The short answer
For an older brick or stone stack, a lime mortar matched to the original is usually the right choice, because it is softer than the masonry and lets damp pass out by way of the joints. Flaunching, which holds the pot and sheds water, wants a tougher, more weather-resistant mix. Hard cement used on soft old brick tends to trap water, so the bricks, not the joints, take the damage.
Why the joints should give way before the masonry does
A stack is exposed on every face, stands clear of the roof and is warmed from inside by flue gases. It gets wet often, and water that gets in has to get out again. In a well-built wall the mortar joints are the exit route.
Hence the old rule that mortar should be softer and more permeable than the brick or stone around it. Moisture leaves by the joints, and any frost damage happens there, where a roofer can repoint it. When a hard, dense mortar goes into soft old brick, the water is forced out through the brick faces instead. Frost then flakes the face off the brick, a failure called spalling, and the joints end up standing proud of the wall.
The aim, then, is to give the repair the same breathing behaviour as the original wall, joint by joint. A stack should lose its repair first at the pointing, which is cheap to renew, not in its bricks, which are not.
Lime and cement mortars compared
The two families behave differently, and most chimney mortars are one of them or a blend.
| Mortar | How it behaves | Where it suits |
|---|---|---|
| Lime and sand | Softer and permeable; lets damp leave through the joint; can move slightly without cracking | Old brick and stone stacks, listed and conservation-area houses |
| Cement and sand | Hard, dense and strong; sheds water well but does not let the wall dry through it | Hard modern brick; flaunching; capping |
| Lime and cement blend | In between, with some strength and some breathability | Mixed or uncertain masonry, where a roofer judges the balance |
Lime comes in different grades, from non-hydraulic putty that sets slowly with the air to hydraulic limes that set by reacting with water and gain strength faster. Which grade suits a stack depends on its exposure and how fast the work has to set. On a stack that is open to the weather on all sides, a faster-setting hydraulic lime often makes sense. We choose at the survey, after looking at the existing joints.
Flaunching needs a different mix
Flaunching is the sloping mortar collar that sits around the pots at the top of the stack. It holds each pot in place and throws rain off the stack top. Unlike the joints below, it is not trying to breathe; it is trying to shed water and to stay put in a place that is windy and wet. A stronger, more water-resistant mix is the usual choice, laid with a good fall and a drip edge.
That is the exception to the general preference for soft mortar. If you ask why a lime stack has a harder cap, this is the reason: different parts of one chimney do different jobs. Our guide to capping a chimney shows how the top of a stack is finished once a flue is no longer in use.
A flaunching that has cracked or shrunk away from the pot lets water into the top courses, so it is worth watching. If your pot is on its last legs, the two repairs are best done together.
Matching the original: colour, sand, texture and finish
A repair that sticks out looks worse than a stack left alone. The roofer matches four things: the mortar's colour, the size and colour of its sand, the depth of the joints and the way they are finished. Sand is the part owners overlook, since it gives a mortar its colour and grain. A small sample patch, left to dry and weather for a day or two, shows whether the match works before the whole stack is touched, and it is far easier to adjust the sand at that stage than after the face is finished.
Joint finish varies too. Many older stacks have a flush or slightly recessed joint. A raised, weathered, ribbon-style joint on an old brick wall traps rain and looks wrong. On a stone stack the joint is often struck back from the face so the stones read as the main thing. Our guide to stone chimneys explains why matching counts so much with Cotswold stone.
On a listed house, repair mortar can fall within the consent process, so ask the council's conservation team before anything is raked out.
What mortar affects the price, and what to ask for
Lime mortar takes longer to prepare and place than a ready-mixed cement one, and it may need protecting from frost and fast drying while it sets. That is why a lime repoint can cost more than a cement one on the same stack. MyBuilder's 2026 chimney repair guide gives an average repointing figure of about £750, with a range of £500 to £2,000, and the spread comes partly from the stack's height, access and condition rather than the mortar alone.
When you compare quotes, ask what mortar each uses and why. A quote that says only "repoint chimney" tells you little. Ours names the mix, the access and any replacement bricks one by one, which makes comparison straightforward. A 10-year workmanship guarantee follows, in writing, once the work is done.
If you suspect the flue itself is the problem, a blocked chimney can cause damp and staining that look like a mortar fault. Stack work as a whole is covered under chimney repairs in Stroud.