The short answer
Crumbling bricks usually mean the stack has been soaking up water and freezing. Nothing shelters a chimney from rain, and brick that cannot dry between storms flakes and powders. Hard cement pointing on soft old brick makes it worse, because the brick then takes the weathering that the mortar should have taken.
Brick faces that flake, powder or drop away point to moisture trapped in the stack, and the cause has to be fixed along with the damage.
Frost, wet and nothing overhead
Of all the brickwork on a house, the stack takes the worst weather. Rain strikes it from every side, wind dries it unevenly, and nothing covers the top. The windward face gets the worst of it, which is why one side of a stack often fails before the others. Brick is porous, so it drinks in water and gives it back as conditions change.
How does frost break a brick?
Water inside the face of a brick turns to ice on a cold night and swells. The swelling pushes the surface off. Builders call the result spalling: flakes or whole faces peeling away, leaving a rough, pitted brick with a powdery red or yellow crust. One winter does a little. Many winters of the same cycle strip a soft brick completely.
Water finds several routes into a stack. A cracked flaunching, the mortar crown on top, lets rain run in. Failed lead flashing at the roof line lets it creep up from beneath. A pot with no cap sends rain straight down the flue and into the inside of the shaft.
While any of these is happening, the brickwork never gets a chance to dry. The crumbling you see is the end of a longer story, and it explains why new bricks alone rarely cure it.
Hard cement on soft brick
Old stacks were laid in lime mortar, which is softer than the brick it holds. A repointing job in the last few decades may have used hard cement mortar instead, and that is a frequent reason for bricks failing.
Why does hard pointing harm soft brick?
Every wall that gets wet has to get rid of the water somewhere. In a lime-pointed stack it leaves by the joints, the softer and more open part. Dense cement pointing does not let moisture pass, so all of it must escape through the brick faces. They then take the full force of frost, and they crumble while the cement stands proud.
You can often spot it from the garden: crisp, intact pointing sitting out from bricks that are rounded, hollowed or flaking behind it. Our answer on lime mortar on chimneys explains why softer mortar protects older brick.
Cement pointing is hard to take out without chipping the brick edges, so this fault is not a quick patch. The old pointing has to be cut back carefully, and the new mortar chosen to suit the brick.
What can be done about it
How much can be saved turns on the extent of the damage, and a survey settles that before anything is priced.
Can crumbling bricks be repaired?
When a few bricks have lost their faces and the rest are sound, those few are chiselled out and bricks of matching age and colour are bedded in their place. Reclaimed bricks often suit an old stack better than new ones, because their texture and shade already match the weathered brickwork around them.
Where the joints have failed, repointing in the right mortar helps. MyJobQuote gives a typical UK range of £450 to £1,200 for chimney repointing (MyJobQuote, Oct 2026), and stack height and access move the figure within it. Our chimney repointing page describes the job.
If bricks are crumbling right across the shaft, or the stack is loose, repairs that keep failing cost more in the end than taking it down and building it again. Ask for the photos from the survey and compare the worst bricks with the best. If the best are not much better than the worst, that tells you where the stack is heading.
Stopping it happening again
Replacing bricks without finding the water gives a short-lived result. The new bricks go the same way if rain still has a route in.
Should the stack be sealed with a coating?
A clear waterproofing liquid sounds sensible, but it can trap the moisture already in the brick and make spalling worse. Fix the entry points first. Our answer on waterproofing a chimney with sealant goes into why.
A sound repair covers the whole path of the water: the flaunching and pot bedding at the top, the pointing down the shaft, the lead flashing at the roof line, and a cap on any flue that is not used. Sand in the gutters or bricks on the lawn mean the stack should be treated as unsafe, so keep people away from beneath it until a roofer has looked. Do the same if a brick drops out and leaves a hole you can see daylight through.
Somerset Building Control lists "unstable chimneys" among the dangerous structures it deals with. For the other work on a stack, see our chimney repairs service. The photos, and the single written price, follow the survey.
More on failing brickwork
Is sand falling from my chimney a warning sign?
Yes, if it keeps happening. Sand is the mortar between the bricks wearing away, and when enough has gone the bricks lose their grip on each other. Look at the gutters and the ground below for grit, and ask a roofer to check the joints before the weather gets deeper into the stack.
Can I paint over crumbling brick?
Paint will not fix the cause, and a film of masonry paint can hold water in the brick behind it. If the surface is powdering or flaking, the damaged brick needs replacing and the water route closing, not covering. A roofer can show you the worst areas in photos.
The full picture: Chimney repairs in Taunton · More short answers