There are few sights more satisfying than a well-pointed Georgian or Victorian brick wall, its joints crisp and its colours singing after a hundred and fifty years of weather. Yet repointing is also the quickest way to ruin a period house. Get the mortar wrong and you will not notice the damage for a decade, by which time the faces of the bricks have begun to crumble and the wall is holding water like a sponge.

Why Lime Mortar Matters More Than Anything Else

Traditional bricks were fired at lower temperatures than modern ones, so they are softer, more porous and more absorbent. They were laid in lime mortar, which is softer still, flexible and, crucially, permeable to water vapour.

Cement mortar is none of those things. It is hard, rigid and effectively waterproof. Point a soft brick wall in cement and several things happen at once:

• Rain that soaks into the bricks cannot escape through the joints, so it drives inward and sideways instead.
• Water trapped behind the cement has nowhere to go, so it freezes in winter, expands and spalls the brick faces.
• The cement is stronger than the brick, so any movement in the wall cracks the bricks rather than the joints.
• Salts carried through the wall crystallise at the surface, blowing the brick apart.

The principle is simple and worth repeating: mortar should always be weaker, softer and more permeable than the masonry it holds.

Work Out What You Are Dealing With

Before ordering anything, identify the existing mortar and the bricks.

Scratch a joint in an inconspicuous spot with a screwdriver. Lime mortar is usually pale, chalky and crumbly, and often contains visible white flecks of lime or shell. Cement mortar is grey, gritty and hard to disturb. If it takes a chisel and a hammer blow to shift it, you have cement.

Also look at the bricks. Handmade, stock and most pre-1919 bricks are soft and will need a non-hydraulic lime putty or a weak natural hydraulic lime (NHL 2). Harder Victorian engineering bricks, terracotta or some late-Victorian facings can take NHL 3.5. If you are unsure, go weaker, never stronger.

Choosing a Sensible Mix

For most period houses, the traditional mix is lime putty and well-graded sharp sand in a ratio of about 1:3 by volume, though 1:2.5 is fine for exposed elevations. Use sharp sand rather than builder's sand, and consider a blended grading so you have a spread of particle sizes. Match the colour and texture to the surrounding work as closely as you can.

Natural hydraulic lime is easier to use in cold or damp conditions and sets faster. Use NHL 2 or 3.5, again around 1:3 with sharp sand, and follow the maker's guidance carefully. Never add cement to speed things up, and never add washing-up liquid or other plasticisers. Both wreck the breathability and the strength.

Mix small batches you can use within an hour, and dampen the wall before you start. Lime needs moisture to cure properly, not to dry out.

Raking Out: Slow, Careful and Deliberate

This is the stage where most damage is done. An angle grinder with a cutting disc will slice through bricks and leave a joint too smooth to key the new mortar. Put it away.

• Rake joints out by hand with a plugging chisel and a small lump hammer, or a mortar rake on a drill at low speed.
• Go to a depth of roughly 15 to 25mm, or about two to two and a half times the width of the joint.
• Remove all loose material, dust and old cement residue with a stiff brush, then a gentle hose or spray.
• Never rake out joints in a wall that is already saturated, and never work in frost.

Take your time and work in manageable sections. A single elevation on a modest terraced house is a weekend's work for two people, not an afternoon's.

Pointing, Tooling and Curing

Wet the raked joints so the mortar does not lose its water to the dry brick. Press the mortar in firmly with a pointing trowel, working in short sections and building up in layers no thicker than about 10mm at a time.

Let each layer firm up before adding the next. When the mortar has stiffened enough to hold a thumbprint without smearing, tool the joint. Use a jointing iron or a piece of dowel slightly larger than the joint width. A flush or very slightly recessed finish suits most period brickwork; deep weather-struck joints look wrong on old walls and collect water.

Then leave it alone. Keep the new work damp for several days in warm weather by misting it and covering with damp hessian. Protect it from direct sun, driving rain and frost. Lime gains strength slowly, so it will feel soft for weeks and continue curing for months. A wall pointed in spring may not be fully hardened until autumn, which is entirely normal.

When to Bring in a Specialist

If the wall is listed, in a conservation area, or shows signs of structural movement, speak to the local conservation officer before you touch it. Many councils expect lime mortar and traditional techniques, and consent may be needed for anything beyond minor repairs. A specialist lime contractor can also advise on shelter coating, salt treatment and repointing sequence on larger elevations.

Do it properly once and the wall will thank you for another century. Rush it with a bag of cement and you will be paying for the same job again, plus new bricks.