Masonry restoration

Repointing and rebuilding of natural stone constructions

Constructions of natural stone and lime-based bedding mortars have a long history. This construction type is a major construction tradition in our earliest buildings, fortresses and castles. Building techniques differ somewhat depending on date of construction, tradition and country. But the use of lime mortar is found as far back as several thousand years ago in these constructions.

The most abundant and useful mortar for stone works

Throughout time, natural stone has been a widely used construction material. Sometimes used with no adaptation of size and form, taken directly from nature (rubble stones), or adjusted blocks carved by a stone mason. To glue this together, bedding mortar made of lime was used - often locally produced. Still today we find thick walls with non-carbonised mortar inside, but also well-carbonised and recarbonised mortars with a high strength. This strength is comparable to standard, lime-cement mortars of today. Nowadays, we would have used concrete and a well-planned drainage and waterproofing scheme to build these structures. As with all mortars rich in lime, natural stone walls are exposed to degrading by water, frost and organic growth. This can result in bulging of walls and total collapse of the structure.

Tear down, rebuild or repoint

As with all conservation projects, a thorough investigation of the structure, reasoning and condition must be performed before starting the work itself. For walls with mortars being washed out and a destabilised, hollow construction, stabilisation with injection mortars can be considered. Repointing should follow the same guidelines as for brick masonry with scratching of the joints to a depth that is at least twice the width of the joints. Removing all loose and decayed material is important to ensure good adhesion. This should be followed by repointing with a strong NHL-based mortar in a minimum of two layers. The surface of the joint should be scratched to ensure water isn't trapped inside the joint. Another option is to tear down and rebuild the wall. Before tearing down, every big stone should be numbered and a picture of the wall should be taken. The wall is then torn down carefully, and rebuilt with a NHL-based mortar, aiming to place the original stone back into its original place, just like a puzzle. Some difference in colour between the old and new mortar is inevitable - but then it is also proof of the renovation work that has been done.