Written by Andrew, Lead Roofer
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Lime mortar vs cement mortar explained. The rule that really decides it, how to tell what you have, and why removing cement pointing can cost you.
Lime mortar and cement mortar are not interchangeable. Lime mortar is soft, permeable and sets slowly by absorbing carbon dioxide from the air, while cement mortar is hard, largely impermeable and sets within hours by chemical reaction with water. On a solid-wall Victorian house, that difference decides whether moisture escapes through the joints or through the brick faces.
Most homeowners arrive at the lime mortar vs cement mortar question because a builder has quoted for repointing and used the word “lime”, or because a surveyor has flagged the pointing on a house they are buying. The answer they usually get is a blanket one: old house, use lime. That rule is right often enough to be repeated everywhere and wrong often enough to cause real damage, because the actual deciding factor is not the age of the building.
This guide covers what each mortar is and how it behaves, the compatibility principle that genuinely determines which one your wall needs, how to identify what is already in your joints, when cement is the correct specification even on a period property, whether existing cement pointing should be removed at all, how to choose between NHL 2, NHL 3.5 and NHL 5, and what the extra cost of lime work reflects.
Lime mortar is a mix of lime binder, sand and water, with no Portland cement. Lime mortar hardens through carbonation, which means the calcium hydroxide in the mix slowly reabsorbs carbon dioxide from the air and turns back into calcium carbonate. Carbonation takes days to weeks rather than hours, and it works from the outer face of the joint inwards.
Two families of lime get used in the UK. Non-hydraulic lime, sold as lime putty or as a hot-mixed mortar, sets by carbonation alone and stays the softest and most permeable option. Natural hydraulic lime, sold as NHL, contains clay impurities from the limestone that give it a partial chemical set in the presence of water, so NHL mortars gain strength faster and cope better with wet or exposed positions.
The property that matters most on an old wall is not softness for its own sake. Lime mortar is permeable, which means water vapour moves through the joint and evaporates from the mortar rather than being forced out through the brick. Lime mortar also has a degree of autogenous healing, so fine cracks caused by seasonal movement tend to reseal themselves as free lime migrates into them.
Cement mortar is a mix of Portland cement, sand and water, sometimes with hydrated lime added as a plasticiser to improve workability. Cement mortar sets by hydration, a chemical reaction that produces a hard, dense material within hours and reaches close to full strength in around 28 days.
Portland cement did not arrive in British housing overnight. Roman and natural cements were in use through the nineteenth century, but large-scale Portland cement production only got going in the 1890s, which is why the mortar in a house built in 1875 and a house built in 1905 can be genuinely different materials despite both being called Victorian or Edwardian by an estate agent.
Cement mortar is the correct material for the vast majority of construction built since roughly the 1920s, because modern bricks, blocks and cavity walls are designed around it. A cavity wall with a damp-proof course and a ventilated cavity manages moisture structurally. A solid Victorian wall manages moisture by letting it move through the fabric, and that is where the two systems stop being compatible.
| Property | Lime mortar | Cement mortar |
|---|---|---|
| Binder | Non-hydraulic lime or NHL | Portland cement |
| Sets by | Carbonation, absorbing CO₂ from air | Hydration, chemical reaction with water |
| Working set time | Days to weeks | Hours |
| Permeability | High, allows vapour to pass | Low, close to impermeable |
| Flexibility | Accommodates small seasonal movement | Rigid, cracks rather than flexes |
| Strength relative to soft brick | Weaker than the brick, sacrificial | Often harder than the brick |
| Self-healing of fine cracks | Yes, to a degree | No |
| Weather window for pointing | Roughly March to October, no frost risk | Wider, though still frost-sensitive |
| Best suited to | Solid walls, soft or handmade brick, stone, listed and pre-1900 buildings | Cavity walls, engineering brick, concrete block, copings, flaunching |
| Curing protection needed | Yes, damp hessian for at least a week | Minimal |
The row that does the real work in that table is permeability, not strength. A lot of the advice online frames the comparison as hard versus soft, which leads people to assume the question is about how much load the wall carries. Domestic brickwork two or three storeys high is nowhere near the compressive limits of either material, so strength is rarely the constraint. Moisture behaviour is.
The governing principle is compatibility with the masonry unit. Historic England’s guidance on repointing brick and stone walls sets it out plainly: replacement mortar should be slightly weaker and more permeable than the brick or stone itself, not merely weaker than the mortar that happens to be there now. Get that relationship right and the joint behaves as a sacrificial layer, taking the frost damage and the salt crystallisation so the brick does not have to.
Read that principle carefully and you can see why “old house, use lime” is a shortcut rather than a rule. A soft handmade red brick from 1820 needs a very soft, very permeable mortar. A hard machine-pressed engineering brick in a late-Victorian basement retaining wall does not, and putting a feeble lime mortar into it can mean the joint erodes back within a few winters. The date on the deeds tells you what is likely. The brick in front of you tells you what is true.
Historic England also declines to prescribe a universal lime grade for historic masonry, because selection depends on the strength and porosity of the masonry, the original mortar, the condition of the wall and how exposed the elevation is. A north-facing parapet in Hampstead that takes driven rain all winter and a sheltered rear elevation on the same building can legitimately warrant different specifications.
Where the original mortar survives in a sheltered spot, that sample is the best evidence available. Matching it for binder, aggregate, colour, texture and joint profile is the starting point on any building of significance, and on a genuinely important building a laboratory mortar analysis and a trial panel are worth the money before anyone touches the elevation.
Cement pointing on a soft brick fails in a specific and predictable sequence, and once you know what it looks like you see it on half the terraces in inner London. Water still gets into the wall, because no pointing is fully waterproof and solid walls take moisture from driving rain, defective gutters and ground contact regardless. What changes is where that water leaves.
With a permeable lime joint, evaporation happens through the mortar. With a dense cement joint, the easiest exit route becomes the face of the brick, so the evaporation front, the dissolved salts and the freeze-thaw cycling all move into the masonry unit. The brick face blows, flakes off in layers, and you get spalling. Look along a stock brick terrace where somebody repointed in the 1980s and you will see the joints standing proud and hard while the brick faces between them have eroded back by several millimetres.
London stock bricks are particularly vulnerable because many of them were made with a proportion of ash and clinker in the clay and fired unevenly, so the outer skin is considerably denser than the core. Once that fired skin is lost, the soft interior weathers fast. Spalled bricks cannot be repaired, only cut out and replaced, which is why cement pointing on the wrong wall turns into a five-figure brick replacement job rather than a repointing job.
Rigidity compounds the problem. A solid wall moves seasonally, and a lime joint absorbs that movement across the whole bed. A cement joint does not flex, so the movement concentrates into hairline cracks at the mortar-to-brick interface, which then admit water directly behind the pointing. The wall ends up wetter than it was before it was pointed, which is the opposite of what the homeowner paid for.
Identifying what is already in your joints costs nothing and takes about ten minutes, and it is worth doing before you brief anyone. Start with a scratch test using a coin or the tip of a screwdriver. Lime mortar gives way under moderate hand pressure and crumbles back to loose sand. Cement mortar resists a screwdriver almost entirely, and if you find yourself reaching for a power tool to make any impression, you are looking at cement.
Colour and texture are the next tell. Lime mortars are usually buff, cream or pale grey with visibly graded sand, and Victorian London mortars are often much darker than people expect because ash and clinker from coal were commonly gauged into the mix, producing a grey-black joint. Cement pointing is flatter in colour, more uniformly grey, and lacks the visible aggregate variation.
Joint profile is the giveaway on repointed work. Original lime pointing on a London terrace is typically flush or very slightly recessed, following the brick edges. Cement repointing is very often strap or ribbon pointing, where the mortar sits proud of the brick face in a raised band with a defined edge, sometimes overlapping onto the brick. Ribbon pointing on a Victorian facade is almost always a later intervention.
Go looking in sheltered positions for surviving original mortar. Under window sills, behind downpipes, in the recess beneath the eaves, inside a chimney breast or on a protected return elevation are all places where the first mortar often survives even when the front has been repointed twice. Comparing a sheltered sample against the exposed elevation tells you what the building started with and what has been done to it since.
Cement mortar has legitimate uses on a heritage building, and a contractor who says otherwise is applying a slogan rather than a specification. The distinction is between mortar that forms part of a breathing wall and mortar that has to shed water as a weathering surface.
Chimney flaunching is the clearest example. Flaunching is the sloped mortar fillet that beds the pots and sheds water off the top of the stack, and it sits fully exposed to rain, frost and thermal cycling with no wall behind it to keep dry. A soft lime mix in that position breaks down within a few winters, which is why a sand and cement flaunching, or a strongly hydraulic lime where a conservation officer requires it, is the sensible call. Cracked flaunching is one of the most common causes of chimney water ingress we find, and it features heavily in our chimney repair cost guide.
Ridge and hip tile bedding follows the same logic. Bedding mortar on a roof is a weathering detail sitting on tile, not a joint inside a solid wall, and BS 5534 now requires ridge tiles to be mechanically fixed as well as bedded, precisely because mortar alone was never a reliable long-term fixing. Copings, cills, verge details and any masonry below the damp-proof course are similarly better served by a cement or strongly hydraulic mix that resists saturation.
The point is not that cement is fine after all. The point is that the correct question is what the mortar has to do in that specific position, which is a different question in a bed joint halfway up a solid wall than it is on top of a chimney stack. Our heritage and conservation roofing work routinely involves lime on the elevations and cement at roof level on the same building.
Not necessarily, and this is where a lot of well-meant advice does more harm than good. The reflexive answer is that cement on an old house should come out. Conservation guidance says something considerably more careful.
SPAB’s advice on inappropriate cement pointing, written by their Technical and Research Director, makes clear that wholesale removal is not automatically the right response. Carry out a small trial area first. Where cement pointing adheres well and the masonry shows no evident deterioration, it may be best left to weather naturally, because removing a well-bonded cement joint chips brick arrises and takes brick faces with it. Selective removal of the failed sections, with sound areas left alone, is often the better compromise.
The Northern Ireland technical note on repointing stone and brick offers a useful field rule along the same lines: if you need an angle grinder to get the mortar out, it is probably sound and does not need replacing. That single sentence would save a lot of London brickwork if it were better known.
So the honest decision framework runs like this. Is the cement pointing cracked, hollow-sounding or separating from the brick? Are brick faces spalling or eroding while the joints stand proud? Is there persistent internal damp or salt staining on that elevation? Where the answer is yes, removal of the affected areas is justified. Where the pointing is sound, well bonded and the bricks either side are intact, leaving it and reviewing it in a few years is a defensible and often better decision than an intervention that damages original fabric to satisfy a principle.
Natural hydraulic lime is graded under BS EN 459-1 into three classes, and choosing between them is where most specifications go wrong. NHL 3.5 has become a lazy default because it is the grade most readily stocked, which is not a technical reason.
| Grade | Traditional term | Character | Typically suited to |
|---|---|---|---|
| NHL 2 | Feebly hydraulic | Softest, slowest set, most permeable | Soft or porous brick and stone, sheltered elevations, internal work |
| NHL 3.5 | Moderately hydraulic | Intermediate strength and set speed | General repointing where the masonry and exposure justify it |
| NHL 5 | Eminently hydraulic | Hardest, fastest set, least permeable | Hard masonry, severe exposure, copings, chimneys, permanently wet positions |
The detail almost nobody mentions is that those numbers are compressive strength classes of the lime binder tested in isolation, not the strength of the finished mortar. Aggregate grading, binder-to-sand ratio, water content, workmanship and curing conditions all shift the final performance substantially. Two contractors using the same bag of NHL 3.5 can produce mortars that behave quite differently on the wall, which is why the Mineral Products Association’s mortar guidance puts as much emphasis on mix consistency and site practice as on the binder specification.
Application matters as much as material. Joints should be raked out by hand to at least twice the joint height, deeper in rubble stonework, and angle grinders avoided because they widen joints and cut into brick arrises. Deep joints get filled in layers of roughly 25mm and compacted from the back so no voids remain. Fresh lime work then needs protecting from sun, wind, driving rain and frost, kept damp under ventilated hessian for at least a week and longer in poor conditions, because lime that dries too fast never carbonates properly and dusts out within a couple of years.
Lime repointing costs more than cement repointing, and the gap is mostly labour rather than materials. Lime binder does cost more per bag than Portland cement, but on a typical terrace elevation the material is a small fraction of the bill. The real difference is that lime work takes longer at every stage.
Hand-raking joints is slower than cutting them out. Building deep joints up in layers is slower than one pass. Protecting and dampening the work for a week or more means return visits and scaffold standing time that a cement job does not incur. The weather window is narrower too, because lime pointing needs to be done between roughly March and October to avoid frost catching the mortar before it has carbonated, which concentrates demand into part of the year.
Access is the other multiplier, and in London it is often the largest single line. Scaffolding a four-storey stucco-fronted terrace in Kensington costs considerably more than reaching a two-storey rear elevation from a tower, and pavement licences add both cost and lead time. Our repointing cost guide breaks the current UK ranges down by mortar type, property type and access, which is the more useful way to look at it than a single per-square-metre figure.
Worth weighing against that: repointing done in the correct mortar on a sound wall lasts decades, whereas cement pointing on a soft brick wall can trigger brick replacement inside 20 years. The cheaper job is frequently the more expensive one.
Repointing a listed building can require listed building consent, particularly where the work involves a change of mortar, a change of joint profile or a substantial area of the elevation. Unauthorised work to a listed building is a criminal offence, so the sequence is to establish the designation status first and speak to the local conservation officer before any scaffold goes up.
Conservation area status does not carry the same consent requirement for repointing an unlisted house, but local authorities across Camden, Kensington and Chelsea, Islington and Westminster publish guidance on appropriate mortar and pointing for their designated areas, and Article 4 directions in some areas remove permitted development rights that owners assume they have. Checking costs nothing and a retrospective enforcement notice costs a great deal.
Conservation officers typically want to see a specification, a sample panel and sometimes mortar analysis before approving work on a significant building. A contractor who can produce those without being asked is usually the one who has done the work before. The same thinking applies to other heritage fabric decisions, which we cover in our guide to roof insulation for heritage properties in London.
Avoid lime mortar where the mortar has to act as a weathering surface rather than a breathing joint. Chimney flaunching, copings, cills, ridge and verge bedding and masonry below the damp-proof course are all better served by cement or a strongly hydraulic mix, because they sit saturated and exposed with no wall behind them to dry out. Lime is also the wrong call on hard modern brick, engineering brick and concrete blockwork, where a feeble mortar will simply erode back faster than the units around it.
Lime was the standard binder in British house building until Portland cement production scaled up in the 1890s, and the changeover was gradual rather than sudden. Cement-gauged mortars became common through the early twentieth century and cement had largely displaced lime in mainstream construction by the 1930s. Anything built before roughly 1900 was almost certainly bedded in lime originally, though most London terraces have been repointed at least once since.
Yes, though mostly on labour rather than materials. Lime binder costs more per bag than Portland cement, but the larger difference comes from hand-raking joints, building deep joints up in layers, the extended curing protection and the narrower seasonal working window. On a full elevation the labour and access costs dwarf the binder cost either way, which is why scaffolding often determines the final figure more than the mortar choice does.
Scratch the joint with a coin or screwdriver. Lime crumbles to sand under hand pressure while cement resists almost entirely. Check the colour and texture too, because lime mortars show graded sand and are often buff, cream or, in Victorian London, a dark ash-gauged grey, whereas cement is flatter and more uniform. Pointing that sits proud of the brick face in a raised ribbon is nearly always later cement work.
Cement-gauged lime mortars exist and were widely used, but adding cement to a lime mix to speed the set or make it “a bit stronger” defeats the purpose on a solid wall, because even a small cement content sharply reduces permeability. Where a faster or harder set is genuinely needed, moving up an NHL grade is the better route than gauging with cement, since it keeps the mortar within a single, predictable material family.
Not in compressive strength, and that is deliberate rather than a shortcoming. The mortar in a wall is supposed to be weaker than the brick or stone around it so that damage concentrates in the replaceable joint instead of the irreplaceable masonry. Domestic brickwork of two or three storeys places nowhere near enough load on the joints for compressive strength to be the limiting factor, so permeability and flexibility matter far more than raw strength.
The lime mortar vs cement mortar decision is not settled by the age of your house. It is settled by what the brick or stone in front of you needs, which comes down to one relationship: the mortar should be slightly weaker and more permeable than the masonry it sits between, so the joint weathers instead of the wall. Get that right and the pointing is a sacrificial layer doing its job. Get it wrong and you have bought yourself brick replacement.
Two things follow from that. First, if a contractor gives you a mortar specification before looking at your brick and your exposure, you have learned something useful about the contractor. Second, if you already have cement pointing and nobody has trialled a small area or checked whether the bricks are actually suffering, resist the urge to have it all hacked out, because removing sound cement causes more damage than leaving it.
If you want a straight answer on what your own elevation needs, we survey brickwork across London and the South East and specify the mortar to the wall rather than to a rule of thumb. Our pointing and brickwork service covers lime repointing on period and listed properties, and we are happy to tell you when the honest answer is to leave it alone for another few years.
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