Why Lime Mortar Behaves Differently

Old walls are not sealed boxes. They are breathing assemblies of brick, stone, timber and lime. Traditional lime mortar is softer, more porous and more flexible than the masonry around it. That matters because moisture will always find a way into a solid wall. With lime, water vapour can migrate through the mortar and evaporate harmlessly. The mortar also flexes with tiny movements instead of cracking, and if a crack does form, lime can re-crystallise and heal it to a degree. Cement mortar, by contrast, is hard, brittle and relatively impermeable. It traps moisture in the softer bricks, which then spall, blister and crumble. For repointing, the aim is not to create the strongest possible joint. It is to create a sacrificial joint that protects the historic fabric.

Choosing the Right Lime and Sand

Start with the masonry, not the bag. For very soft, old bricks and sheltered elevations, a non-hydraulic lime putty is often ideal. It sets slowly by carbonation, needs air, and stays flexible. For general external work, a moderately hydraulic lime such as NHL 3.5 is a practical choice. Use a weaker NHL 2 for delicate historic brick, and reserve stronger NHL 5 for exposed, hard stone where a faster set is essential. Never add cement to make the mix stronger. Strength is the enemy here.

Sand matters as much as lime. Use a sharp, well-graded sand that matches the original mortar's colour and texture. Washed pit or river sand is usually better than soft building sand, which can be too fine and clay-rich. A good pointing mix by volume is:

• Lime putty: 1 part putty to 2.5–3 parts sand for soft brick; 1:3 for general repointing.
• Hydraulic lime: 1 part NHL 3.5 to 2.5–3 parts sand, adjusted to the wall's softness.
• Pozzolan: if using a pozzolanic additive, follow the maker's ratio, often 1 lime : 1 pozzolan : 3 sand for damp or slower-setting conditions.

Test a small panel first. Colour, strength and working characteristics vary enormously.

Mixing in Practice

Measure by volume, not by eye. For lime putty, add the sand to the putty gradually and mix until evenly coloured. For dry hydraulic lime, mix the lime and sand dry first, then add clean water slowly. Aim for a consistency like thick porridge or firm butter. Good pointing mortar holds its shape on a trowel and does not slump. If it is too wet, it will shrink, crack and stain the brickwork. If it is too dry, it will not bond.

Let the mix stand for 15–30 minutes, then knock it up again before use. This improves workability. Use small batches. Hydraulic lime mixes should be used within a few hours. Putty mortars can be covered and kept for a day or two, but do not let them dry out. Add only water, never cement, resin or antifreeze. In cold weather, use warm water, not hot, and protect the work.

Preparing Joints and Packing Correctly

Preparation is where most repointing jobs succeed or fail. Rake out the old mortar to a depth of at least twice the joint width, usually 15–25mm, and remove all cement, dust and loose material. The joint edges should be square, not feathered. Dampen the masonry with a fine spray before pointing. This stops the wall sucking moisture from the mortar too quickly.

Pack the mortar in thin layers, around 10–15mm at a time, using a pointing iron or churn brush. Do not fill a deep joint in one go, as the mortar will shrink and crack. Press firmly to fill voids and key into the brick. Keep the face of the brick clean as you go. When the mortar is thumbprint-dry, finish the joint with a churn brush or a wooden stick. For historic walls, a flush or slightly recessed joint is usually best. Avoid proud, strap-pointed joints that trap water.

Curing and Aftercare

Curing is not a quick dry. Lime needs moisture to set and air to carbonate. After pointing, keep the work damp for at least three to seven days, and longer in hot or windy weather. Mist with water and cover with damp hessian or breathable sheeting. Protect the wall from direct sun, driving rain, strong wind and frost. Do not seal the mortar with acrylics or masonry paint; it needs to breathe.

Carbonation takes months, not days. The mortar will continue to harden slowly. Avoid washing with acid or pressure-washing new work. If frost is forecast, cover the wall and delay pointing. Below 5°C, lime sets very slowly and may fail.

Common Mistakes and When to Get Help

• Using cement or a too-strong lime mix.
• Wrong sand: too fine, too dirty or poorly graded.
• Filling deep joints in one pass.
• Tooling too early or too late.
• Letting the mortar dry out too fast.
• Cleaning with acid or harsh jets.

If your building is listed, in a conservation area, or has unusual historic fabric, take advice before you start. A small test panel and a patient approach will tell you more than any bag label. Done well, lime repointing is not a cosmetic fix. It is part of how an old wall continues to breathe, move and survive.