Why are my chimney bricks crumbling?

Spalling is the loss of a brick's outer face. The hard, fired skin cracks and lifts, and the softer body underneath erodes quickly, so a spalled brick looks hollowed out and pitted, with orange powder lying on the ledges below.

Ordinary weathering is slower and kinder. Spalling is quicker and patchy, and tends to hit individual bricks while their neighbours stay sound. That pattern is a clue. When one brick fails and the next is perfect, the cause is usually local: a spot where water collects, a joint that has gone hard, or a brick that was soft when it was laid.

Few parts of a house take more weather. Wet on every face, with no warm room behind it to dry it from within, a stack tests bricks that would last for generations in a garden wall.

Brick is porous. After rain it holds water in thousands of tiny pores, and as that water freezes it pushes the pore walls apart. One freeze does little; repeated freezing and thawing across a winter does. The face is the thinnest part of the brick, so it gives first.

That is why damage often shows in late winter, on the faces that stay wet longest and under a broken flaunching (the mortar bed that holds the pots). Our guide to chimney damage in winter covers the seasonal side; here the point is narrower. Frost is the trigger, but a brick has to be wet and weak before it fails.

Softer, older bricks, including some handmade ones, are more vulnerable than hard engineering bricks, and an under-fired brick will spall before its neighbours.

Older brick and stone stacks were built in lime mortar. Lime is softer and more open than the brick it holds, so water leaves the wall through the joints. If the joints are sacrificial, wearing away slowly and needing repointing every few decades, the bricks are spared.

Modern cement mortar reverses that. Hard and dense, it is stronger than an old brick and almost waterproof, so water that gets into the stack can only escape through the bricks. They take the strain, freeze and flake, while the cement joints stand proud around them. If your stack has crisp grey pointing and ragged, receding bricks, that is the classic sign of a cement repoint that went onto the wrong wall.

The remedy is to take the cement out carefully, without chipping the bricks, and repoint in a mortar matched to the age and softness of the stack. We say more in our notes on the best mortar for chimney repairs. MyBuilder's 2026 chimney guide puts repointing at about £750 on average, in a range of £500 to £2,000 depending on the house, and that is a national figure, not our price.

A third cause sits inside the chimney. Burning coal, wood and gas produces acidic and sulphur-bearing gases, and these condense in a cold flue. Over the years, salts build up in the mortar and the brick on the flue side. When the stack gets wet, these salts dissolve, travel to the surface and crystallise as the water dries. The crystals grow in the pores and force the face off the brick, much as frost does.

Salts show as a white or brown bloom, and sometimes as staining that creeps through into the chimney breast indoors. An unused flue can still suffer, because the salts remain in the masonry and rain still reaches them. A sulphate attack on the mortar can also swell the bed joints on the flue side, which is one reason some stacks lean. If you see a lean, read our guide to the leaning chimney problem before you book a repointing job alone.

A flue liner, or a cap and ventilation on an unused flue, can reduce the problem. Which of those is right depends on what the flue is for, and it is a question for the survey.

Replacing a spalled brick without asking why it failed is a short-term fix. On a survey we look at where the water is getting in and where it stays. Typical culprits are:

• a cracked or missing flaunching, so water pools around the pot
• open or failing joints high on the stack
• flashing or a back gutter that has let go, which feeds water into the lower courses
• missing or broken pots and an uncapped flue
• cement render or paint that seals the surface and holds water behind it

Flashing, in particular, is worth checking because water that runs down behind it soaks the base of the stack. Our answer on when chimney flashing needs replacing lists the signs. Photographs taken from the roof give us, and you, a clear record of what is failing and where.

It also helps to know whether the problem is the brick or the stack's use. If a flue is blocked or misbehaving, a chimney sweep may be the first call, and our note on whether to call a sweep or a roofer sets out the split.

Light spalling on a sound stack can sometimes be left under watch, once the water source is fixed. Badly spalled bricks are cut out one at a time and replaced with bricks matched for size, colour and hardness, bedded in a mortar suited to the stack. The surrounding joints are raked out and repointed at the same visit, so the new work sheds water evenly.

Where spalling has gone through many courses, the stack is no longer worth patching and a partial or full rebuild makes more sense. MyBuilder's 2026 chimney repair guide gives a stack rebuild a range of £2,000 to £10,000 or more, which is why the survey exists: to tell you which end of the scale your stack sits at. Our written quote then covers the scaffold, brick replacement, repointing and any flashing.

Surface treatments, sealers and paint are no substitute. A coating that stops the brick breathing can make spalling worse. If you want to understand the options, start at our chimney repairs page, then send photographs of the stack and we will say what we can see from them. Finished work is cleared away, and it carries our 10-year workmanship guarantee in writing.