The short answer
Cement mortar is harder and less breathable than Bath stone, so it traps moisture in the stone and pushes damage onto the soft limestone around the joints. Water that should leave through the mortar leaves through the stone face instead, which flakes and crumbles. Lime mortar is softer and more permeable than the stone, so it wears first and protects the stone, which is why it is used to repoint Bath stone stacks.
A soft stone and a hard mortar
Bath stone is an oolitic limestone, quarried around Bath and used for the city's terraces, crescents and chimney stacks. It cuts easily when fresh and hardens on exposure, but it remains a relatively soft, porous stone. Water moves through it, in and out, with the weather.
Mortar is the sacrificial part of a masonry wall. The joint is meant to be the weakest and most permeable element, letting moisture escape via the joint and confining decay to it, where it can be repaired by repointing. The stone, which cannot be replaced without cutting out blocks, is meant to last.
Cement turns that arrangement upside down. Portland cement mortar sets hard, much harder than the stone beside it, and it is far less permeable. The joint becomes the strongest part of the stack and the stone the weakest, so the stone takes the strain.
How the damage happens
Water gets into every masonry stack, through the stone, through hairline cracks and down from the top. With a lime joint, it passes back out through the mortar as the weather dries. With a cement joint, the water is held in the stone behind the pointing, and its only way out is through the stone face next to the joint.
When that water evaporates at the surface, it leaves dissolved salts behind. Salts crystallise in the pores of the stone and push the surface apart. In winter, trapped water freezes and expands, and the stone face shears away in flakes along the line of the joint. Over time, the arrises, the sharp edges of each block, crumble and round off, and the cement stands proud like a grid on a worn surface.
Cement also does not move with the stack. A stone stack expands and contracts with temperature and takes up small movements. A rigid cement joint cracks, and the crack lets in more water behind the pointing. On a listed stack in Walcot, Bath Preservation Trust recorded its concern that cement had probably aggravated the decay of the stone.
Why lime works
Lime mortar is made from burnt limestone, slaked and mixed with sand. It sets slowly, stays softer than the stone and remains permeable, so the wall can dry through it. When a stack moves or the weather bites, the joint takes the wear and the stone is spared.
That suits Bath stone because the stone and the mortar share a chemistry. Both are lime-based, and they weather in compatible ways. The Trust has asked for lime mortar so the old stonework can breathe, and has backed lime for flaunching on listed stacks too. Historic England guidance on traditional buildings takes the same line.
Lime is not a miracle. It needs skilled application, it takes longer to cure, and it has to be protected from frost and fast drying while it does. It lasts well when done properly, and it can be raked out and renewed in future without harming the stone. Our lime repointing page describes the method.
Spotting cement on your stack
Cement pointing is often easy to spot from the ground. It is grey, dense and hard-looking, where lime tends to look lighter, warmer and slightly rougher. It is often raised or smeared over the edges of the stone, in a ribbon that stands proud of the face, and the joint may be cut into a neat, hard line.
Look at the stone beside the joints. Stone that is flaking, scaling or pitted along the edges, with a clean sharp joint standing between them, is the classic pattern. Powdery, sugary stone and a white crust of salts add to the picture.
Do not scrape at it yourself. Chipping cement out with a hammer can take the stone edge with it, and the dust is harmful to breathe. Cement removal on soft stone is done carefully, by hand or with fine tools, to avoid doing more harm. Some stacks have had a cement render added too, which hides the stone and holds the damp in a wider area; see rendering a chimney for how that goes wrong.
Putting a cement-pointed stack right
The cure starts with taking the cement out, as far as can be done without harming the stone, and repointing in a lime mortar matched to the stone in colour and texture. If the stone has already decayed, individual blocks may need cutting back or replacing with matching stone, and in a severe case part of the stack may have to be rebuilt.
Check what sits at the top. Flaunching, the sloping bed that holds the pot, is often laid in cement as well, and it is a frequent route for water. Redoing it in lime stops that route; see chimney flaunching. A cracked or tilted pot sitting in it is a separate fix, and our answer on replacing a chimney pot explains how that goes.
Listed stacks need particular care. Changing from cement to lime is usually welcome, but check with B&NES conservation before altering materials on a listed building, as we explain on listed chimneys. For timing, read how often a chimney needs repointing. Other stack questions are answered under chimney repairs.
Cost and what to ask a roofer
Stone and lime work cost more than a quick cement patch, and they are worth it. MyBuilder's chimney repair guide (2026) gives typical UK ranges: repair £500 to £2,000, repointing averaging about £750 and a stack rebuild typically £2,000 to £10,000 or more. Scaffold, stack height and lime work all affect the figure. None of these are Empire prices; our surveyed quote is a fixed figure given in writing.
When you ask for quotes, ask what mortar will go in, how deep the joints will be raked and whether the old cement will be removed or pointed over. A quote that says only repoint with no mortar specified deserves a question. For a Bath stone stack, the answer should be lime.
We match materials, and on Bath stone that means lime and stone that suits the original. Ask any roofer which mix they intend to use for the pointing and the flaunching, and ask whether the old cement will be raked out by hand rather than cut with a grinder that widens the joints and chips the edges of the stone.
