Why Are My Chimney Bricks Spalling?

A brick is hard on the outside and porous in the middle. Rain soaks in, and the brick dries out again when the sun and wind come back. Spalling begins when the water cannot get out fast enough. In a hard frost the trapped water turns to ice and takes up more room, and the brick surface cannot take the force, so a thin layer lifts off. Repeat that over several winters and the face of the brick falls away in flakes or small scales, leaving a rough, pitted surface.

No other brickwork on a home takes the weather like a stack. Rain strikes it from every side, and the wind dries it unevenly. On top of that, the flue inside carries warm, damp air from a fire, a boiler or a sealed-off cap. The brick is often wet from inside as well as out.

Fallen flakes in the gutter, on the roof or in the garden are the first thing a homeowner spots. Our answer on signs your chimney needs repair shows how spalling sits alongside the other warning signs.

In a well-built old stack, the mortar is the softer part, and moisture leaves through the joints. When someone repoints in hard cement, the joint becomes tighter than the brick, and the water has to leave through the brick face instead. The salts it carries dry in the surface, the frost does its work, and the face lets go. The same thing happens when a cement fillet is smeared across the base of the stack, or across a join that should be flexible.

Clear sealants and waterproof sprays cause a similar problem. They can seal the surface while water is already inside the brick, so the moisture stays put and the freeze-thaw damage carries on behind the coating. Wait until the cause is found before buying any coating, and be wary of any product that promises to stop flaking without a repair to the joints or the cap.

The fix is to put the right mortar back, a softer one than the brick, and to find out where the water is coming from before you spend on a coating. Our answer on lime mortar chimney repointing explains why the choice of mortar matters so much.

It depends how deep the damage goes. If only the odd brick has lost its face, and the rest of the stack is sound, a bad brick can be chiselled out and a replacement bedded in the gap, with the neighbouring joints dressed to blend in. The roofer chips out the damaged brick and the mortar around it, fits a matching brick or a half-thickness slip, and repoints the joints. The new brick should be a similar colour and hardness, and ideally no harder than its neighbours, or it will shed water onto them.

Where bricks can be turned round, that is sometimes done on older work, but the back face of an old brick may be rougher or softer, so it is a judgement call. If a large part of the stack is flaking, patching can look worse than the problem, and rebuilding the top section is the sounder repair. Our page on rebuilding a chimney stack explains when that point is reached.

Whichever route is chosen, the cap, the flaunching and the flashing deserve a look in the same visit. Replacing bricks under a cracked cap that keeps letting rain in is a short-lived job.

A roofer should send you close-up photos of the worst bricks before quoting.

As a national guide, Checkatrade's April 2026 chimney repair table gives repointing at £300 to £1,000, and its October 2026 repointing table has an average of around £750. Swapping a few bricks sits inside the same kind of job. Your own figure turns on how many bricks are affected, how tall the stack is and how it is reached, so check whether VAT and scaffolding are inside any price.

Do not leave spalling for years. A few flaking bricks are cheap to deal with. Once the face has gone, the softer centre of the brick erodes faster, and whole sections can loosen. Frost is the main trigger, so a repair done before winter saves a worse one the next spring.

The stack gets a proper look and the damaged bricks are photographed up close, after which a single written price follows. Our chimney repairs hub lists the other stack jobs. A photo of the worst patch, taken from the ground, tells us where to begin.