Lime repointing
What is lime repointing?
​Repointing is the process of removing and replacing mortar in the joints between bricks or stone in a wall. This might be necessary because of the building's age, damage to the wall or because modern inappropriate materials like cement or masonry paint have been used.​
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Pointing mortar is a 'sacrificial' construction material, meaning it is supposed to gradually decay over time instead of the surrounding brick or stone. Moisture is transferred from the brick or stone to the pointing, where it evaporates naturally. Good quality lime mortar used for pointing should last at least 100 years, although there are examples of lime mortar surviving for thousands of years!
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Unfortunately, many old buildings have been repointed or rendered with cement over the years. Cement is not breathable and traps moisture in the brick or stone - the exact opposite of what's needed. This inevitably results in a build-up of damp in the wall, causing the brick or stone decaying instead of the mortar, as well as issues with cold and damp transferring to the inside of the building.​
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The way to fix this is by removing the problem materials, whether that's cement pointing, impermeable render or painted masonry, and using lime to repoint the joints. The below process is an overview of a typical lime repointing job.
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The lime repointing process
Lime repointing is fairly straightforward so long as the correct preparation is undertaken and appropriate materials are used. It isn't a one-size-fits-all approach: different walls will need different lime mixes. For example, a wall made of old, soft brick will need a much weaker mix than a wall made of hard flint. That's why it's crucial to assess the building structure and history before undertaking works.
Identifying the correct repointing mix
Before starting any work, it's important to decide on the most suitable lime mortar to use for the job. Where repointing must match existing mortar, a sample can be taken for analysis to work out the closest match in terms of colour, grade and material. We can produce test samples for you to compare with existing mortar, as shown in the picture.
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If the wall is made up of softer stones or old bricks, then it's most likely that a weaker non-hydraulic lime mix is most appropriate. This is the same lime used in plastering mixes and provides the best level of breathability. The overall mix must always be weaker and softer than the bricks or stones it is in contact with.
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In cases where a hard stone or brick is used, a stronger mix can be used instead - either a non-hydraulic lime with added pozzalan or a Natural Hydraulic Lime (NHL) mix.
Chimney stacks and retaining walls will also need a stronger mix that is able to stand up to higher exposure or prolonged wet conditions. In these cases an NHL is likely to be used.
Preparing the walls
Removing failed or inappropriate mortar is the first step, usually known as 'raking out'. Old mortar is raked out with a hammer and chisel or power tools as needed to a depth of about twice the width of the joint. For example, a 10mm joint in brickwork should be raked out to 20mm depth.
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If joints are particularly large, such as in a rubble stone wall, then they are partly filled with small stones to reduce the amount of lime mortar needed. Very deep joints may be filled in layers of around 15mm, with time in between each layer to allow it to cure.
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Dirt and dust is removed from the joints so that the lime mortar can stick properly, then the joints are dampened down to make sure that the mortar does not dry out too fast. This is known as reducing the suction of the wall.
Filling the joints
The mix for lime pointing is dryer than that used for plastering but is otherwise similar to a base coat mix without fibres.
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It is applied with a small pointing trowel, pushing the mortar in well to the back of the joints. This stage doesn't need to be neat - the important factor is that the mortar is applied well into the joints without any gaps.
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Once all joints have been filled and have started to stiffen up, it's time to compress it with a wooden stick. I find an old broom handle works well. The handle is dragged along the joints with some pressure, pushing the mortar back and filling any small gaps and cracks that might have appeared as the mix shrinks. This process also tidies the work up and removes excess mortar, allowing it to be finished flush with the brick or stone work.
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Finally, a stiff churn brush is used to beat the mortar back further and give the final finish. This exposes some of the coarser aggregate and opens up the surface allowing it to cure effectively.
Aftercare
The mortar must be kept damp for at least a week to enable a gradual set. If it dries out too fast, it will shrink and crack, leaving visible gaps between the brick or stone and the mortar. The speed of drying is controlled by covering the wall with damp hessian sheets and spraying with water as needed. In summer, this can mean regular spraying with the mist function on a hose.
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Non-hydraulic mortars will always set slower than NHL-based mortars, at approximately 1mm per month - so even while the surface may have hardened, the deeper layers are still curing. Non-hydraulic mortars will only reach their full strength after several months.
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NHL-based mortars set through contact with water and are much faster, taking only a few days.
