Old Bricks Are Softer Than You Think

Walk down any older terraced street in Britain and you are looking at walls that have stood through two centuries of rain, frost and coal smoke. Georgian, Victorian and Edwardian bricks were fired at relatively low temperatures in small kilns, which left them porous, slightly irregular and comparatively soft. That is not a flaw. It is a system.

Traditional brickwork was built to move and to breathe. The bricks absorb moisture during wet weather and release it again when conditions allow, while the lime mortar between them flexes gently with the building. Understanding that balance is the whole secret of repairing historic brickwork without causing damage — and it is why the materials you choose matter far more than the finish you achieve on the first day.

Why Cement Repointing Wrecks Old Walls

The single most common mistake made on historic brickwork is repointing with cement. A standard cement mortar is significantly harder, denser and less permeable than the bricks around it. Once that hard mortar is packed into the joints, rainwater that soaks into the wall has nowhere to escape through the mortar bed. It moves through the brick instead, and when the temperature drops, that trapped water freezes and expands.

The result is spalling — the face of the brick lifting and flaking away in sheets until the soft inner body is exposed and decays quickly. Cement is also rigid, so it cannot accommodate the small movements that old buildings make, and hairline cracks soon appear and funnel water straight into the wall.

• Harder than the brick: the mortar should always be the sacrificial material, never the brick
• Trap for moisture: water enters easily and cannot evaporate out
• Frost damage: trapped water expands and blows brick faces apart
• Salt staining: damp held in the wall carries salts to the surface

Cement render is even worse. Slapping an impermeable skin over historic brickwork holds water against the wall for years and can cause decay deep into the structure.

What Lime Mortar Actually Does

Lime mortar sets by carbonation, absorbing carbon dioxide from the air over weeks and months rather than hardening in hours. That slow cure gives it three valuable properties: it is vapour-permeable, it is slightly flexible, and it is softer than the brick it surrounds. In a properly built traditional wall, the mortar is meant to be the weak link — to crack before the brick does.

Not all lime is the same, and matching the mortar to the masonry is essential:

• Non-hydraulic lime putty is the most breathable and forgiving, ideal for soft, sheltered historic brick in mild exposure
• Natural hydraulic lime (NHL) is graded 2, 3.5 and 5 — the higher the number, the harder and faster the set
• For most old brick, lime putty or NHL 2 to 3.5 is the sensible starting point
• Exposed, coastal or heavily weathered walls may justify a slightly stronger mix, but never a cement one

Aggregate matters as much as the binder. Choose a well-graded sharp sand that matches the colour and texture of the surviving original mortar, and take the trouble to find an inconspicuous patch of historic pointing to copy. Some local mortars contained ash, crushed brick dust or shell, and reproducing that blend helps the repair sit invisibly alongside the old work.

Getting the Mix and the Joint Right

A typical repointing mix is one part lime putty to two and a half or three parts sand, or one part NHL 2 to three parts sand by volume. Always follow the manufacturer's guidance for the lime you buy, and mix small batches.

The technique matters just as much as the recipe:

• Rake out joints to at least 20mm, or one and a half to two times the joint width, whichever is greater
• Never use an angle grinder — it shatters brick edges and is a common cause of irreversible damage
• Square off the joint sides, remove all dust, then dampen the joint before packing in mortar
• Build up deep joints in layers of 10 to 15mm, letting each layer firm up slightly before the next
• Match the original joint profile: flush, bucket handle or weatherstruck. Avoid deep recesses, which hold water
• Keep the new work damp for several days and protect it from sun, wind and frost

Do not repoint in freezing conditions, in heavy rain, or in a heatwave. Lime needs gentle, moist curing, not a quick bake.

Renders, Paints and Other Traps

Breathability applies to everything that touches the wall. Lime render moves and dries in sympathy with the brick beneath; cement render does the opposite. Limewash and mineral paints are the traditional finishes, and if you want a modern paint, check that it is explicitly described as vapour-permeable.

Also look at the wider building. Raised soil or paving bridging the damp course, leaking gutters, overflowing downpipes and cracked rainwater goods all push water into old walls. Silicone water repellents are another trap — they seal the surface and trap moisture behind, sometimes making damp problems considerably worse.

When to Call in a Specialist

If your wall is bowed, badly eroded, or carries load from above, do not treat it as a pointing job. A conservation-accredited building surveyor or structural engineer should assess it first. If the building is listed, you will almost certainly need listed building consent before repointing or rendering, and in a conservation area some works require permission too.

When choosing a contractor, ask for evidence of work on historic brick, look at completed jobs a few years old, and insist on a written specification naming the mortar type, mix ratio and aggregate. A cheap cement repoint will look neat for a season and cost you a fortune in replacement bricks within a decade. Done properly with lime, the repair will quietly outlast most of us, which is exactly the point.