Why Old Buildings Hate Cement

The surprising science behind lime mortar, breathability, and why well-intentioned repairs destroy historic walls

If you own a historic building, the instinct is simple.

Something looks worn.So you repair it with the strongest modern material you can find.

Unfortunately, that is exactly how thousands of historic buildings are slowly destroyed.

Across the UK conservation specialists follow the principles of the Society for the Protection of Ancient Buildings (SPAB). Their approach is based on one idea that sounds simple but turns out to be revolutionary.

Old buildings need to breathe.

Understanding that single idea explains why lime mortar works, why cement damages historic walls, and why many well-meaning repairs fail.

The Hidden Job of Mortar in Old Walls

Most people assume mortar simply sticks bricks together. In historic buildings it actually performs two critical jobs.

First, it acts as a structural cushion. Buildings move slightly with settlement, temperature changes and moisture. Lime mortar absorbs this movement so the bricks or stones do not crack.

Second, it acts as a moisture escape route. Traditional lime mortar is deliberately softer and more porous than the bricks or stones around it. Moisture naturally evaporates through the mortar joints instead of the masonry units.

This means the mortar slowly erodes over decades while the bricks remain protected.

Conservation experts describe mortar as a “sacrificial material.” When it wears away, you simply repoint it.

When Repointing Is Actually Needed

Many buildings are repointed unnecessarily. SPAB guidance is clear. You should only repoint when the mortar has genuinely failed.

Typical signs include: • mortar missing to a depth of around 25–40 mm • joints that crumble when scraped with a screwdriver • powdery sand-like mortar • loose mortar that pulls away from the brick or stone

If the mortar is sound, it should be left alone. Historic mortar is part of the building’s story and removing it unnecessarily damages the fabric.

The Biggest Mistake People Make

The most common mistake is using cement mortar. Cement is harder, stronger and far less permeable than lime. That sounds good but it causes a serious problem.

Because the mortar is now stronger than the brick, the brick becomes the sacrificial element instead of the mortar. Moisture gets trapped in the wall. When winter arrives the water freezes and expands. The result is a classic conservation failure called spalling, where the face of the brick literally pops off.

Many historic buildings across Britain have been quietly damaged this way during the last fifty years.

The Science of Breathable Buildings

Traditional buildings operate like an overcoat. They absorb moisture and then release it again.

Three physical processes allow this to happen.

Vapour permeability – moisture vapour passes through the materia lHygroscopic buffering – materials absorb and release humidity Capillary action – water is drawn to the surface where it evaporates

Lime mortar and lime render allow this cycle to work. Modern cement renders and plastic paints block it. Once moisture becomes trapped inside a historic wall the building slowly begins to decay.

The Lime Cycle: Why Lime Is Such a Clever Material

Lime mortar works because of a remarkable natural chemical cycle. Limestone is heated in a kiln to produce quicklime. Quicklime is mixed with water to create lime putty. When lime mortar is applied to a wall it slowly reacts with carbon dioxide in the air and turns back into limestone again. This process is called carbonation.

In other words, the mortar literally turns back into stone over time while remaining flexible and breathable.

Different Types of Lime

Not all lime behaves the same way. Conservation work uses several types depending on exposure.

Non-hydraulic lime (lime putty)Very breathable and flexible. Best for plaster and sheltered masonry.

NHL 2 A weak hydraulic lime suitable for softer bricks and sheltered walls.

NHL 3.5 The most common choice for external pointing.

NHL 5 Used only in very exposed locations such as chimneys or coastal structures.

Choosing the correct lime is essential because using a mix that is too strong can cause exactly the same problems as cement.

Why Cement Render Can Be a “Time Bomb”

Many older houses were coated with cement render during twentieth-century renovations. At first everything looks fine.

But cement behaves very differently from lime. It is rigid, so hairline cracks inevitably form as the building moves. These cracks allow water in but the render does not allow moisture to escape.

Water becomes trapped behind the render where it quietly soaks the wall structure.

Over time this leads to damp interiors, decaying timber and masonry damage.

Removing cement render often reveals a wall that has been saturated for years.

How Historic Brickwork Should Actually Be Repaired

Conservation practice follows a principle known as minimal intervention.

The aim is to repair only what is necessary and preserve as much historic material as possible.

Common techniques include

Reversing bricks If the front face has weathered but the brick is sound, it can sometimes be turned around and re-laid.

Tile stitchingCracks can be stabilised using thin ceramic tiles set in lime mortar. This provides flexible reinforcement without rigid metal bars.

Like-for-like replacementIf bricks must be replaced they should match the original in size, colour and firing characteristics.

Second-hand bricks are usually avoided because they often contain hidden salts that can damage the wall later.

Repairs That Should Never Be Done

Conservation specialists strongly warn against several treatments that remain surprisingly common.

Sandblasting brickworkThis strips away the protective fired surface of the brick and accelerates decay.

Water-repellent coatingsThese trap moisture inside the wall.

Injected damp-proof coursesOften installed to treat “rising damp” that is actually caused by blocked breathability or high ground levels.

Why Flint Walls Are Even More Sensitive

Flint walls behave differently again. Flint itself is almost completely impermeable.

That means all moisture must evaporate through the mortar joints. Using cement mortar in flintwork is therefore particularly damaging because it blocks the only escape route for moisture. Traditional lime mortar is essential for these walls to function properly.

The Real Lesson of Historic Building Repair

The biggest insight from conservation practice is surprisingly simple. Historic buildings were designed to work with soft, breathable materials.

• When we replace those materials with stronger modern ones we unintentionally break the system

• The building cannot breathe.

• Moisture becomes trapped.

• And the damage begins.
  

This is why conservation specialists repeat the same rule again and again.

The repair should never be stronger than the building itself.