In short
- Older solid walls manage moisture by letting it evaporate, and lime mortars and renders allow that to happen.
- Cement is harder and less permeable than soft brick, ragstone and flint, so moisture and decay are pushed into the masonry instead of the joint.
- Non-hydraulic lime sets slowly by absorbing carbon dioxide, while natural hydraulic limes, graded NHL 2, NHL 3.5 and NHL 5, also set with water and are stronger but less permeable.
- Cement should be raked out by hand where it can be removed without damaging the brick or stone around it.
- Lime work needs protecting from frost, heavy rain and drying sun, so it is generally planned outside the coldest months.
Why breathability matters in a solid wall
Most houses built in Kent before about the 1920s have solid walls with no cavity. Rain soaks into the outer face, damp rises a little from the ground, and moisture from inside the house works its way outwards. None of this is a problem as long as the water can leave again, which it does by evaporating from both faces of the wall, mainly through the joints and any render or plaster.
Traditional lime mortars, renders and plasters are porous and allow that evaporation. They are also slightly flexible, so they accommodate the small seasonal movements of an old building without cracking badly. The SPAB describes lime's ability to let structures breathe and move gently as essential for old buildings. Approved Document L makes a similar point in its guidance on historic and traditional dwellings, describing vapour permeable constructions using lime render or mortar that absorb moisture and readily let it evaporate.
The idea of breathability is sometimes over-simplified. Lime doesn't make a wall dry on its own. What matters is the balance between how much water gets in and how easily it can get out, and every change to the wall, from a new render to a raised path, shifts that balance.
How cement mortar and render damage soft masonry
Ordinary Portland cement mortar is much harder and less permeable than lime, and usually harder than the soft handmade bricks, ragstone and chalk-bedded flint of older Kent buildings. The SPAB warns that cement and other modern materials used in place of lime can seriously harm historic fabric. The common types of damage are:
- Faces lost from bricks and stone: water that can't escape through hard joints evaporates through the masonry instead, carrying salts that crystallise just below the surface and push the face off. Frost does similar damage to saturated masonry.
- Cracked units instead of cracked joints: when the wall moves, a rigid cement joint doesn't give, so cracks run through the bricks or stones.
- Trapped water behind render: hairline cracks in cement render let rain in, but the dense render slows drying, so the wall behind stays wet. On timber frames this can rot sole plates, studs and infill panels unseen.
- Damp inside: a wall that can no longer dry outwards may show damp patches, salts and mould on the inner face.
- Loose flints: flints bedded in lime and then pointed in cement can lose their bond as the softer lime behind decays.
Non-hydraulic, hydraulic and hot-mixed lime explained
Lime isn't a single material. Building limes are made by burning limestone or chalk to make quicklime, then combining it with water. How pure the limestone is, and how the lime is prepared, decides how it behaves. The SPAB and Historic England both advise matching the lime to the original materials and the exposure of the wall, so treat the table below as a general introduction.
| Type | How it sets | Typical uses |
|---|---|---|
| Non-hydraulic lime (lime putty or dry hydrate) | Hardens slowly by absorbing carbon dioxide from the air. Classed as CL under European standards. Putty is lime slaked with excess water and matured | Internal plaster and limewash, and pointing and render on sheltered walls. Often used with a pozzolan, such as brick or tile dust, to give an earlier set where needed |
| Natural hydraulic lime, NHL 2, NHL 3.5 and NHL 5 | Made from limestone containing reactive silica and alumina, so it also sets with water. The numbers are strength classes, with NHL 5 the strongest | External pointing and render where some early strength and weather resistance are needed. Higher grades are stronger but less permeable and less flexible |
| Hot-mixed lime | Quicklime is slaked together with damp sand in one operation, producing heat and a lime-rich mix that is used fresh or after a short period | Pointing, bedding and render. The SPAB notes that mortars were widely mixed hot in the past, and it now runs training in hot-mixed lime work |
The SPAB points out that greater hydraulicity brings more strength but lower permeability and flexibility. A common principle is to use the weakest mix that will survive in that position. A sheltered ragstone garden wall and an exposed chimney stack on the Thanet coast won't need the same mortar.
Spotting cement repointing on your walls
You don't need a laboratory to make a reasonable judgement. Look closely at the joints in several places, particularly on the most weathered elevations:
- Colour: cement mortar is usually flat grey, while lime mortars range from off-white to buff or warm yellow, depending on the sand.
- Hardness: lime is often soft enough to mark with a key or a fingernail. Cement is very hard and rings when tapped.
- Texture: lime joints often show small white lumps of lime and a sandy, open texture. Cement looks dense and smooth.
- Profile: cement repointing is often smeared over the brick edges or finished as a raised strap or ribbon joint that stands out from the face.
- Masonry around it: the clearest sign is brick or stone faces eroded behind hard joints, so the joints stand proud.
- Cracks: cracks running through bricks rather than along joints suggest the mortar is stronger than the masonry.
Renders can be harder to judge. Cement render often sounds hollow when tapped where it has come away, shows fine map cracking, and may be painted with a plastic masonry paint. If you are unsure, a specialist can take a small sample.
Raking out by hand without damaging the masonry
How the old mortar comes out matters as much as what goes back in. Angle grinders and heavy power tools can widen joints and chip the arrises, the edges of bricks and stones, in seconds, and that damage is permanent. Historic England's repointing guidance is aimed at avoiding exactly this. Good practice usually follows these stages:
- Assess and recordLook at how much of the wall genuinely needs repointing. Sound lime joints can often be left alone. Photograph the original joints and keep a sample of the mortar.
- Rake out by handRemove loose or unsuitable mortar with hand tools such as plugging chisels and hacking knives, to a depth of roughly two to three times the joint width, or until sound material is reached. Very hard cement may need fine, careful cuts, but power tools should be the exception, not the method.
- Clean and dampenBrush or blow out dust, then dampen the joints so the masonry doesn't draw water out of the fresh mortar too quickly.
- Fill in layersDeep joints are packed in layers, letting each firm up before the next, rather than filled in one go.
- Finish the jointWhen the mortar has stiffened, it is finished to match the agreed profile, often by beating it back with a stiff brush to expose the aggregate.
- Protect while curingCover with damp hessian or similar, and keep the work protected from rain, sun, wind and frost for as long as the specialist advises.
Weather, seasons and frost risk for lime work
Lime sets slowly, and fresh lime mortar and render are vulnerable while they cure. Frost is the biggest risk: water freezing in uncured mortar can make it crumble or come away. Drying too fast is the next problem, because lime needs moisture to carbonate properly, so hot sun and drying winds can leave a weak, powdery surface.
- Many specialists avoid external lime work when frost is forecast during the curing period, and plan larger jobs between spring and early autumn.
- In hot or windy weather, work is kept damp and shaded, and walls in direct sun may be worked at cooler times of day.
- Driving rain can wash lime out of fresh joints, so scaffold sheeting is common on exposed elevations.
- Hydraulic limes cure faster than non-hydraulic lime and are sometimes chosen where the season is shorter, but they still need protecting.
Kent's coastal and exposed sites, from the cliffs at Dover and St Margaret's at Cliffe to the seafronts of Margate and Herne Bay, can see strong winds and salt-laden rain. Ask anyone quoting how they will protect the work, and accept that a responsible specialist may suggest moving a job to a better time of year.
Matching mortar colour, joint profile and render finish
Good repointing should be hard to spot once it has weathered. The colour and texture of lime mortar come mainly from the sand, so specialists often analyse or visually match the original and source a local sand, sometimes blending two or more. Ask for small trial panels and let them dry fully before choosing, because lime mortar lightens a great deal as it dries.
- Joint profile: copy the original. Flush or slightly recessed joints are common on older brickwork. Avoid raised strap or ribbon joints, which shed water onto the brick below and are rarely historic.
- Tuck pointing: some Georgian and Victorian fronts have fine tuck pointing, a thin raised line of putty over coloured mortar. It is specialist work and should be repaired, not replaced with ordinary pointing.
- Galletting: small stone or flint chips pressed into wide joints are a traditional detail on some ragstone and flint walls, and should be kept.
- Render finish: lime render is usually applied in two or three coats over several weeks, and may be floated smooth, roughcast or finished with limewash.
- Paint: limewash or a breathable mineral paint suits lime render. Plastic masonry paints can trap moisture much like cement.
Kentish ragstone, flint, stock brick and tile hanging
Kent's walls are unusually varied, and each material has its own needs.
- Kentish ragstone: a hard, grey sandy limestone quarried around Maidstone and the Medway valley, often laid as rubble with wide joints. The stone itself is durable, but the joints carry much of the moisture, so hard cement pointing over wide joints can trap water in the wall core.
- Flint: knapped, squared or cobble flint is common across the chalk of north and east Kent and along the coast, often with brick quoins and dressings. Flint doesn't absorb water, so walls depend almost entirely on their lime mortar, and loose flints are a sign that the bedding mortar has failed.
- Yellow stock brick: soft, yellowish bricks made in north Kent were widely used in Victorian towns along the Thames and Medway. They weather badly behind hard cement joints.
- Red brick: soft handmade red bricks, sometimes with blue-grey glazed headers in patterns, are common on older houses throughout the county. Matching size and colour matters when individual bricks are replaced.
- Tile hanging: clay tiles hung on battens on upper walls are typical of the Weald and west Kent. They are fixed with nails, not mortar, but the brick or timber behind them still needs to dry, and cement fillets at junctions can trap water.
Tell anyone quoting what your walls are made of and send photographs of each elevation. Specialists who work regularly with a material will know where to source matching brick, stone, sand and lime.
What affects the cost of lime repointing and render
There are no reliable published price guides for lime work, and prices depend heavily on the building, so this guide doesn't give figures. These are the factors that make one quote differ from another:
- How much of the wall genuinely needs work, and whether sound joints can be left.
- How hard the existing cement is, because slow hand removal takes most of the time.
- The joint width and wall type. Wide rubble ragstone and flint joints use more mortar and labour than fine brickwork.
- Scaffolding, access and protection, especially on tall or exposed elevations and narrow streets.
- The lime type, sand matching, trial panels and any mortar analysis.
- Brick, stone or flint repairs needed once the joints are open.
- The season, as protection and slower curing in cooler months add time.
- Consent, drawings or method statements for a listed building.
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Sources
- SPAB: Lime: www.spab.org.uk/advice/lime
- Historic England: Repointing Brick and Stone Walls: historicengland.org.uk/images-books/publications/repointing-brick-and-stone-walls/
- Historic England: Repairing Walls of an Older Home: historicengland.org.uk/advice/your-home/looking-after-your-home/repair/walls/
- SPAB: Conservation Mortars for the 21st Century, conference report: www.spab.org.uk/sites/default/files/documents/MainSociety/Advice/Mortars-SPAB-conference-report.pdf
- SPAB: Traditional Building Limes (Including Hot Limes), course outline: www.spab.org.uk/whats-on/traditional-building-limes-including-hot-limes
- GOV.UK: Approved Document L, conservation of fuel and power, Volume 1 dwellings (paragraphs 0.8 to 0.13): www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l
- Planning Portal: Listed building consent: www.planningportal.co.uk/planning/planning-applications/consent-types/listed-building-consent