Why the wrong mortar can damage a historic building
Repointing might seem like one of the simplest maintenance jobs you can carry out on an older property.
The existing mortar is deteriorating, so it is removed and replaced with new mortar. Simple enough.
However, when dealing with historic buildings, the type of mortar used can be extremely important.
One of the issues I regularly encounter when surveying older properties is traditional brick or stonework that has previously been repointed using hard cement mortar. Although the pointing itself may appear strong and durable, it can actually contribute to deterioration of the surrounding masonry.
What is pointing?
Pointing is the exposed mortar visible between individual bricks or stones.
Its purpose isn't simply cosmetic. Mortar helps distribute loads, accommodates small amounts of movement and plays an important role in how moisture behaves within a traditionally constructed wall.
Over time, mortar naturally weathers and deteriorates.
In many historic buildings, this was intentional. The mortar was effectively the sacrificial element of the wall, designed to deteriorate before the surrounding masonry.
Why was lime mortar traditionally used?
Before modern cement became commonplace, buildings were generally constructed using lime-based mortars.
Lime mortar is typically softer and more flexible than modern cement mortar.
This allows traditional walls to accommodate small amounts of movement without placing excessive stress on the bricks or stone.
Lime mortar is also vapour permeable, helping moisture within the wall evaporate through the mortar joints.
Both characteristics are particularly important in solid-walled historic buildings.
What happens when cement mortar is introduced?
Modern cement mortar is generally harder and less permeable than traditional lime mortar.
When used to repoint softer historic masonry, the relationship between the mortar and the surrounding brick or stone changes.
Rather than the mortar joint being the softer, sacrificial part of the wall, the masonry itself may become the weaker element.
Moisture can also be encouraged to evaporate through the face of the brick or stone rather than through the mortar joint.
Over repeated wetting, drying and freezing cycles, this can contribute to deterioration of the masonry.
What does the damage look like?
One of the most recognisable signs is spalling, where the outer face of a brick begins to break away.
Other signs can include:
Cracking around mortar joints
Recession or erosion of masonry
Dampness within walls
Localised salt deposits
Sections of hard mortar separating from surrounding masonry
The pointing itself can sometimes remain in excellent condition while the bricks around it gradually deteriorate.
This is why very hard pointing isn't necessarily a sign of a good repair.
Does all cement pointing need removing?
Not necessarily.
Removing hard cement mortar can itself damage historic masonry if it is undertaken aggressively or unnecessarily.
The condition of the wall needs to be considered as a whole.
If inappropriate pointing is clearly contributing to deterioration, carefully removing it and repointing using a suitable mortar may be appropriate.
However, the specification should be based on the particular building and masonry rather than simply assuming that one type of lime mortar is suitable for every historic property.
Repointing is about more than choosing lime
Good repointing also depends heavily on workmanship.
Mortar joints need to be carefully prepared, the replacement mortar appropriately specified and the finished joint detailed to suit the masonry.
Poorly executed lime pointing can still perform badly.
For listed buildings, repairs may also require additional consideration, particularly where they affect significant historic fabric or alter the appearance of the building.
Why this matters when buying a period property
Previous repointing can provide useful clues about how an older property has been maintained.
During a building survey, I consider not only whether the mortar is deteriorating but also whether previous repairs appear compatible with the original construction.
Extensive inappropriate pointing doesn't automatically mean a property has a serious problem.
It does, however, help identify areas where maintenance may be required and where deterioration should be monitored.
Final thoughts
Mortar may only occupy a relatively small proportion of a wall, but it plays an important role in the performance of historic masonry.
Using a mortar that is harder or less permeable than the surrounding brick or stone can gradually contribute to deterioration rather than protecting the building.
When maintaining a historic property, the strongest or most modern material isn't necessarily the best one.
Understanding the original construction and choosing compatible repair materials is usually the better approach, helping preserve both the appearance and long-term performance of the building.