Why are my chimney bricks spalling, and what is the fix?

You see it first as small flakes on the path, the lawn or the roof below the stack, often after a cold snap, then as brick faces that look rough, pitted or peeled. In bad cases whole outer layers fall away to expose a softer, powdery interior. The mortar between the bricks may stand proud, because the brick has worn back further than the joint. Look at the lower courses, close to the roof, and the top courses under the pots, since those two zones often fail first.

Spalling is not the same as ordinary weathering. Brick darkens and softens with age, and that is harmless. Spalling means the surface is breaking away in pieces, which exposes more brick to the weather and speeds up the damage. A chimney is the first place to show it, since a stack has no roof or wall to shield it and every face is wet and cold in winter.

Brick is porous, so rain is drawn into its surface, and on a freezing night the moisture inside turns to ice. Ice takes up more space than water, so it pushes outwards against the face of the brick. Do that on enough winters and the face gives way. Our page on chimney winter damage shows the pattern in more detail.

Chimneys suffer more than walls because water reaches them from above and the flue adds its own moisture. Flue gases carry water vapour and acids, and a cold, unused flue lets condensation build up inside the masonry. A missing cap or cracked flaunching lets rain run straight down the inside. The more water a brick holds as winter arrives, the worse the damage, which is why top-of-stack repairs matter so much.

Many spalling stacks were repointed at some point in hard cement mortar. The logic seemed sound: a strong mortar for a long-lasting repair. But old bricks are often softer than the cement. When frost and moisture move through the wall, the weaker material takes the strain, and with cement joints that is the brick. The joint survives while the brick face flakes off around it.

A softer, lime-based mortar sacrifices the joint instead, and joints are far cheaper to repoint than bricks are to replace. Our guide to lime mortar on chimneys explains why it is the usual choice for older stacks. The same logic explains why sealing or painting a spalling stack tends to fail: it traps the moisture that caused the damage. Our page on chimney waterproofing explains when a breathable repellent is worth using and when it is not.

The sequence starts with stopping water entering. That means checking the cap, flaunching, flashing and pointing, and fixing whatever is failing. Next, bricks that have lost their faces are chiselled out and swapped for similar bricks, or in lighter cases the face is left alone if the brick behind is sound and dry. Repointing follows, using a softer mix than the brick.

A badly spalled stack, especially one that leans, can need part of it or all of it rebuilt, because loose faces and weak joints no longer hold the weight together. MyBuilder's chimney repair guide, updated in August 2026, puts repointing at around £750, within a range of £500 to £2,000, and a stack rebuild at £2,000 to £10,000 or more. Those are national figures. Your own price comes in writing after a survey, backed by pictures of the stack. Chimney-height scaffolding is £400 to £900 in MyBuilder's scaffolding guide.

Spalling bricks sometimes point to a problem inside. A sweep can check whether the flue is sound and clear, and our page on chimney sweep or roofer explains which faults belong to which trade. Where a chimney breast has gone from the room below, the stack above may lose support, and our notes on a removed chimney breast describe what to look for.

A damaged pot can also let rain straight into the flue and down the inside of the stack, wetting the brick from within, and chimney pot replacement is often done in the same visit as the brick repairs. Bundling the work saves a second scaffold.

Once the stack is mended, the best protection is a sound cap, tight flashing and joints that are checked every few years. Clearing moss and algae from the top helps the brickwork dry, and so does trimming back any branches that keep the stack shaded and damp through the colder months. A stack in permanent shade can stay wet for weeks. Take a photograph of the stack once a year from the same spot so changes show up early. Keep an eye on the base of the stack too, where water collects around the flashing and where spalling often starts. If flakes keep appearing on the path after the repair, tell us; it can mean damp is still entering from the top or from the flue.

A spalled stack is repaired rather than rebuilt whenever the structure allows, because the lighter job costs less. Repairs are backed by a 10-year workmanship guarantee in writing, and the roof and garden are left clear. The chimney repairs hub links the rest.