When surveying old stone buildings, we often refer to the preference of using traditional lime renders and mortars.
The photographs above show what can happen to stonework where inappropriate materials are used.
This is an old stone boundary wall, but the principle is the same for the wall of a building.
The stone used to build this wall, like that used in many old houses and cottages, is a Killas type of stone. Killas is a sedimentary rock that has undergone metamorphism usually by extreme heat and pressure and very common in Cornwall.
In this example, the weathering back of the stone has left the pointing standing proud of the surface of the wall leading to a greater risk of water seeping into the wall where it sits on the raised cement mortar pointing.
This wall has in the past been repointed with sand and cement which is harder than the rock and less permeable. What has occurred over time is that as the wall has become wet as it has absorbed the moisture from, for example, driving rain which has soaked into the stone.
Where the pointing is impervious it has trapped the water, and if this freezes in the cold winter months, the ice formed expands and causes the face of the rock to weather and become crumbly as the water hasn’t been able to freely evaporate through the mortar joints.
In the past, it was not common to repoint or even render old buildings with sand and cement as there was little awareness of the problems being stored up.
This is why we always recommend if you are going to repoint or re-render an old stone building, you employ a tradesman with appropriate experience to carry out the work using traditional lime renders and mortars.
Whilst they may not be as hard wearing as sand and cement, lime mortar joints to pointing allow moisture in the walls to evaporate much more readily and avoid the situation we have found with the wall in the photos.
One of the great advantages of using lime mortar is that it reduces the risk of penetrating dampness that can be the cause of so many problems and issues in old stone property.