The short answer
Match the mortar to the brick. Older soft bricks and lime-built stacks do best with lime mortar, which is softer than the brick and lets moisture escape. Modern hard bricks laid in cement can be repointed in a cement-lime mix. Hard cement on soft old brick is the combination that does the most harm.
Why mortar choice matters
Mortar and brick work as a pair, and the weaker of the two is meant to be the mortar. Water moves through a wall by evaporating from its surface, and it prefers to leave through the softest, most porous material. If the mortar is the sacrificial part, the water dries out through the joints, and in time the joints wear and are repointed. If the mortar is harder and denser than the brick, the water leaves through the brick instead. The faces of the bricks flake and crumble, which is damage you cannot repair by repointing, and the stack ends up needing rebuilding.
A chimney stack makes this effect worse than a wall. It is exposed on all sides, it freezes and thaws constantly, and flue gases add moisture and acids from inside. The mix you choose has to cope with those conditions for as long as possible.
Lime mortar
Lime mortar is made from slaked lime and sand, sometimes with a small addition of cement for early strength. It was used in buildings until cement became common in the twentieth century. Its main properties are these.
- It is softer and more flexible than cement, so it moves with the stack instead of cracking.
- It is vapour-open, so moisture passes out through the joints rather than the brick.
- It hardens slowly by taking in carbon dioxide, which means it needs protection from frost and rain for longer.
- It can partly heal fine cracks over time.
- It is the traditional choice for older houses, and is expected on listed buildings.
The drawbacks are slower working, more skill in mixing and applying, and a slightly higher price. In the open conditions of a chimney top, a lime mortar usually needs a little more time to cure before it sheds weather reliably. Our page on how long chimney repointing takes covers the timescales.
Cement mortar
Cement mortar uses Portland cement, sand and water, often with a plasticiser. It is strong, sets fast and is simple to use, which is why it became the default for modern building. On modern, hard-fired bricks, a sand and cement mix is often acceptable, and builders have used it for decades. Its limits show on old brick. A strong cement joint is brittle, so a moving stack cracks it, and the cracks let water in behind the hard surface where it cannot easily escape. The brick faces take the punishment. We see this on older houses where a past repair used cement across the joints of soft bricks and left the brickwork flaking around them.
Which to choose for your stack
| Your stack | What usually suits |
|---|---|
| Pre-1920s, soft handmade brick, lime bedded | Lime mortar |
| Listed building or conservation area | Lime mortar, matched to the original |
| Mid-century hard machine-made brick | Cement-lime mix, or lime if the original was lime |
| Modern stack in hard engineering brick | Cement mortar or a cement-lime mix |
| Mixed ages, previous cement patching | Judged on site, matched to the brick, avoiding hard patches |
The way to decide is to look at what is there. We check the brick type, scrape a sample of the old mortar, and look at past repairs to see how they have fared. The mortar colour and the joint profile also matter for appearance. On a visible stack in an older street, the pointing is part of the look of the house.
Many Basingstoke houses have post-war brick stacks built in a mortar that sits between the two extremes, and the answer is often a cement-lime mix. Older village properties in the surrounding area are more likely to need lime. If you are unsure, a roofer should be able to explain why they recommend what they do. The page on chimney repointing sets out the full process.
Getting the finish right
The mortar itself is only half of it. The colour of the sand decides how the joints look, and a pale or yellow sand against warm red brick stands out for years. We match the sand to the original where we can, and try a small test patch before the whole stack is done. The joint profile matters as well. A flush or slightly recessed joint sheds water well, while a deep recess collects it and a raised, ribbon-style joint is vulnerable to frost. The depth of raking out and the dampening of the brick before pointing affect how well the new mortar bonds, so we treat the preparation as seriously as the mix.
Cost, and a note on rules
Lime mortar can cost somewhat more in labour and materials, which adds a modest amount to a repointing job. Cost guides give chimney repointing a spread from £450 to £1,200, with about £750 as a typical figure, and these are national numbers not specific to the mortar chosen. The price for your stack follows the survey. For listed buildings and conservation areas, like-for-like repairs are usually fine, but changes to materials or appearance may need consent, so check with the council's conservation officer first. For all chimney services, see chimney repairs in Basingstoke.
Every roof is different. A free survey with photos settles it for yours.
Get a free survey