Repointing Brickwork with Lime Mortar Safely

Why Lime Matters on Old Brickwork

If you own a Georgian townhouse, a Victorian villa or a humble brick-and-flint cottage, the mortar between those bricks is almost certainly lime. It was the standard binder for centuries, and it was chosen for good reason. Lime mortar is softer, more porous and more flexible than the modern cement mixes that became common in the twentieth century. It allows the wall to move gently with the seasons, to shed rainwater and to dry out from the inside.

Cement mortar does the opposite. It sets hard, traps moisture and forces any movement into the bricks themselves. On a period property, that often means spalled faces, cracked bricks and damp patches on internal walls. So when mortar starts to crumble or fall away, the answer is rarely to reach for a bag of cement. It is to repoint in lime, and to do it carefully.

Reading the Wall Before You Start

Begin by understanding what you have. Rake out a small area with a plugging chisel and look at the mortar. Old lime is usually pale, chalky and easily scratched; it may contain lumps of unslaked lime or ash. Note the colour, the texture and the size of the aggregate — sharp sand, soft sand, or something pit-sand grey.

Then examine the joints. Period brickwork rarely has neat, uniform pointing. You might find flush joints, slightly recessed ones, or a subtle struck profile. Copy what is there rather than imposing a modern finish. A raised, ribbon-like joint on a soft eighteenth-century brick looks wrong and holds water against the face.

Choosing and Mixing the Right Lime

Most period mortars were non-hydraulic lime putty mixed with sand, sometimes with a pozzolan such as brick dust or ash. For general repointing, a non-hydraulic lime putty or a natural hydraulic lime (NHL 2 or 3.5) is usually appropriate. Avoid anything stronger than the original mortar.

• Mix by volume, not guesswork. A typical starting point is one part lime to two and a half or three parts well-graded sand.
• Match the sand. The aggregate gives the mortar its colour and texture. Bagged sand from a builders' merchant is often too uniform; a local sharp sand or a blended pit sand will sit better.
• Add colour carefully. If the original has a warm buff or grey tone, test small batches. Brick dust, ochre or a darker sand can all shift the shade.
• Keep it workable. Lime putty benefits from being knocked up and left to stand. It should be soft, buttery and hold its shape on a trowel.

Always make a sample panel. Point a square metre, let it cure for a few days and look at it in both dry and wet weather. It is far easier to adjust a mix now than to redo a whole elevation later.

Raking Out Without Damaging the Bricks

The most common mistake is being too aggressive. Old bricks are often soft, and a mechanical grinder will cut into the arrises and leave a scarred face that cannot be undone. Work by hand wherever possible.

Use a plugging chisel and a lump hammer, or a good quality mortar rake, and remove the mortar to a depth of roughly twice the width of the joint — usually around 15 to 25mm. Take out any loose or hollow material, but leave sound mortar in place. Brush the joints out with a stiff bristle brush and damp them down before pointing. A dry, dusty joint will pull the water out of the lime and weaken the bond.

Pointing Techniques and Joint Profiles

Pack the mortar in firmly with a pointing trowel or a small hawthorn, working it into the back of the joint rather than smearing it across the face. Fill in stages where the joints are deep. Once the mortar has begun to stiffen — usually after an hour or two — finish the joint.

Use a churn brush or a piece of sacking to work the surface, which pushes the lime into the brick and leaves a slightly open, sandy texture. For a traditional struck joint, a wooden or metal jointer run along the mortar will do the job. Whichever profile you choose, keep it consistent and avoid over-tooling, which compresses the surface and can reduce porosity.

Curing, Weather and Aftercare

Lime does not dry like cement; it carbonates, hardening slowly as it draws carbon dioxide from the air. That process needs moisture, so freshly pointed work should be protected from drying out too quickly. In warm or windy weather, mist the wall lightly with water over the first few days, or hang hessian sheeting.

Equally, avoid pointing in frost or in relentless rain. Late spring and early autumn are usually kindest. As a rule of thumb, allow one day of curing per millimetre of joint depth, and do not pressure-wash or seal the wall afterwards. Lime needs to breathe, and a breathable mineral paint or limewash is the right finish if you want to colour the wall.

Take your time, match what is already there, and the brickwork will thank you for another century.