Why Traditional Lime Mortar?
Helitec Site Supervisor and heritage Stonemason, Mark Downton discusses the advantages of using traditional and compatible mortars in heritage restoration work.
Damaged to stone can often result when incompatible mortars are used
The use of traditional lime mortars has been with us for quite some time, I’m not talking a century or two, I’m talking thousands of years!
A material that has been used for such a lengthy period of time has to have major significance, right? Of course it does, our ancestors didn’t get it wrong. They knew that building with materials such as Stone and Bricks required a sacrificial mortar which is soft, breathable, flexible and porous.
There are various important reasons for this and micro movement is one of them. This can occur in any building, particularly older buildings where footing depths may not be as deep or foundation settlement, moisture level changes and other changing conditions may have had an impact on the building over time. Being soft and flexible lime mortar has an excellent ability to cope with micro movement, so if a building wants to move, lime mortar will usually let it.
If the mortar is less flexible and harder, as is the case with most cement based mortars, when the building wants to move, cracks will usually form, taking the weakest path which is often through the masonry stone or brick unit itself.
When micro movements occur in a building built or repaired with lime mortar, the movement tends to occur through the mortar joints. Lime mortars generally also have an amazing ability to chemically react and repair themselves through a process called ‘self healing’. Cement based mortars do not have the same ‘self healing’ ability and the cracks tend to stay open and expose the masonry to further exposure and deterioration.
Another important reason for the use of lime mortar is its breathability. This is vital in heritage buildings because rain and moisture will penetrate the porous stone or soft brick then the lime mortar allows the moisture to evaporate through the soft permeable mortar joints.
If the mortar joints are filled with cement based mortars, then the breathability can be reduced and the moisture tends to evaporate through the easiest path i.e. through the porous stone or brick. This process can cause the surrounding masonry to deteriorate and spall.
Compared with many modern buildings, Heritage buildings often have a fantastic life span if they are maintained and kept in their original state. If non-compatible modern materials are introduced, these can often have dire consequences.
The use of correct restoration materials and techniques is critical in the preservation of Heritage buildings.
Although sometimes alternative materials and can appear to make work easier or offer savings in time and labour for building contractors and tradesmen, a traditional or heritage trained craftsman will recognize, appreciate and understand the use of correct and compatible lime mortar materials for the preservation of, not only the aesthetic appearance and uniformity of colour and texture, but also the structural performance and life of heritage buildings.