Why Historic Brickwork Fails — and Why the Mortar Is Usually to Blame

If you own a Victorian terrace, a Georgian townhouse or a stone-and-brick cottage, you will almost certainly face repointing at some point. The instinct is to assume the bricks are worn out. In truth, the bricks are usually the toughest thing in the wall. It is the mortar between them that gives way first, and it is the mortar that decides whether the wall survives the next fifty winters or quietly crumbles from within.

Old bricks were fired at relatively low temperatures and are softer, more porous and slightly irregular. They were never designed to be waterproof. Instead, the whole wall was built as a system: rain soaks in, travels through the brick and mortar, and evaporates back out again when the weather dries. Break that cycle and you get problems.

The Damage Cement Mortar Does to Old Walls

From the 1950s onwards, cement-based mortars became the default repair. They are strong, quick and cheap. On a modern cavity wall that is fine. On a solid-walled period property, they are a slow disaster.

Cement mortar is harder, denser and far less permeable than the bricks around it. Three things follow:

• Moisture gets trapped. Rain that enters the brickwork can no longer escape through the joints, so it sits in the brick and drives inward.
• Frost and salt do the rest. Wet bricks freeze, expand and flake. Soluble salts crystallise just beneath the surface and blow the face off the brick — the classic spalling you see on repaired gable ends.
• The wall cannot move. Period properties shift very slightly with thermal and moisture changes. Rigid cement joints do not flex, so the brick takes the stress instead and cracks.

Irritatingly, cement mortar often looks neat for a decade before the damage shows. By the time the bricks are spalling, they need replacing, not just the pointing.

How Lime Mortar Lets a Wall Breathe

Lime mortar is what these walls were built with, and it works in the opposite way. It is softer than the bricks, more porous and slightly flexible.

That means when movement happens, the mortar joint sacrifices itself rather than the brick — which is exactly the trade you want. It means water vapour can pass through the joint in both directions, so the wall dries out naturally. And lime has a useful trick: it absorbs carbon dioxide from the air over months and years, slowly hardening as it cures while remaining vapour-permeable.

Choose the right lime and the wall keeps working as designed. Choose the wrong one and you have simply swapped cement for a different problem.

Know When to Repoint — and When to Leave It Alone

Repointing is not routine maintenance and it is not something to do while you are at it. Every time joints are raked out and refilled, the arrises of the bricks are at risk. Repoint too often and you will damage more than you repair. As a rule of thumb, repoint only when:

• Mortar is deeply recessed, crumbling or falling out in visible quantities
• Water is clearly tracking into the wall through open joints
• Joints are sound but have been wrongly replaced with cement and are causing spalling

If the pointing is weathered but intact, leave it. A slightly eroded historic joint is doing its job. And before you blame the brickwork, check the real culprits: overflowing gutters, failed downpipes, blocked hoppers, cracked render, perished mortar flaunching around chimneys, or slipped slates and failed flashings. Water almost always arrives from above. Fix the roof and the drainage first, or you will repoint a wall that keeps getting soaked.

Getting the Mix, Depth and Profile Right

If you do repoint, the details matter far more than the effort.

• Mix: For most pre-1919 brickwork, a non-hydraulic lime putty with well-graded sharp sand at roughly 1:2.5 by volume, or an NHL 3.5 for more exposed elevations. Avoid cement, gypsum and any advice to add just a bit of cement to make it go off.
• Sand: Match the original in colour and grit. Sharp, well-graded sand gives strength; too much clay or fine builder's sand makes a weak, shrunken mortar.
• Depth: Rake out to at least 15–20 mm, roughly twice the width of the joint. Shallow pointing simply falls out.
• Profile: Finish flush or very slightly recessed and rub back with a churn brush. Avoid proud, weatherstruck bands that trap water on the brick faces.
• Conditions: Never point in frost or in direct sun. Dampen the wall first, work in small areas, and keep new work covered with hessian for a few days.
• Consent: On listed buildings, and often in conservation areas, repointing needs listed building consent or prior approval. Speak to your conservation officer before you start.

If you are unsure, do a test panel on a hidden elevation and leave it through a winter. It will tell you more than any specification.

Looking After the Whole Building

Repointing sits within a wider rhythm of period property care. Clear gutters twice a year, check chimney flashings and mortar flaunching each autumn, keep an eye on slate and tile roofs after storms, and make sure ground levels outside sit at least two brick courses below the damp course. Investigate damp internally before assuming it is rising — nine times out of ten it is a gutter, a cracked sill or defective pointing higher up.

Do this work patiently, with materials that match the building, and a period house will comfortably outlast you. Fight it with hard modern materials and you will spend the next twenty years paying for the argument.