How to repoint ashlar masonry

Ashlar masonry is built with large, regular, squared stone blocks, a method brought to Scotland by the Romans. Ashlar was later used to create the fine stone façades of great 17th and 18th-century country mansions. By the mid-1800s, such façades were widely used on city tenements and villas and on rural farmhouses.

A range of stone types was used across Scotland in the construction of traditional ashlar buildings, but sandstone was used most often in most areas.

Ashlar wall construction

Rough stone taken from the quarry was carefully cut into masonry blocks of precise dimensions and given tooled or smooth polished external faces. Edges were cut square and true so that each block would fit precisely with its neighbour when set into the wall.

Rows of stones cut to exactly the same height, in random lengths, were laid with a ‘broken bond’. This is where blocks are staggered so that the middle of each block on the next row sat directly above the vertical joint on the row below. This gave the wall maximum load-bearing strength.

An ashlar wall has very fine joints just a few millimetres wide, as a result of the accuracy of block cutting in the past. During this period, the external face of the ashlar block may also have been adorned with a chamfer – creating the illusion of V-shaped joints when the blocks were put together – or tooled with decorative patterns.

Traditional buildings must be allowed to breathe so that water vapour can escape. Despite being such a small part of the ashlar masonry façade, lime mortar joints give a building its ability to breathe. They are thus vital to how the building handles moisture.

An ashlar wall constructed of squared stone blocks is inherently stronger than a rubble stone wall. As a result, it can withstand considerable erosion and distress before damaged stones must be replaced. Indent repairs to the sandstone may eventually be necessary where erosion is severe.

Signs that an ashlar wall may need repointing include:

• washed out or decayed mortar
• loose or missing ashlar blocks
• loose or missing cope stone
• plant growth in the fine joints
• stone decay caused by cement mortars

Mortar used in the original construction of traditionally built ashlar masonry will deteriorate over time and repointing work will eventually be necessary.

Ensuring that ashlar repointing is carried out using suitable methods and materials will reduce the risk of moisture penetrating the masonry.

Failure of pointing

Repointing is necessary when the bedding or jointing mortar washes out and the ashlar blocks are at risk of becoming unstable. Left as is, rainwater will enter the joints, promoting further deterioration.

Plant growth can also damage pointing, particularly where roots enter the wall.

Cement mortar use

Cement mortars should not be used for repointing work as they can damage the stonework and interior finishes by trapping water inside the wall. Other impermeable mortars and/or coatings must also be avoided.

Lime based mortars should be used instead as these will allow the building to breathe and water vapour to escape.

Lack of breathability in a building can lead to:

• masonry decay
• internal damp
• rotting timbers

Lime mortar repointing work shouldn’t be carried out during periods of heavy rainfall or frost. A suitable cover may be needed to protect the work.

Ashlar masonry was built with great care and attention, and the precision required to repoint ashlar correctly demands considerable time and skill.

Survey existing pointing

A survey of the existing pointing mortar should be carried out before work begins to assess how much repointing work is required.

Repointing of an entire façade may not always be necessary if only part of the original mortar has decayed. Mortar that is sound and shows no signs of deterioration should be left in place.

Patch repointing in selected areas is often all that’s needed, and using a mortar mix similar to the original can help to maintain an even appearance.

Complete façade repointing may sometimes be essential, for example, where the weathering of softer sandstone has led to widespread façade erosion.

Care must be taken, and suitable tools used, to avoid damage when preparing ashlar masonry for repointing.

Decayed mortar can be raked out by either:

• carefully picking it out with a thin steel hook
• easing out the old material by inserting a handheld hacksaw blade into the joint and pulling gently

Power tools shouldn’t normally be used to cut out decayed or loose mortar from the fine joints as they may damage the stone. Oscillating mechanical tools can yield good results in the hands of highly skilled operatives, however. But rotary tools such as angle grinders should never be used.

Even hand tools such as chisels can cause damage if not used properly or if too wide to provide the precision that’s needed.

Raked out areas should be flushed out with water so that any last bits of loose and decayed mortar are washed down and off the wall face. Traces of dust and debris can stop the new mortar bonding with the stone.

Removing cement mortars

Cement mortars are unsuitable for repointing ashlar and can sometimes be removed if used for pointing in the past.

A hammer and chisel may be needed to remove cement mortars, often used as superficial over-pointing. Carefully chipping away the cement will expose the lime mortar beneath. Oscillating mechanical tools may be used, but never rotary tools such as angle grinders. Avoid damaging the edges of underlying stones.

You must weigh up the pros and cons of removing cement pointing that is still sound or which resists removal. To prevent damage, the best option may be to leave sound existing cement mortar in place and accept how it looks.

Regularly inspect any cement mortar to check if it has started to cause damage to the masonry. Removing it at this stage may be better for the wall’s well-being even if some damage is caused in the process.

Repointing mortar must be compatible with the existing masonry so as not to cause any unnecessary masonry deterioration.

The fineness of ashlar joints means that mortars made using coarse sands must not be used for pointing ashlar.

Added ingredients

Ashlar pointing mortar was historically made using lime putty and whiting (crushed chalk) with a little linseed oil – a mix that’s still used today. If the mortar must also include fine sand or stone dust as an aggregate (to match the original pointing mortar), the particle size should be no larger than one third of the minimum joint width.

Repointing mortar must be carefully made up, paying close attention to the exact quantities of the lime and other materials used. This is so that further batches made of the mortar will possess the same qualities.

Specialist analysis of the existing mortar can help to ensure that its replacement looks right and performs equally well.

Pre-application tips

Before applying the mortar:

• damp down the stone to stop the mortar from drying too quickly
• cover the arrises (edges) of stones with heavy duty tape to protect them from staining and damage during repointing

Be careful when choosing tape for this purpose – some brands may leave an adhesive residue on the stone’s surface, which can be difficult to remove. Test the tape first on a part of the wall that isn’t easily seen.

An alternative to using tape is to sponge down the stone once the mortar has been applied but before it has dried fully.

Repointing mortar mustn’t be spread over the face of ashlar masonry, but applied only to the thickness of the joints around each block.

Lime mortar can be inserted into fine ashlar joints in three ways:

• using a suitably thin tool that easily fits into the joints, like a pointing key or steel blade
• pressing in the mortar from the face of the stone
• careful injection using a proprietary tool

Each method has its pros and cons, and a single job may combine two or more of these methods. However the lime mortar is applied, an adequate depth of mortar must be inserted into the joint – usually at least 30–40mm.

Ashlar walls were often built with slightly tapered blocks to help achieve very fine joints on the face. Vertical – and sometimes horizontal – joints may be much wider within the depth of the wall. Excessive voids may result in mortar disappearing endlessly into the wall: these should be filled with a suitable grouting lime mortar.

The setting mortar should be tamped back with the tip of a bristle brush to get rid of any shrinkage cracks. Once firmed up enough, the mortar surface can be finished by lightly scraping it with a small wooden spatula or similar tool.

Preventing damage during pointing

Protective tape should only be removed from the arrises (edges) of stones once the mortar is dry but not yet hard. Any mortar dislodged when removing the tape can then be easily pressed back into place.

Care should be taken to avoid spilling new mortar onto stone faces below the work area, as this can cause staining that is difficult to remove. Temporary protection should be put in place to contain any spillages. Any accidental spillages or smearing should be washed off with water.

Permanent staining is less likely if the:

• mortar is of the right consistency
• stone surfaces are dampened before pointing work begins