How to Repoint an Old Stone Wall: A Practical Guide to Choosing the Right Mortar

“What should I use to repoint an old stone wall?” is one of the most common questions when renovating old houses. It is also probably one of the questions that receives the most conflicting advice.

Cement and sand. Lime, cement and sand. Lime only. Quartz sand and tile adhesive. Then varnish. Or a water-repellent treatment. Most of these suggestions sound logical if we think of a joint simply as material that has to fill the space between the stones and become “strong”.

With a wall that is 100–200 years old, however, stronger does not necessarily mean better. The aim is to create a joint that is strong and durable, while at the same time remaining compatible with the old stone, brick and existing mortar.

Why are the joints so important?

Stone or brick and the mortar between them form a single porous system. Mechanical stresses are distributed through this system, liquid water and water vapour move through it, and when moisture is present, soluble salts may also be transported.

A joint is not simply a decorative line between stones. Its porosity, capillarity, vapour permeability and mechanical behaviour influence the way the entire wall performs.

How does moisture move through an old wall?

Old masonry is a porous system. Liquid water may enter through deteriorated joints, cracks, defective details, roofs and gutters, or directly through surfaces heavily exposed to driving rain.

Once water enters the wall, it is distributed through the interconnected pore structure. Drying is often much slower than wetting and depends on the pore structure of the materials, temperature, relative humidity and air movement.

If the water contains soluble salts, they may be transported with it. As the water evaporates, the salts may crystallise within the pores or close to the surface, generating stresses that can gradually damage mortar, stone or brick.

Mistake No. 1: “Use cement so it will be strong”

This is probably the most common piece of advice. Cement develops high strength, is readily available and gives the impression that “this will never move again”.

In old masonry, however, high strength is not an advantage in itself. If a hard and dense cement mortar is placed between relatively soft and porous stones or historic bricks, the new joint may begin to behave very differently from the original system.

The weaker original material may then be forced to absorb a greater share of the mechanical and moisture-related stresses. If evaporation through the new joint is restricted, the zone where evaporation and salt crystallisation occur may also shift into the adjacent stone or brick.

What about a lime-cement mix?

The presence of lime in a mix does not automatically make it suitable as a restoration mortar. The final behaviour depends on the quantity and type of binders, the aggregate, additives, water content and workmanship.

Therefore, even when a product comes from a reputable manufacturer, the right question is not “Is it a good-quality product?” but “Was it developed for this type of historic masonry, and are its properties compatible with it?”

Mistake No. 2: “If cement is the problem, just use lime and sand”

This is closer to the logic of traditional masonry, but it is still too general.

There are air limes, natural hydraulic limes of different classes, lime-pozzolan systems and ready-made restoration mortars. They are not interchangeable and may have significantly different mechanical and hygric properties.

“Use lime” is a direction, not a sufficient technical specification.

Mistake No. 3: “Add tile adhesive so it bonds better”

This advice appears extremely frequently on social media: “add a little tile adhesive”, “cement, quartz sand and adhesive” or “add adhesive so it doesn’t fall out”.

With old masonry, this can be a serious mistake. Tile adhesive is developed for a very different function and different service conditions. Adding it to a pointing mortar does not simply increase adhesion - it changes the properties of the entire mortar.

This can lead to several problems:

• Incompatible strength and deformability. The resulting mortar may become excessively rigid compared with the old lime mortar, historic brick or softer stone. During movement and loading, stresses may be transferred into the original material rather than being accommodated by the joint.
• Changes in moisture behaviour. Polymers and cementitious components may alter porosity, capillary water absorption and water-vapour transport. This is particularly important in old masonry because joints actively participate in moisture transport and evaporation.
• Additional risk where salts are present. Cement-containing materials may introduce additional soluble ions into the system. In the presence of moisture, these ions, together with salts already present in the masonry, may migrate and crystallise. The visible result may be efflorescence - the characteristic white salt deposits, but the more serious problem is crystallisation within the pores below the surface, where stresses may develop and contribute to powdering and loss of material.
• Unknown final composition. When tile adhesive is added to mortar arbitrarily, the actual properties of the resulting mix - strength, shrinkage, vapour permeability, water absorption and behaviour in the presence of salts - are no longer known.

Mistake No. 4: “If the wall is wet, it must be rising damp”

Not every damp wall is affected by rising damp. Water may come from a leaking roof, defective guttering, driving rain, incorrectly graded ground, damaged plumbing, condensation or a combination of several mechanisms.

This is critically important because an incorrect diagnosis leads to the wrong intervention. New pointing will not solve an active moisture problem if the source of wetting remains unresolved.

Why are salts so dangerous?

Soluble salts can migrate with water through porous masonry. When the water evaporates, the salts may crystallise within the confined pore structure.

Repeated cycles of dissolution and crystallisation, and in some salts transitions between different hydrated forms, may generate significant crystallisation pressures.

The result may be powdering of the mortar, scaling, loss of surface material from stone or brick, and characteristic white deposits.

The joint does not have to last forever

In historic masonry, it may sometimes be preferable to replace the mortar locally after many years rather than lose original stone or brick.

A compatible mortar may also perform a sacrificial function - absorbing part of the weathering and moisture-salt stresses instead of allowing them to become concentrated in the historic masonry units.

So, what SHOULD I do?

If you want a durable, technically sound and visually appropriate result, follow this sequence:

What if the wall was built with earth or clay mortar?

This is common in old rural houses and traditional masonry. The presence of earth- or clay-based bedding mortar does not automatically mean that it should be removed and replaced deep within the wall with new mortar.

It is important to distinguish between surface repointing and intervention within the core of the wall. If the original earth mortar remains stable, the aim should be to preserve as much of the historic material as possible and repair only the deteriorated areas.

Clay and earth mortars may be highly sensitive to wetting. The selection of a new material and application method must therefore take into account not only mechanical compatibility but also the amount of water introduced into the masonry.

If there are substantial voids or missing bedding material behind the deteriorated joints, surface repointing alone may not be sufficient.

What should we do if there are large internal voids?

If missing mortar extends significantly into the wall, the problem is no longer simply one of repointing but of restoring cohesion within the masonry core.

Before injection, open joints and cracks generally need to be closed with a compatible mortar so that the injection material remains within the voids. For surface repointing, a restoration mortar such as ANCIENT JOINTING MORTAR/N may be considered when its properties are suitable for the specific masonry.

For filling internal voids in traditional and historic masonry, MICRO-INJECTION MORTAR BM 10 may be used. It is a micro-injection grout based on natural hydraulic lime NHL 5, intended for consolidation of masonry, vaults, arches and areas where internal mortar has deteriorated.

With walls built using earth or clay mortar, however, the sensitivity of the original material to water must be assessed beforehand. BM 10 is mixed with water and should therefore not automatically be assumed suitable for every earth-built masonry system.

For highly water-sensitive substrates, specialised mineral systems with reduced water content may be considered, such as CaLoXiL^® Lime Injection Grout Extra Fine, developed for water-sensitive substrates.

Injection and repointing are different operations. Before injection, the cause and extent of the voids, the condition of the original bedding material and the possible need for structural intervention must be assessed.

Ready-made mortar or your own mix?

Both approaches are possible, but they require different levels of control.

With a ready-made restoration mortar, composition, grading and key technical properties are predefined by the manufacturer. This makes it possible to evaluate its characteristics against the requirements of the specific masonry and reduces variation between batches.

The alternative is to prepare the mortar on site using a suitable binder, aggregate and, where necessary, specialised additives.

For restoration mortars, for example, S.T. Calce NHL 5 may be used - a natural hydraulic lime for preparing mortars and renders for the repair and restoration of historic masonry.

But selecting the lime is only the beginning. The type and grading of the aggregate, binder-to-sand ratio, water content and mixing method all determine the final behaviour of the mortar.

For certain cement-based and cement-lime formulations, a specialised plasticiser such as QMix DH may also be used. The manufacturer specifies it for masonry and plastering mortars, including pointing and repointing. It improves workability and introduces controlled air entrainment into the mix, potentially reducing mixing water, bleeding and drying shrinkage.

However, adding a plasticiser does not automatically transform any formulation into a suitable restoration mortar. In old masonry, the entire mix must be evaluated against the original mortar, stone or brick, moisture conditions, salts and exposure.

Ready-made restoration pointing mortar: ANCIENT JOINTING MORTAR/N

One advantage of a technically formulated ready-made mortar is that its properties are known and controlled. This allows a technical assessment against the requirements of the particular masonry instead of relying on a mix prepared “by eye”.

ANCIENT JOINTING MORTAR/N is a ready-made pointing mortar based on natural hydraulic lime NHL 5 and selected aggregates, specifically formulated for pointing brick, stone and tuff masonry and for the restoration of old buildings. The product contains no resin.

How is ANCIENT JOINTING MORTAR/N applied?

1. Remove loose material, dust, gypsum, oils and other contaminants that may interfere with adhesion.
2. Properly dampen the substrate before application.
3. Mix the mortar with approximately 20% clean water.
4. Mix for approximately 3 minutes using a drill and paddle mixer or approximately 5 minutes in a concrete mixer until a homogeneous, lump-free mix is obtained.
5. Apply the mortar into the joints and compact it well against the substrate.
6. At the end of the plastic phase, finish the joint using a suitable tool.
7. According to the technical instructions, after approximately 2 hours brush the surface with a suitable brush to expose the aggregate texture.
8. Once dry, remove residual material with a suitable brush.
9. Protect freshly pointed masonry from rain for the first 48 hours.
10. Do not apply below +5°C or above +30°C, on frozen substrates, or where freezing is expected within the following 24 hours.

What should the finished joint look like?

There is no single universal joint profile that is correct for all historic masonry. The original geometry of the wall, the character of the stone, the width of the joints and the historic workmanship should all be considered.

A common mistake is to transform an old wall into a decorative “rustic” surface by deeply recessing the joints so that every stone stands out as much as possible. This does not always correspond to the historic appearance of the masonry and, on external walls, may create unwanted areas where water can collect.

The aim is not to make the wall look “older”, but to ensure that the intervention is technically correct and visually compatible with the original masonry.

Should the stone be varnished or treated with a water repellent after repointing?

Varnishing should not be regarded as a standard final treatment for old mineral masonry. Film-forming coatings can significantly alter the water and vapour-diffusion behaviour of the surface.

Hydrophobic impregnation is a different technology. It is not automatically required after every repointing intervention. The need for such treatment depends on the type and porosity of the stone, exposure to rain, the condition of the joints, the presence of salts and active sources of moisture.

The choice of impregnation should be based on the specific material and exposure conditions, not simply on the desire to “stop the wall absorbing water”.

The most common mistakes - in brief

Conclusion: make the joint strong, but not at the expense of the historic wall

A correctly executed joint should be sufficiently strong, properly compacted and durable under the conditions in which it will perform. But this does not mean making it as hard, as dense or “stronger than the stone” as possible.

In old masonry, we are looking for balance: mechanical compatibility, controlled moisture movement, good bond with the substrate, correct preparation and proper curing of the mortar.

This information is of a general technical nature. For buildings of cultural heritage value, structural cracks, severely deteriorated masonry, active moisture or significant salt contamination, the specific solution should be adapted to the conditions of the site and assessed by an appropriately qualified specialist.