The short answer
The guiding rule is that the mortar should be softer and more breathable than the brick. On older and solid-wall chimneys built with soft, porous brick, lime mortar is usually correct: it flexes with the masonry, lets the wall breathe and dry, and sacrifices itself ahead of the brick when it weathers. Hard cement mortar on soft historic brick is a common, damaging mistake — it is rigid and impermeable, so it traps moisture, cracks because it cannot flex, and forces frost and spalling damage into the brick faces instead of the joints. Cement-based mortar can be appropriate on newer chimneys built with modern, harder, frost-resistant bricks. The right answer depends on the age and hardness of the brickwork, so it is worth confirming with a competent mason.
Choosing the wrong mortar is one of the most common and most damaging repointing errors. The decision is not about which mortar is stronger, but which is compatible with the brick it surrounds.
Lime vs cement
- Core rulemortar softer than the brick
- Older / soft bricklime mortar
- Modern / hard brickcement may suit
- Lime givesbreathability and flex
- Cement risk on old bricktraps damp, spalls brick
How the two mortars behave
Lime mortar and cement mortar behave very differently in a wall, and on a chimney those differences matter more than usual because of how exposed the stack is. Lime mortar is comparatively soft, flexible and breathable (vapour-permeable). It allows moisture to move through the joints and evaporate, lets the wall accommodate small movements without cracking, and is designed to weather sacrificially — wearing away ahead of the brick so the brick is preserved. It is the traditional mortar of older British buildings and is compatible with the soft, porous bricks they were built from.
Cement mortar is hard, rigid and relatively impermeable. It is strong and sets quickly, which suits modern construction, but it does not flex and it resists the passage of moisture. On a wall built to work with cement — modern, dense, frost-resistant brick — that is fine. On a wall built to work with lime, it is a problem: the rigid joint cannot move with the masonry, so it cracks; and because it is impermeable, any moisture that does get in is trapped and pushed into the brick rather than escaping through the joint.
Why the wrong choice damages a chimney
The classic and damaging error is hard cement pointing on a soft-brick chimney. Because cement is harder than the surrounding brick, the rule that the mortar should be the sacrificial element is reversed: now the brick weathers ahead of the joint. Trapped moisture cannot escape through the impermeable cement, so it sits in the brick, freezes, and breaks the faces off — spalling caused directly by the wrong mortar. The rigid cement also cracks as the stack moves and heats, opening hairline gaps that let water in behind the pointing, where it does further hidden damage. The result is a chimney that looks repointed but is quietly deteriorating faster than before.
Lime mortar on an appropriate stack does the opposite: it keeps the wall breathing, flexes rather than cracks, and protects the brick by weathering first. The trade-offs are that lime is slower to set and cure, needs protection from frost and rapid drying while it cures, and demands more skill to use well — which is why it is sometimes avoided in favour of a quicker cement job that stores up trouble. On the right building, that extra care pays off in a far longer-lasting, more sympathetic repair.
Comparing the mortars
The table summarises the practical differences. The correct choice follows from the age and hardness of the brickwork rather than a general preference for one mortar.
| Factor | Lime mortar | Cement mortar |
|---|---|---|
| Breathability | high, lets wall dry | low, traps moisture |
| Flexibility | flexes with movement | rigid, can crack |
| Weathers | sacrificially (protects brick) | harder than soft brick |
| Suits | older / soft, porous brick | modern / hard, dense brick |
| Curing | slower, needs protection | faster, less fuss |
Indicative guidance on mortar selection. Sources: SPAB / Historic England lime mortar guidance.
Choosing for your chimney
The decision rests on one question: what is the brick like? If the chimney is on an older or solid-wall property and the bricks are soft and porous — typical of Victorian and earlier housing — then a lime mortar is almost always the right choice, and on listed or conservation-area buildings it may be required. Matching the type of lime, the mix and the joint profile to the original is part of doing it well. If the chimney is on a newer house built with modern, hard, frost-resistant brick, a cement-based mortar (often a gauged mix) can be appropriate and is what the wall was designed for. Where you are unsure of the brick's age or hardness, it is worth getting a competent mason to assess it before any repointing, because the cost of choosing wrong is not just a poor repair but active damage to the brickwork. As a default for any traditional chimney, lean towards lime and confirm rather than reaching for cement out of habit.
Frequently asked questions
Why is lime mortar better for old chimneys?
Because it is softer, more flexible and breathable than the brick. It lets the wall dry, flexes with movement rather than cracking, and weathers sacrificially so the brick is protected. Hard cement on soft old brick traps moisture and makes the brick spall instead.
Can I use cement mortar on any chimney?
Only on chimneys built with modern, hard, frost-resistant brick that the cement is compatible with. On older chimneys with soft, porous brick, cement is too hard and impermeable — it traps damp and damages the brickwork. Match the mortar to the brick.
Is lime mortar harder to work with?
Yes, somewhat. Lime sets and cures more slowly, needs protecting from frost and rapid drying while it cures, and takes more skill to use well. That extra care is worthwhile on a traditional building, where lime gives a longer-lasting, more sympathetic repair.
Sources & further reading
- SPAB — lime mortar and repointing advice
- Historic England — repointing brick and stone
- Checkatrade — repointing costs
Figures on this page are typical UK ranges drawn from published sources and depend on your specific chimney. They are guidance, not a quotation.