracks, collar firm | Rake out and fill | The bed is sound and only the surface needs sealing |
| Mortar hollow or crumbling | Remove and rebuild | Loose mortar cannot be patched into a lasting bond |
| Pot loose or leaning | Rebed the pot in new flaunching | The pot must be reset level and held firmly |
| Pot cracked as well | Replace the pot, then new flaunching | A cracked pot lets water in whatever the mortar does |
How the work is done
Because the stack is high and exposed, access comes first. Some stacks can be reached from a roof ladder and boards, but a tall stack on a steep slope normally needs scaffold or a tower, and the decision is covered in our answer on scaffolding for chimney repairs. The sequence then runs as follows.
- Survey and photographs. We look at the collar, the pots and the top courses of brick, and photograph each pot so you can see the cracks.
- Remove the old mortar. Loose and failed flaunching is chipped away carefully down to the stack top. We do not bash it, because heavy blows can send shock down into old brickwork or crack the pot.
- Check the base. We look at the bed of the stack, because there is no sense in putting new mortar on a crumbling brick top. Where the top courses are weak, they are dealt with first.
- Reset the pots. Each pot is bedded plumb and level on a fresh layer of mortar, with the same height and spacing as before. A pot that leans is reset rather than propped.
- Build the new collar. Mortar is packed around each pot in a firm mass and shaped with a trowel into a smooth slope, with a drip at the edge so water falls clear of the brickwork. The collar is built in stages to avoid slumping.
- Cure and finish. The mortar is protected so it sets slowly instead of drying out too fast and cracking. We then clear the debris from the roof and the garden.
The mortar mix matters
A sensible mix is dense and fairly dry when applied, using sharp sand rather than soft building sand, and gauged to suit the stack. On a period stack we often match the existing mortar so the repair does not trap moisture in the older brick behind it; a hard cement collar on a soft old stack can push damp into the brickwork, so a lime-based mix is sometimes the better match. On a post-war stack a cement mix is usually fine. Whichever we use, we explain the choice at the survey. The two mortars are weighed against each other in the piece on lime or cement for chimney repointing.
Some roofers add a waterproofing agent or a bonding additive to the mortar. These can help adhesion, but they do not replace a good fall and clean preparation. The shape of the collar does more to keep a stack dry than any additive.
Why flaunching is often done with repointing
Crumbling flaunching rarely stands alone. The same weather that cracks the collar is working on the mortar joints in the top courses below it, and a stack with failed flaunching often has joints that have eroded as well. Because the access is already in place, it makes sense to look at both. Our guide to chimney repointing sets out what the second job involves. Doing them together saves a return visit and a second scaffold hire, though only if the survey shows both are needed. We will not add work the stack does not need.
What a lasting collar looks like
When the job is finished, the collar should look plain rather than clever. The slope is even all round each pot, the surface is smooth and tight with no pitting, and the mortar meets the clay without a gap you could slide a knife into. The pot stands upright and the line of pots across the stack is level. Photographs taken before and after let you compare those points with the old state of the stack. A collar that looks lumpy, thin or hollow now will fail early, however good the weather has been.
What the work costs
Reported UK figures for a typical repointing job come to about £750, and flaunching is usually priced inside a wider chimney repair. Flaunching on a single pot is a smaller job than a full repair, but the figure for a stack varies with the number of pots, the height of the stack and access. Scaffold or a tower is often the largest single cost, so a stack that needs two small jobs is cheaper done at once. Your fixed quote follows the survey, with a price for each item so you can see what drives it.
Can you do it yourself?
The question usually comes from the cost of access, and it is a fair one to ask.
Many homeowners ask whether a patch of mortar from a bag will do. From a ladder at the eaves it will not reach, and climbing onto a roof to a chimney top is among the more dangerous domestic jobs. A bagged repair applied to wet, dirty mortar also tends to fall off within a season. The work needs a stable platform, the right mix and a trained hand, which is why we advise against it.
What we do at the survey
We look at the collar, the pots and the brick below them, then send photographs of what we found and a fixed written quote. If a patch will do, we say so; if the whole collar needs to go, you see why. Once the job is finished the site is left clear, the waste goes with us, and our own workmanship is covered by a ten-year guarantee in writing. The full set of stack services sits on our chimney repairs in Basingstoke page. Ring 01256 213956 or drop us a WhatsApp message to book a visit.
