Why Chimney Bricks Flake: Spalling Explained

Spalling: the face comes off and the brick stays

Spalling is the loss of the fired outer skin of a brick, usually in thin plates or small curved flakes, leaving a rougher, paler and softer surface underneath. The brick is still there, but the part that did the protecting is not. It is quite different from joints receding, which is ordinary wear on the mortar, and different again from a brick that has cracked right through. Small red flakes on a roof slope, in a gutter or on the path below a stack are the usual first sign.

Two forces do most of the work. Water soaks into a brick and then freezes, and ice takes up more room than water, so the pores are forced apart from within. A stack stands clear of the roof and is wetted and chilled on every side, so it goes through that cycle more often than any wall in the house. Salts do the same job without needing a frost: they dissolve, travel with the water, and crystallise just behind the surface, where the growing crystals push the face off. On an exposed coastal stack, as in chimney repairs in Hoylake, the two act together, which is why the faces that take the wind and spray tend to be the worst.

Why one brick fails and the one beside it does not

Bricks are not all alike, even within a single stack. Hard, dense, well-fired bricks take up little water and shrug off a great many wet-and-dry cycles. Softer ones, underfired or made from a more open clay, draw water in readily and hold on to it. Older stacks very often contain both in the same course, because bricks came from different batches and the softer ones were used where nobody expected to look. The result is a stack where one brick is crisp and its neighbour has lost half its face.

Firing matters in another way too. The outer skin is the hardest, most compact part of a brick, and once it goes the interior is far more absorbent than the surface ever was. That is why spalling tends to accelerate once it starts: the exposed core takes in more water and more salt, which does more damage, which exposes more core. A stack that has been losing faces slowly for some time can then seem to deteriorate quite suddenly.

Mortar takes part in this as well. A pointing mix that is harder and denser than the bricks traps water inside them, so the bricks end up doing all the drying out, with all the salt and frost damage that comes with it. The mortar should be the weaker, more porous partner so that moisture leaves through the joints instead; the reasoning is laid out in our guide to Repointing a Chimney: Why the Mortar Mix Matters More Than the Finish.

Reading how far it has gone

From the garden, judge depth rather than count. A shallow loss where the brick is still flat and its edges are sharp is early. A face that has gone hollow, with rounded edges, a sandy surface and the brick standing back from the mortar line, is well advanced. The top few courses, and the ones just under the projecting head, usually go first because they are the most exposed and the slowest to dry.

Look at what has landed below, and from which side. Red dust and flakes gathering on a bay roof, a lean-to or a path mean the loss is happening now rather than being old history. Compare with a neighbour’s stack facing the same way: if theirs is crisp and yours is not, the brick is probably the difference, not the weather. A phone photograph zoomed in from the ground is enough for us to form a first view, and our notes on What to Look For on a Chimney From the Ground cover how to do it without leaving the ground.

When spalling matters and when it can wait

A few spalled faces on an otherwise sound stack are cosmetic for a good while. The bond is intact, the brick still carries its load, and nothing is about to move. What turns it into a repair is the pattern: a run of faces lost in the same course, several faces going on one corner, or loss at the head where the stack is thinnest and most exposed. At that point the stack is losing the section it needs, and water is being let deeper into the joints than it should be.

The second trigger is what the loss exposes. A bare core soaks up water and carries it through the wall of the stack, where it can reach the chimney breast inside and show as damp. Telling whether the stack is the source of damp indoors is a separate question, and a careful look at the chimney breast helps. A brick that has begun to loosen, a hollow sound when tapped from a safe working position, or a stack that seems to lean are reasons to ask for a look sooner rather than later.

What can be done about it

The range runs from nothing to a rebuild. Where only a handful of bricks have gone, they can be cut out and replaced individually, and that method has its own guide in this series. Where a whole face is affected, a few courses may be taken down and rebuilt, and where the loss runs the full height of the stack it is normally a rebuild. The decision between those routes is discussed in Repoint, Rebuild or Remove? How the Decision Gets Made.

What does not help is anything that seals the surface. Paint, waterproofer and cement-rich skim coats hold salt and moisture inside the brick and make the next frost worse. Please do not pick off loose flakes yourself, and do not climb up to look. Roofline Solutions works on the outside of the stack, is fully insured and gives free written quotations, so send a photograph or ring and we will tell you what we can see.