Spalling brickwork and lime mortar
If the faces of your bricks are flaking, crumbling or falling away, the cause is very often the mortar around them rather than the bricks themselves — and specifically, mortar that is harder than the brick it surrounds.
This is the technical argument underlying most of what we do on chimneys and old brickwork. It explains why a repair carried out in the 1980s can be the reason your stack needs rebuilding now.
What spalling is
Spalling is the loss of the outer face of a brick. It starts as fine flaking or a crumbly surface and progresses to whole faces breaking away, exposing the softer, more porous core beneath. Once the face is gone it does not come back, and the brick absorbs water far faster than it did before — so spalling accelerates once it starts.
The mechanism is almost always freeze–thaw. Water gets into the brick, freezes, expands by roughly nine per cent, and pushes the face off from the inside. All it needs is a brick that stays wet through a frost.
Why Victorian brick and cement mortar do not mix
This is the part that surprises people, and it matters more on a chimney than anywhere else on a building.
Traditional London stock brick is comparatively soft and porous. It was built with lime mortar, and that combination was designed to work as a system:
- The mortar is softer than the brick, deliberately. It is the sacrificial element.
- Moisture leaves through the joints, because they are the most permeable path.
- The mortar erodes instead of the brick. When joints wear out you repoint them — a modest, repeatable maintenance job.
- The wall accommodates movement through many small mortar joints rather than cracking.
Now repoint that wall in a strong sand-and-cement mortar. You have reversed the relationship: the joints are now harder and less permeable than the bricks. Moisture in the wall can no longer leave through the mortar, so it leaves through the brick faces instead — and takes them off in the next frost.
The visible signature. A stack repointed in hard cement typically shows crisp, intact, proud mortar joints standing slightly forward of eroded, crumbling brick faces. That is the wall telling you exactly what happened: the mortar outlasted the brick, which is precisely backwards. Once you have seen it you will notice it all over South East London.
Why chimneys suffer worst
Every factor that drives spalling is amplified on a chimney stack:
- Exposure on all four faces, with no shelter and nothing above it
- It cannot shed water like a slope — rain sits on and in it
- Disused flues no longer warm and dry the stack from inside
- Failed flaunching lets water in from the top, saturating the masonry from within where nothing can dry it
- Sulphate attack — flue gases and prolonged damp react with mortar, causing expansion, horizontal cracking and leaning. See chimney stack repair.
When repointing works, and when it does not
| Condition | What it needs |
|---|---|
| Sound brick faces, eroded or open joints | Repointing in an appropriate mix |
| Light surface flaking on a few bricks | Repointing, and monitor |
| Faces blown off scattered individual bricks | Cut out and replace those bricks, then repoint |
| Widespread spalling across a face | Rebuild the affected section — there is nothing sound to point against |
| Hard cement pointing standing proud of eroded brick | Careful removal of the cement, then repoint in lime |
| Horizontal cracking, expansion, leaning | Structural — see stack repair and rebuild |
The honest test is simple: is there enough sound brick left to point against? Repointing works by keying new mortar into a joint between two solid faces. If those faces have gone, there is nothing to key into and no amount of pointing will hold.
Replacing individual bricks
Where spalling affects scattered bricks rather than a whole face, those bricks can be cut out and replaced. Two things matter:
- Matching. Reclaimed London stocks of the right colour, size and texture are almost always a better match than modern equivalents on a period property, and on a visible stack the difference is permanent.
- Turning bricks. Occasionally a spalled brick can be removed, turned to present a sound face, and re-bedded. Genuinely useful where matching bricks are hard to source, but it depends on the brick being sound behind the damaged face.
What about waterproofing sealants?
Brick sealants and water-repellent coatings are widely sold as a solution to damp brickwork. Be very cautious applying them to old, soft brick.
The problem is that these products reduce water entering the face but do not stop water reaching the masonry from elsewhere — through failed flaunching, defective flashing, or from inside the building. Water that gets in behind a sealed face then cannot evaporate out through it. The result can be worse spalling than before, because the moisture is trapped and freezes behind a sealed surface.
Fixing the source of the water is the answer. A coating is not a substitute for flaunching that has cracked or pointing that has washed out.
Choosing the right mortar
The principle is that mortar should be softer than the masonry it bonds. For soft London stock brick that generally means a lime-based mix rather than a strong cement one, matched in strength and appearance to the original.
Lime mortars need more care: they cure more slowly, cannot be laid in frost, and need protecting while they set. That makes them slightly less convenient, which is exactly why they are often skipped. On a stack you intend to keep for another fifty years, the convenience is not worth what it costs the brickwork.
We assess the existing brick and mortar and specify accordingly rather than defaulting to one mix. Where the original is genuinely a harder brick and a cement mortar is appropriate, that is what we will recommend.
Brickwork crumbling on your chimney?
A close photograph of the brick faces and the joints usually tells us whether you are looking at repointing, brick replacement or a rebuild.
Brickwork questions
Can spalled bricks be repaired rather than replaced?
Not really — the lost face cannot be restored. Filling with mortar or a repair compound is cosmetic, does not match, and traps moisture behind it. The realistic options are replacing the affected bricks, turning them where the back is sound, or rebuilding the section.
How do I know if my chimney was pointed in cement?
Look at the relationship between joint and brick. If the joints are hard, crisp and standing proud while the brick faces are eroded and recessed, it is almost certainly a hard cement mortar on softer brick. Original lime pointing tends to be flush or slightly recessed, softer to the touch, and lighter in colour.
Should hard cement pointing be removed?
Where it is causing brick damage, yes — but carefully, because removing hard mortar can damage the brick arrises if done roughly. It is slow work and it is why re-pointing over cement-pointed brickwork costs more than pointing that has simply eroded.
Is lime mortar weaker? Will the chimney be less stable?
Lime mortar is softer in compression, which is the point — but a lime-built stack is not unstable. Buildings have stood in lime mortar for centuries. Strength in masonry comes from mass, bond and geometry, not from having the hardest possible mortar. What genuinely destabilises a stack is water getting into it and frost breaking the brick apart.