Why old walls need lime, not cement

If you own a cottage, a farmhouse or a converted barn, the chances are your walls were built to move. Traditional masonry is not a rigid box; it is a series of separate units — bricks, stones, cob or timber panels — held together by a mortar that was always intended to be softer than the material it binds. Lime mortar is that material. It is permeable, slightly flexible and, crucially, sacrificial: when a wall settles, gets wet and dries out again, it is the mortar that gives way first, not the brick or the stone.

Cement changed all that. It is hard, brittle and largely waterproof, so when it is forced into the joints of a lime-built wall it seals the surface while trapping moisture behind it. That moisture has to go somewhere. It finds the path of least resistance — through the face of a brick, the edge of a limestone quoin or the timber frame hidden behind the render. The result is spalled brickwork, crumbled stone and rotten lintels, often blamed on the age of the building rather than the cement that caused it.

A short guide to the limes you will meet

Lime is not one product. Before you start, it is worth knowing what you are buying and where it belongs.

• Non-hydraulic lime putty — pure calcium lime, slaked and stored wet. It sets by carbonation, absorbing carbon dioxide from the air, so it needs air movement and dries slowly. Beautiful for internal plaster and sheltered, soft masonry.
• Naturally hydraulic lime (NHL) — sold as NHL 2, NHL 3.5 and NHL 5. These set partly by reaction with water, so they will cure in damper conditions. NHL 2 is the gentlest and is usually the right choice for old brick; NHL 3.5 suits general external work; NHL 5 is the strongest and closest to cement in behaviour, so use it sparingly.
• Gauged mixes — lime putty blended with a pozzolan such as brick dust or volcanic ash to speed up the set, useful in cold or damp weather.

The guiding principle is to match the mortar to the wall. Soft, hand-made Victorian brick wants a weak, forgiving mix. Hard gritstone or limestone can take something a little stronger. When in doubt, go softer.

Repointing: the details that actually matter

Most repointing goes wrong before any mortar is mixed. Rake out the joints properly — to a depth of at least 1.5 to 2.5 times the width of the joint, and no less than 15mm externally. Use hand tools on soft brick and stone; an angle grinder will do more damage in an afternoon than a century of frost.

Once raked, brush out the dust and dampen the wall. Dry masonry will suck the water out of the mortar and leave it weak and crumbly. Pack the lime in firmly in layers, letting each one firm up before the next, and finish the joint flush or slightly recessed. Avoid proud, weatherstruck ribbons that trap water on the brick face — they look neat in a photograph and cause trouble within a decade. Finally, cure it slowly. Mist it with water and cover it with damp hessian in hot or windy weather, and never let it dry out too fast.

Lime plaster and render

Inside a period property, lime plaster is what makes the walls feel dry and generous. It is applied in three coats — a scratch coat, a float coat and a thin finish — each allowed to set before the next goes on. The finished surface is subtly uneven, warm to the touch and genuinely breathable.

The most common mistake is skimming old lime plaster with gypsum. Gypsum is harder and less permeable, and once it is on, the wall beneath it can no longer manage moisture. Patch repairs should be done in lime, feathered into the existing work and left slightly proud before being rubbed back. Outside, lime render over solid brick or stone performs the same job: it sheds most of the rain while allowing the rest to evaporate away.

Barn conversions and timber frames

Barns are the hardest working old buildings and the most rewarding to repair. Their walls are often a mix of timber frame, brick infill, wattle and daub, or stone, all moving at slightly different rates. Lime is the only sensible binder here because it flexes with the frame rather than fighting it.

Where oak posts and beams are exposed internally, leave them. Where panels are rendered, use a lime render that can accommodate seasonal movement, and finish externally with a breathable limewash rather than a modern masonry paint. Limewash is cheap, easy to reapply and lets the wall breathe, which is exactly what a converted barn needs if it is to stay free of damp.

Practical mixing and aftercare

Get the basics right and lime is wonderfully forgiving. A few rules worth pinning to the workshop wall:

• Mix by volume, not by eye. A typical repointing mix is one part lime to two and a half or three parts well-graded sharp sand.
• Use the right sand. Washed, sharp, with a spread of grain sizes. Builder's sand alone makes a weak, shrink-prone mortar.
• Keep it cool and damp. Protect new work from sun, wind and frost for at least a week; a fortnight is better.
• Do not over-work the surface. Trowelling lime too soon brings the binder to the face and weakens the mix.
• Test a small panel first. Colour, texture and set time all vary, and a sample patch will tell you more than any specification.

Work with lime rather than against it and a period property will reward you with walls that dry out, timber that lasts and interiors that feel calm, solid and genuinely old. It is slower, gentler work — and that, on an old building, is usually the point.