A beginner's guide to lime mortar repairs

Why lime, and why it matters in an old wall

If your cottage was built before the twentieth century, there is a good chance its walls were put together with lime. Lime mortar is soft, slightly flexible and — crucially — porous. It lets moisture move through the wall and evaporate away, and it allows the whole structure to shift a little with the seasons without cracking. Cement mortar does none of those things. It sets hard and brittle, it traps water inside the wall, and it forces any movement to find the weakest point, which is usually the brick or stone itself.

That is why repointing an old wall with a cement-rich mix does so much long-term damage. Water sits behind the hard pointing, freezes, and blows the face off the bricks. Once that happens, you are into replacement masonry rather than a straightforward repair. Using lime is not nostalgia — it is simply working with the building rather than against it.

Reading your walls before you touch them

Before mixing anything, spend half an hour looking. Prod the mortar with a screwdriver. Good lime mortar should crumble or powder under pressure; if it rings hard and resists, someone has already been at it with cement. Look for sandy joints, hollow patches, and areas where pointing has fallen away leaving a gap of more than about 15mm.

Match what is already there as closely as you can. A few useful rules:

• Soft, sheltered walls and old brickwork: non-hydraulic lime putty, mixed roughly 1 part putty to 3 parts sand by volume.
• Exposed walls, or stone that is moderately hard: natural hydraulic lime (NHL) 3.5, typically 1 part lime to 2.5–3 parts sand.
• Very exposed or damp situations: NHL 5, though this is stiffer and less forgiving, so start elsewhere if you can.
• Sand: a well-graded sharp sand, usually 0–4mm, chosen to match the colour and texture of the original. Bagged building sand with lots of fine silt is a poor substitute.

Do a small test patch somewhere out of sight, let it cure for a fortnight, and see how it looks against the surrounding mortar.

Raking out and damping down

Work on a manageable area — a metre square at a time, no more. Rake the joints out to a depth of roughly twice their width, aiming for 15–20mm on exposed walls. A plugging chisel and a small lump hammer do the job, but take it slowly; the aim is to remove soft mortar, not to widen the joint or chip the arrises. Brush out dust with a stiff brush, and never be tempted to use a disc cutter, which will scar the masonry in seconds.

Then, and this is the step most beginners skip, dampen the wall thoroughly. Dry stone and brick will pull the water straight out of your mortar, leaving it weak, crumbly and poorly bonded. Spray the joints with water and let them soak. On a hot day, keep a plant mister to hand and dampen again as you go.

Mixing a small batch of lime mortar

Lime rewards small batches. Mix in a bucket with a trowel or a paddle mixer, and only make what you can use in an hour or two. A half-bucket of sand and the appropriate lime is plenty for a first attempt. Add water gradually until the mix holds together when squeezed but does not slump — it should feel like a firm, slightly tacky paste rather than wet concrete.

Let it stand for ten minutes, then give it a quick stir. Lime putty mortar benefits from a longer rest, even overnight, but NHL mixes should be used within a couple of hours once water is added. Never add cement or a plasticiser to speed things up.

Packing the joint and finishing

Use a pointing iron or a wooden stick rather than a steel trowel. Push the mortar firmly into the joint in layers for anything deeper than 20mm, letting each layer firm up before adding the next. Work from the top of the wall downwards, and pack it in properly — voids at the back are what cause failures later.

Once the mortar has firmed up to what is often called the leather-hard stage, tidy the surface. A churn brush dragged across the joint closes the face and exposes the sand grains, giving a texture much closer to historic work than a smooth, shiny finish. Avoid tooling it flat and tight, which compresses the surface and reduces its ability to let water through.

Aftercare, and knowing your limits

Lime cures slowly, by carbonation, and it needs moisture to do so. In warm or windy weather, cover the work with damp hessian and mist it daily for the first few days. In cold weather, protect it from frost with sheeting — lime mortar should not be laid below about 5°C, and it will not set at all if it freezes. Keep an eye out for hairline cracks as it dries; small ones are normal, but deep ones usually mean the mix was too dry or the joint was not dampened enough.

Finally, be honest about scale. A few square metres of pointing is a satisfying weekend job. Structural cracks, failed lintels, deeply eroded stone and anything above head height are best left to a specialist who works with lime regularly. Ask what mix they intend to use, and be wary of anyone who reaches for a bag of cement.