Why Historic Brickwork Needs a Softer Mortar

Bricks made before the twentieth century were fired at lower temperatures than modern ones, which leaves them softer, more porous and more absorbent. They were also laid in lime mortar, a material that stays slightly flexible for decades and allows water vapour to pass straight through the wall. The two work together as a system: the brick takes the load, and the mortar absorbs the movement, the moisture and the minor stresses.

Lime mortar is what conservationists call sacrificial. It is deliberately softer and weaker than the bricks around it, so if the wall moves or freezes, the mortar gives way first. A cracked joint is easy to cut out and repoint. A cracked brick face is not, and once a brick spalls its fired skin, that damage is permanent.

That single principle — mortar should be softer than the masonry it holds — explains almost everything about repointing a period property.

What Cement Repointing Does to an Old Wall

Portland cement mortar is hard, strong and largely impermeable. Pointed into historic brickwork, it behaves like a waterproof seal around every brick. Rain that soaks into the face of the brick can no longer escape through the joints, so it has to dry outwards or inwards. Outwards means the face of the brick blows off in frost. Inwards means damp patches on internal plaster, salts blooming on the surface, and the sour smell of a wall that never dries.

There is a second problem. Cement is rigid, and old walls are not. As the building settles and the brickwork expands and contracts with temperature, the rigid joints concentrate stress into the bricks. You will often see a neat pattern of hairline cracks running through the bricks rather than the joints — the classic signature of cement pointing on a soft Victorian or Georgian wall.

Look out for these telltale signs:

• Mortar joints that sound hard and ring when tapped with a chisel handle
• Brick faces flaking or dropping away in thin, shell-like slivers
• Neat grey or dark cement ribbons smeared across the brick faces
• White salt deposits, green algae and persistent damp on internal walls
• Hairline cracks passing through bricks rather than following the joints

If you spot several of these together, the pointing is very likely the cause.

Choosing the Right Lime

Broadly, you have two choices. Non-hydraulic lime putty, often called fat lime or air lime, sets by absorbing carbon dioxide from the air. It is the most flexible and most breathable option, which makes it ideal for sheltered elevations and internal work. It also needs air to reach it, so it cures slowly — think months rather than days.

Hydraulic limes, graded by strength from NHL 2 through NHL 3.5 to NHL 5, set partly by reaction with water and are better suited to exposed elevations, chimneys and areas catching driving rain. For most general repointing on period brickwork, a moderately hydraulic lime such as NHL 3.5 is a sensible default. Avoid the strongest grades unless the brickwork itself is unusually hard.

The sand matters just as much as the lime. Use a well-graded, sharp, washed sand with a spread of particle sizes, and avoid mixes dominated by fine material. Aim to match the colour, texture and aggregate size of the original mortar, ideally by taking a small sample of the existing joint to a supplier who understands historic work.

Mixing, Pointing and Curing in Practice

A traditional putty mix is usually around 1 part lime putty to 2.5 or 3 parts sand by volume. With hydraulic lime, a 1 to 3 ratio is a common starting point. Mix thoroughly, add water gradually, and let the mortar stand before use so the lime can fully wet the sand.

Preparation is where good jobs are made. Rake out the joints to a depth of roughly 20 to 25mm, or about twice the width of the joint, and square off the edges of the brickwork. Remove all cement residue and dust, then dampen the joints — not soak them, just take the suction off so the new mortar does not dry out too fast.

Pack the mortar in firmly in layers, working it into the back of the joint rather than smearing it across the face. Match the original profile, whether that is flush, struck or a slight bucket-handle curve. Once the mortar has begun to firm up, tidy the joints and brush the brick faces clean, but never tool while the lime is still wet.

Aftercare is everything. Keep the new work damp for several days with a fine mist and damp hessian, and shield it from direct sun, drying wind, rain and above all frost. Lime needs weeks of gentle curing, not a single dry afternoon.

Weather, Season and Workmanship

Point in mild, settled weather, ideally between late spring and early autumn. Avoid working when temperatures are falling towards 5°C or lower, when frost is forecast, or in direct midday sun on a south-facing wall. Elevations in constant shade may need longer protection.

Ask any contractor to run a sample panel first, so you can judge colour, texture and finish before the whole elevation is committed. Good repointing looks slightly recessed and even, with clean brick faces and a joint that dulls rather than glistens. If the finished work is uniform grey, rock-hard within a day, or smeared across the bricks, it is cement — and it will need removing again.