What flaunching is and what it does
At the top of a brick stack sits a flat slab of masonry called the stack top, and set into it are the chimney pots, the clay or metal tubes that finish each flue. The gap where each pot meets the brickwork is filled with a bed of mortar, built up into a slope. That mortar is the flaunching, and it is all that stands between each pot and the weather.
It does two things. First, it holds each pot upright and fixed so wind cannot knock it over. Second, its slope sends rain outward and away from the pot, so water falls off the stack instead of pooling round the base and seeping into the joints below.
Sound flaunching is a small piece of work with a large job. It is the roof of the stack, and when it fails, rain has a direct route into the middle of the structure. That is why a roofer checks the top of a chimney first. See chimney cowls and pots for what sits on top of it.
Why it cracks
Flaunching occupies the highest, barest spot on the building and has no shelter at all. Several things work on it year after year:
- Frost: water enters hairline cracks, freezes, expands and widens them.
- Wind and movement: pots shift slightly in gusts, which loosens the bond between pot and mortar.
- Shrinkage: mortar that dries too fast or is mixed wrongly cracks as it cures.
- Age: a mortar that has weathered for decades loses its surface and eventually its grip.
Manston's Met Office record for 1991 to 2020 shows 21.43 days of air frost a year and about 613 mm of rain, a fair guide to the wet and freezing a stack top goes through. It is a single station's record rather than a forecast for any particular house.
Cracks are easy to miss from below. Look for dark lines where the mortar meets the pot, bits of mortar sitting in the gutter, or a pot that seems to lean or rock. A pot in that state needs attention before the next storm, because the next strong wind may finish what the frost began.
How flaunching is repaired
A small crack or a thin patch can be repaired by raking out the loose material and pointing it. A flaunching that is cracked right through, hollow underneath or breaking away from the pot has to be renewed.
Renewing it follows a short sequence:
- The pots are lifted off, or their position is marked, and the old mortar is chipped away down to sound brick.
- The stack top is cleaned and checked for damage, because water that has been getting in may have started to spall bricks beneath.
- The pots are reset level and bedded in fresh mortar.
- The mortar is built up and shaped to a slope that sheds water, and left to cure out of the worst of the weather.
The mortar should be chosen to suit the brick and its weathering. Older stacks may need a matching mix, which the lime mortar for chimneys answer covers. A flaunching mixed too strong can crack away from the pot in its first winter, and one mixed too weak will not last, so the mix is part of the decision, not an afterthought. A pot that has already tipped or dropped is reset in new mortar at the same time, never propped back in place.
Doing it safely, and what it costs
Because flaunching is the last course before the pots, repairing it means working at height with proper access. Where the poles stand on the footway, the scaffold company has to obtain a licence from Kent County Council first, and the roofer plans the visit around that date. It is not a job to try from a ladder against the stack or by leaning from a roof window.
It is worth treating cracked flaunching early. Water that gets in through the top can flake bricks, loosen the joints and darken the chimney breast inside, and by then the work grows from a mortar repair into repointing, brick replacement or a rebuild. Compare the stages in repairing cracked chimney flaunching.
Flaunching is often done together with other stack work, such as repointing, because the access is already in place, which also spares a second visit.
The number of pots, the height, the access and the brick below all decide what a stack top costs, and the survey weighs them. One written total follows. Other stack work is under chimney repairs.
