The short answer
Chimney pots crack because frost, heat and flue gases act on old clay, and because the flaunching that beds them weathers and lets water in. A cracked or loose pot is lifted off, the old flaunching is cleared, and a sound pot is rebedded on fresh mortar. Many pots can be saved by rebedding; some need replacing.
What a chimney pot actually does
A pot extends the flue above the top of the stack and helps the smoke or fumes leave cleanly rather than swirling back down. It also gives the stack a finished, weatherproof top. Pots are usually clay, sometimes terracotta, and come in many shapes: round, square, tall, short, plain or decorative. Some stacks carry a cowl or a cap on top, which our guide on chimney pots and cowls explains.
Each pot sits in a bed of mortar on the stack's crown, with a sloping fillet of mortar around its base. That fillet is the flaunching. It holds the pot in place and throws rain outwards so that water runs off the top rather than into the brickwork. If the pot is the visible part, the flaunching is the working part, and the two are almost always dealt with together. The wider chimney repairs section shows where pots fit in the whole stack.
Why pots crack, tilt and come loose
Several things act on a pot at once. Rain soaks into the clay; in cold weather the water freezes and expands, splitting layers off the surface. Heat from fires, and the acids and sulphates in flue gases, attack the inside of the clay and the mortar beneath it. Add the sun, wind and many years of weathering, and an old pot can crack down its length or shed flakes.
The flaunching often fails first. Once it cracks or pulls away from the pot, water runs under it and into the stack, and the pot loses the grip that holds it upright. A pot can then tilt or rock with the wind, and a rocking pot is a falling pot. A fallen pot is a danger to anyone below, which is why a visibly loose pot is something to deal with promptly. Our chimney flaunching guide goes into the mortar side of the problem.
Old repairs sometimes make things worse. A pot set in hard cement can trap water behind the mortar, and a cracked layer then peels away in the next frost. Matching the mortar to the age and type of the brickwork matters.
Signs you can see from the ground
No climbing is needed for a first check. On a dry day, standing in the garden or on the pavement, look for pots that lean differently from their neighbours, vertical cracks, pieces missing from the rim, and a dark or stained stack top where water has run down. A white or green crust on the brick below the pot may point to water entering over a long period. Pieces of clay or mortar in the gutter or on the ground below are another sign.
If you can see the flaunching, check that it is a firm, sloping cap with no cracks and no gaps around the pot. A flat, crumbly or dished top holds water instead of shedding it. Binoculars or a phone zoom picture help. Our page on sweep or roofer tells you who to ask about which part. If a pot is leaning badly, keep people away from the area beneath it until it has been looked at.
Rebedding a pot or replacing it
Where the pot itself is sound and only the bedding has failed, the work is to lift the pot off, chip away the old flaunching, clean the top of the stack and bed the pot again on fresh mortar. A new flaunching is then formed in a smooth slope that sheds water. If the pot is cracked, flaking or too weak to lift without breaking, it is replaced with a new pot, matched to the old in size and style where the house calls for it. Older terraces and listed houses often need a pot matched closely in colour and shape.
A pot that is replaced should be bedded so it stands upright and sits clear of the brickwork edge. Flue use matters too: a flue serving a gas appliance or a stove may need a particular type of terminal, so tell us what the flue is used for. The work is usually reachable from a ladder or scaffold, and our page on chimney removal permission covers the case where the whole stack is coming down instead.
Where several pots stand on one stack, it makes sense to check all of them while the access is there, even if only one looks poor from below.
Cost depends on how many pots there are, how high the stack is and whether the flaunching and brickwork need work as well. The written quote after survey fixes it, and any scaffold or VAT should be named in it.
What to expect at survey and after
At survey, the pots, the flaunching and the brickwork around them are photographed, and so is the loft side of the stack, to check for damp. A written quote follows, aimed at the least work that cures the problem. A rebedded pot may be all that is needed; where the brick below has also spalled, our page on spalling chimney bricks explains why that repair comes first. A pot on a leaning stack is a different matter, covered in our leaning chimney page.
At the finish we sweep up and remove the waste. A 10-year workmanship guarantee in writing covers it, though it is not insurance-backed. A simple check afterwards: after the next heavy shower, look at the ceiling and any chimney breast in the rooms below, and let us know if a patch appears.
Open pots can hold nests, which are protected, so in spring and summer we check before moving a pot.