Chimney flaunching
What is chimney flaunching, and why does it crack?
Flaunching is the mortar at the very top of a stack. Here is what it does, why it cracks and what the repair is.
The short answer
Flaunching is the sloping bed of mortar round the base of a chimney pot, and it holds the pot steady and carries rain away from the pot to the edge of the stack. It cracks because it is a thin, exposed layer that dries, soaks and moves with the weather. A cracked bed is relaid, since a patch leaves the crack beneath it in place.
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What flaunching does at the top of the stack
At the top of a stack, the pots stand on the final course of brick and are bedded in mortar. That mortar is built up around each pot and finished with a slope that runs outwards and downwards. The slope is the flaunching, and it does three jobs at once.
- It holds each pot upright and stops it rocking in the wind.
- It sheds rain outwards to the edge of the stack, instead of letting it sit around the pot.
- It covers the top of the brickwork, so the joints below stay out of the worst of the weather.
Standing as it does above the ridge, the flaunching is the part of the stack that is wetted and dried most often. It is seen from below only as a grey line under the pot, and cracks are hard to judge from the ground. A flaunching can be failing while the pot above it looks perfectly straight.
Where two or more pots share a stack, each one has its own bed, and the beds can be joined into a single surface across the top.
Why flaunching cracks and what a crack means
Mortar is a hard, brittle material in a thin layer, and the top of a stack gives it a hard life. It is soaked by rain, dried by wind, warmed by the flue and left standing in all weathers. Over time it opens up hairline cracks, and then wider ones, and pieces can lift away from the pot or the brick.
A crack in the flaunching matters for what it lets in. Rain that runs into the crack reaches the top courses of the stack and the flue below, and the survey treats a cracked bed as a likely route for damp showing on the breast indoors, which is explained on the damp chimney breast page. It can also leave a pot loose, and a loose pot is a risk to anyone below.
What to watch from the ground:
- a pot leaning to one side
- grey or pale pieces on the roof or in the gutter
- a dark stain running down the stack from the top
None of these proves the flaunching has failed, but each is a reason to book a survey.
Patch it or relay it
A thin crack in an otherwise firm bed can look like a candidate for a patch. The survey weighs it against what is under the crack.
| What the survey finds | Repair that follows |
|---|---|
| A hairline crack, bed firm, pot steady | Monitor, or a repair after a check of what lies below |
| Cracks running right round a pot | Relay the bed |
| Pieces missing or lifting off | Relay the bed and check the top courses |
| Pot loose or cracked itself | Lift the pot, replace it if needed and relay |
Relaying means chipping out the old bed, cleaning the brick, resetting the pot if it has moved and building the new mortar bed with a fresh slope that carries water clear of the pot and the brick joints. A patch laid over a failed bed can part from it, so we relay instead. Where the pot is cracked as well, see replacing a chimney pot. Where the top courses of brick are flaking as well, they are dealt with in the same visit.
Access, planning and the written price
Flaunching is worked on from the top of the stack, so the job needs a safe platform. The survey decides whether that is scaffold or an equivalent, and the one written total price covers it. Where scaffold overhangs the highway, a county licence is needed, and we arrange it with our scaffolder. What the licence costs in Kent and in East Sussex is set out on our scaffolding for chimney repairs page, with the period each one runs for.
Relaying the flaunching on a listed house needs the same care as any work to a listed stack: consent covers works that affect the character of the building, so ask the council whether a like-for-like repair is included, and keep its reply. In a conservation area, check whether an Article 4 direction applies.
The written price comes once the survey is done. If the flaunching is the only fault, the job is short and the pot is reset while the bed is dry and clean, and it makes sense to do it at the same visit as any other work at the top, since the platform is already up. Everything we do on stacks is gathered on our chimney repairs page.
More questions
Can I seal cracked flaunching with something from a tube?
You cannot reach it safely from the ground, and a sealant is a coating, not a new mortar bed. A coating bridging a failed bed can hide movement from the next inspection. The survey looks at what lies beneath the crack, and relaying the bed is the repair that deals with the cause, with the pot reset at the same time if it has moved.
Can a cracked bed round the pot make a room damp?
It can let rain into the top of the stack, and from there the wet may reach the flue and the breast below. Damp indoors has other possible sources, so the survey checks the flashing, the pointing and the pot too. Our page on a damp chimney breast explains how the cause is traced.
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