The short answer
A spalled brick has lost its fired face to flaking or peeling, and on a chimney that is usually caused by water getting into the brick and freezing, often made worse by hard cement mortar or render that traps moisture. The cure is to stop the water entering, replace the bricks that have lost their faces, and repoint in a mortar softer than the brick, so the joints weather first.
Frost, water and a brick that cannot dry
A brick is not solid. It is full of tiny pores, and in the weather it soaks up rain and then dries out again. A chimney stack is wet more than any other wall on the house because every face is exposed and the top courses are soaked from above. When water in a brick freezes it expands, and if the brick is saturated, the pressure pushes the outer skin off in flakes.
That flaking is spalling. It shows as bricks with rough, pitted faces, loose grit and powder on the roof or the ground, and a lighter, softer layer where the fired surface has gone. The surface of a brick is its hard outer skin, and once that is lost the softer core underneath weathers quickly.
A stack on an older or listed house may be built of soft bricks, which are more vulnerable. Older bricks were fired at lower temperatures and are more porous.
The scale of damage varies. A few bricks near the top may be affected where the water is heaviest. In a bad case, whole faces are crumbling, and the stack is no longer sound.
Hard cement and what it does to old brick
The mortar between bricks is meant to be the weaker partner. Water should move out through the joints, and when the joints eventually wear, they are renewed. A strong cement mortar reverses that. If the joints are harder and less porous than the bricks, moisture cannot pass through them and leaves through the brick faces instead. The faces take the weathering that the joints should have taken.
Cement render has a similar effect on a whole stack. If it traps moisture behind, the bricks inside stay wet for longer, and spalling follows. Often the damage only shows once the render is stripped.
Many older stacks were repointed at some stage with a cement mix, and the result shows up later as flaking bricks with sharp, standing joints between them.
Frost is the trigger, but mortar is often the cause. A wet brick with a free path for water out will survive many winters. The same brick locked between hard joints will not. Our page on chimney repointing in lime mortar explains the alternative.
Replacing the bricks that have gone
Where only the face of a brick has gone, it is sometimes possible to turn it, if the brick is deep enough and the other face is sound. More often the damaged bricks are cut out one at a time, and new or reclaimed bricks are bedded in their place. Cutting out needs care, since heavy blows can shake the neighbours loose, so a small chisel and patience are used.
The replacement should match for size, colour and porosity, which is how easily it takes up and releases water. A brick far tougher than those around it simply moves the problem to them. Reclaimed bricks of the same age are the usual choice on older houses.
Then the joints are raked out and the stack repointed. A mortar softer than the brick is chosen, so the joints weather first, and the new work is protected while it cures. If the flaunching at the top is cracked, it is renewed so that water stops entering from above.
When many bricks have failed, or the stack is leaning or cracked, partial rebuilding is the safer course, as described in our guide to a chimney rebuild. A few failed bricks, by contrast, can be dealt with in a single visit from a scaffold or tower.
Lime, consent and what we check
Lime mortar is the usual match for old soft brick, because it is more porous than the brick and lets moisture escape through the joints. It is also easier to renew later without damaging the masonry. On a newer stack with harder brick a different mix may suit, which is a decision made on the survey, from the brick in front of us.
Planning has a part to play. For a house in a conservation area, ask the council whether an Article 4 direction applies. A listed building needs listed building consent where changes affect what makes it special, and replacing bricks can count. Ask the planning team before the work is booked.
At survey stage we go up to the stack, record every face, press the bricks to find which are sound, and study the mortar, flaunching and lead. We record the cause as well as the symptom, which is why we look at the roof and the stack together.
The price follows from what we find: how many bricks, how much access is needed, and whether the repair is a few bricks and pointing or a larger rebuild. It comes as one written total. See also our chimney repairs hub for the wider list of work.
