Few parts of a house are as weathered as its chimney stack. It rises clear of the roof, takes rain and wind from every side, and dries slowly in winter. On a Bath stone stack, the mortar joints are the part meant to give way first. They are the sacrificial layer: softer than the stone, more open to moisture, and cheaper and easier to renew than the blocks beside them. Repointing with lime keeps that arrangement working. Repointing with cement reverses it, trapping water in the stone and sending frost damage into the faces of the blocks. Lime suits older stacks for good reasons, a proper repointing runs through raking out, filling and curing, a stack gives clear signs when it needs the work, and done properly the repointing protects the stone you are paying to keep for the next generation.
Why lime and not cement on Bath stone
Bath stone is an oolitic limestone, and it is relatively soft. It takes up water and gives it back, and the way it does so depends on mortar that behaves in a similar way. A lime joint gives way before the stone and passes moisture readily. When a wet stack dries out, most of the moisture leaves through the joints. Cement mortar is harder and far less permeable. Water that cannot leave through the joints leaves through the stone faces instead, carrying salts with it, and the face of the block flakes, powders or peels in frost.
That is the reason conservation bodies ask for lime on historic masonry. Bath Preservation Trust made the point when commenting on a chimney stack at 7 Walcot Buildings, asking for lime so the historic stonework could breathe. Its comments added that cement used earlier on that listed stack had probably worsened the damage to the stone.
The second reason is movement. A stack is tall, narrow and moved by sun, wind and frost. Lime mortar is more flexible than cement and takes small movement as fine hairline cracking in the joint, which can be left or repaired. A rigid cement joint tends instead to crack away from the stone and let water in behind it.
None of this means lime is a fussy or exotic choice. It was the ordinary mortar for the houses it is now used to repair. What goes wrong when cement is used is covered in our page on cement and Bath stone, and the wider chimney repairs service sets out the other work on a stack.
Spotting a stack that needs repointing
You can see most of the signs from the ground, with binoculars if the stack is tall. Look at the joints between the stones. Healthy lime pointing is slightly recessed or flush, with a sound face. Failing pointing is sandy and crumbling, or has fallen out altogether so that the stone edges stand proud.
The other signs are on the stone. Look for blocks with a flaking or powdery surface, especially at the corners and at the arrises where the stone meets the joint. Look for green or black staining on the windward face, which shows where water sits. Look for weeds or small plants growing from the joints, a sure sign that mortar has gone soft enough to root in. Inside the house, a damp patch or brown staining on the chimney breast can mean that the stack is letting rain through, though the lead round the base and the flaunching are also suspects.
Condition decides the work, not a calendar. HomeBuilding's repointing guide (2022) suggests chimneys typically need repair at intervals of 30 to 50 years, a rough rule and no more. The New Forest National Park Authority takes a more careful line: only badly eroded or missing mortar should be repointed, and old pointing is often still in good condition after a very long time. A survey therefore looks for the joints that have really failed and leave the sound ones alone.
Our guide to chimney repair costs puts figures on the options, which helps when the stack sits close to the line between patching and a bigger job.
How a lime repointing is done, step by step
The job begins with access. A stack can rarely be reached safely from a ladder, so scaffold or a platform goes up round it before anything else is touched.
Raking out
Loose and failed mortar is removed by hand, with a small chisel or a plugging tool. Power tools are avoided because a disc can cut into the edges of soft stone and widen the joint. The joint is raked back to sound mortar, to a depth that gives the new mix something to grip. Stone that has weathered back is not chiselled square; the aim is to take out the old mortar and leave the stone alone.
Cleaning and wetting
The joints are brushed out and the dust removed. Because the stone is thirsty, the face is dampened before pointing so it does not suck the water out of the fresh lime and weaken it. Dampening is done with a spray, not a hose, and the stone should be damp rather than wet.
Pointing
The lime mix is pressed into the joint in layers, deep joints in stages, and finished to match the existing profile. On most older stacks that means a flush or slightly recessed finish, matched to the sand colour and texture of the surrounding joints. A slightly different sand makes a patch show as a lighter or darker scar, so the mix is tested on a small area first.
Curing
Lime sets slowly by taking up carbon dioxide from the air, and it cures best kept damp and shaded. Fresh pointing is misted and covered in hot or windy weather, and protected from frost, which can destroy lime before it has set. This slow cure is why lime repointing is planned for the milder part of the year.
Repointing and the other parts of the stack
Repointing deals with the joints, and the joints are only one of the ways water gets into a stack. A repair that stops at the pointing can leave the real fault in place. Three neighbouring parts deserve a look at the same visit.
The first is the flaunching. That is the sloping mortar cap bedding the pots, the top line of the stack's defence. Cracks in it let water into the head of the stack and into the flues. It should be renewed in lime too, for the same reasons. For the Walcot Buildings flaunching the Trust likewise preferred lime, and the same reasoning holds on any stone stack.
The second is the lead round the foot of the stack, where chimney meets covering. A flashing, soakers and a back gutter seal the junction between the stack and the roof. If they have lifted, split or been bridged with a cement fillet, repointing above will not stop a leak below. What the lead should look like, and how it fails, is described in our guide to the lead round a chimney.
The third is the stone itself. Where individual blocks have decayed beyond saving, the repair may include cutting out the bad stone and fitting a new piece, called an indent, in matching Bath stone. This is a stonemason's job, and the quote should show it as a separate item.
Doing the lot in a single visit saves a second set of scaffold and means the stack is left in a consistent condition. If the stack is so decayed that repointing cannot hold it, the question changes from repair to rebuild, weighed in repair or remove a chimney.
What repointing in lime protects
The purpose of repointing is not cosmetic. A stack with sound joints sheds water instead of absorbing it, and that protects four things.
It protects the stone. Water that stays out of the joints stays out of the blocks, so there is less freezing and thawing inside the stone and less surface loss over time. Bath stone is soft, and once the face is lost the stone beneath weathers faster. Good pointing slows that.
It protects the structure. A stack is a stack of loose blocks bedded in mortar. Where the mortar has gone, the blocks can shift, and a stack that has begun to lean is a far larger job than repointing. Keeping the joints sound keeps the stack in line.
It protects the house below. Water that gets through a stack travels down the flues and shows as damp on the chimney breast, staining wallpaper, plaster and ceilings. Many internal damp patches on a chimney breast start with failed pointing, though they can also come from lead or flaunching. A survey separates the causes.
It protects the appearance. On a Bath terrace, the chimneys are a visible part of the roofline, and listed and conservation area stacks are part of the character of the street. A lime repair matched to the original joints looks like the original work. A grey cement repair looks like a patch.
Consent, timing and what to ask for in a quote
Like-for-like repointing in lime is repair, and repair does not normally raise questions. Even so, Bath contains around 5,000 listed buildings, and the council wants owners of listed houses to contact it before anything is done. Its listed buildings guidance stresses that the two permissions differ, and that unauthorised work is an offence. Not certain of your house's status? Ask the council before booking anyone. Rebuilding a stack, altering its height or replacing stone with a different type is more than a repair and needs a conversation.
On timing, aim for spring through early autumn. Lime needs mild, damp conditions to cure and fears frost. Work is not usually planned for deep winter, and if frost is forecast soon after pointing, protection is needed. Our page on chimney repairs in winter covers what can be done in the cold months.
When you read a quote, check four things. It should name the mortar as lime, with the sand matched to the existing joints. It should say how much of the stack is being repointed: all of it or the failed sections. It should say whether the flaunching and the lead are included. And it should show whether scaffold is in the price. Our fixed price arrives in writing after the survey, with the workmanship under a 10-year written guarantee.
Ask, too, to see the stack before and after. Photographs of the joints you cannot see from the ground are the easiest way to check that what was priced was what was done.