Around Stroud, Painswick, Minchinhampton and the villages along the Cotswold edge, plenty of houses carry stone chimneys. They are built from oolitic limestone, the same family of rock that makes up the older stone slate roofs nearby. Limestone is a soft, porous stone. It breathes, taking in damp and letting it out again through its surface and its joints, and that behaviour is what keeps a stone wall healthy. A repair that interrupts it does damage that may not show for years.
This guide is about stone stacks specifically: what lime mortar does, how weathered stone is repaired or replaced, what dressing means, and why Portland cement and hard mixes harm a soft stack. Mixes for chimneys of any kind are compared in our answer on the best mortar for chimney repairs. If your house is listed, read listed chimney repairs too. The wider service is on our chimney repairs page.
Limestone stacks: what they are built from
The district council's 2000 stone slate guidance says the stone slates on Cotswold roofs come from oolitic limestone, either Forest Marble or the frost-split Stonesfield Slate. Chimney stacks on the same houses are cut from limestone of a similar kind, quarried near the house where possible. The stone varies from one quarry to the next, in colour, grain and hardness, and a stack that looks uniformly golden from the lane will turn out, at close range, to contain blocks of different texture.
Why it behaves as it does
Oolitic limestone is made of tiny rounded grains cemented together. It is porous enough to take water in and soft enough to be worked with hand tools. Rain soaks into the surface, and warmth and wind draw it out again. That cycle is gentle if the joints can take some of the moisture and the stone can dry through them. It is damaging if something stops the drying.
The stack's own weaknesses
A chimney is the most exposed piece of masonry on the house, with nothing above it to shelter the stone. The top courses and the flaunching take the worst of it. Soot and flue gases add their own chemistry from inside. Bedding planes, the natural layers in the stone, can open up where water has frozen in them, and the face of a block can peel away in thin sheets, leaving a hollow in the stack. This is called spalling, and it is the commonest sign that a limestone stack needs attention. We cover its causes in the context of brick in spalling chimney bricks.
Why lime mortar and not cement
The mortar in a stone stack is meant to be the weaker partner. A joint should be softer than the stone beside it, so that if the stack moves slightly, or water has to leave the wall, the joint takes the strain and the moisture, not the stone.
What lime does
Lime mortar is made from lime and sand. It is flexible, it lets damp pass through, and it wears away before the stone does. It is also forgiving: a lime joint can be raked out and repointed generations later without disturbing the stone. Those properties are exactly the ones a soft, porous limestone needs.
What cement does
Portland cement mortar sets hard and holds damp in. Used in a limestone stack, it makes joints stronger than the stone. Water that used to leave through the joints now has to find another way out, and the only route is through the stone itself. The faces of the blocks decay, because the stone has become the sacrificial part. The result is stones that crumble at the arris (the sharp edge) and joints standing proud of eroded faces, a pattern that is easy to spot once you know it.
If the joints of a stone stack stand proud of the stone, and the stone looks worn behind them, the mortar is too hard for the wall.
Choosing the mix
Lime mortars come in several kinds, and the right one depends on how exposed the stack is and how soft the stone is. On a stack in a conservation area, or on a listed house, the council or conservation officer may have a view too. We choose the mix on the stack, test a small area first where we can, and match the sand to the colour of the existing joints.
Raking out, repointing and what to remove
Repointing a stone stack is a slower process than repointing brick, and it is easy to do harm with the wrong tools.
Taking out the old mortar
Joints are cut out by hand to a depth that gives the new mortar enough to grip, usually about the width of the joint or more. Power tools, such as angle grinders, cut into the soft stone edges as readily as the mortar, and we avoid them. Where the old joints are hard cement, removing them carefully is the most delicate part of the job. The stone beside them is often already weakened, and a heavy hand makes it worse.
Packing and finishing
Once raked, the joint is dampened, so that the stone does not draw water out of the new lime too fast, then filled and compacted in layers. The finished surface is brushed back or lightly scraped rather than smoothed over, which leaves a texture that matches an old stack. Lime cures slowly and has to be kept from drying out too quickly in sun and wind, which is a good reason to schedule the work for a mild spell and to cover it as it goes.
Pointing styles
Old stone stacks often have flush or slightly recessed joints. Raised ribbon pointing, where mortar is built out over the stone in a band, holds water against the face and is the opposite of what a stone stack wants. We follow the style of the original work where it can still be read.
For the cost of this kind of repair, see our guide to chimney repair costs. Stone is slower than brick, and the figures for brick are a floor, not a guide.
Matching stone and replacing what has failed
Where individual stones have split, spalled or eroded beyond saving, the choice is between cutting out the damaged part and putting a new piece in, or replacing the whole block.
Matching the geology
Cotswold District Council's 2000 guidance reproduces English Heritage policy on roof slates, and its standard carries over to chimney stone. A replacement block should share the old one's geology, colour, grain, size and depth, and the two rocks, limestone and sandstone, are never swapped. A sandstone block in a limestone stack weathers differently, holds water differently and shows from the road.
Sourcing new stone
Stone for repairs comes from working quarries, from stone merchants, and sometimes from salvaged material. The council's guidance observes that second-hand stone has been taken from other old buildings, occasionally through theft, and that reclaimed material belongs on the building it came from. We take that principle seriously. Stone that comes off your stack and is sound is kept and reused on it; we do not strip another building to patch yours.
Piecing in and replacing
A small defect can be repaired by cutting out the damaged stone to a clean edge and fitting a new piece, called a dutchman or a piece-in, bedded in lime. For larger damage, the block is replaced. Each new stone is cut or dressed so that it sits in the course with the right joint width, and so that its bed runs the same way as its neighbours, because a limestone bed set on its edge weathers badly.
Dressing
Dressing is shaping the face of a stone with chisels so that it takes on the texture of the old work. A new block that is left smooth and pale shows from the street, whereas a hand-dressed one blends in and starts to weather like its neighbours. It takes time, and a stack rebuilt without it looks patched.
When the stack needs taking down and rebuilding
Repointing and piecing in suit a stack that is fundamentally sound. When the shaft has moved or cracked right through, or the stone has failed across most of a face, the answer is the same as for any other stack: take it down to firm masonry and build it back. Our chimney rebuild guide gives the stages.
Recording and numbering
Stone stacks are the strongest case for recording before dismantling. We photograph each face and number the stones where they matter, so that the stack goes back in the same order, with the same bond and the same corbelled courses. A rebuilt stone stack should look like the one that was there.
Pots, caps and the top
The top of a stone stack carries pots and often a stone cap or drip course. The pots go back, or are replaced to match, and the flaunching is formed in lime rather than a hard mix, so that the crown can breathe and shed water. Cowls and bird guards can protect an unused flue, and our guide to chimney pots and cowls explains the options. Stone caps are heavy, so they need a sound bed.
Leaning stone stacks
A stone stack that leans is looked at as carefully as any other, because the causes include moisture, mortar failure and movement below. Our guide to leaning chimneys sets out the possible causes.
Consent, cost and what we put in the quote
A like-for-like repair to a stone stack, in lime and matching stone, is not normally a planning matter, since work that does not materially affect the outside appearance of a building is not development. A stone stack is also often a feature that makes the house what it is, so be careful about changing anything. On a listed house, Stroud District Council says every part is legally protected, front and back, whatever the grade, and altering it without consent is an offence, so ask the conservation team first. There are 41 conservation areas in the district, five with Article 4 Directions; the council has not said if those reach chimneys, so put that to its planning team.
What it costs
No national cost guide separates stone stacks from the rest, so the ranges we can cite are for chimney work in general. MyBuilder's guide, updated August 2026, gives repointing at about £750 on average, across a spread of £500 to £2,000, and a stack rebuild at £2,000 to £10,000 or more. For stone, expect the upper part of these ranges, since lime, dressed stone and slower work all add to the labour. Scaffold is on top: about £1,125 in MyBuilder's 2026 re-roof figures, £700 to £1,300 in Checkatrade's October 2026 guide. Checkatrade gives stone slate roofing, generally, at about £160 to £210 per m², varying with the source of the stone, which shows how far stone carries a premium.
The written quote
After the survey, we write up the work: scaffold, raking out, lime mortar, new stone, dressing, pots and flaunching, then waste and making good. VAT is shown. Survey photographs of your own stack come with it, the site is left clear, and a written 10-year workmanship guarantee covers the work.