Using the correct mortar is crucial for durability
To ensure appropriate levels of strength, permeability and longevity it is necessary that the mortar used for repointing walls is similar in composition to that of the original mortar. A golden rule of repointing is never to allow the strength of the mortar to be greater than that of the bricks.
Buildings constructed before 1920 are unlikely to have mortar with cement content and therefore, to avoid serious problems, should preferably be repointed with a lime mortar rather than a cement mortar.
More modern buildings are more likely to have been built with cement based mortar.
- Lime mortars flex and breathe, they absorb moisture but also allow it to evaporate.
- Cement mortars are rigid, strong and waterproof
When repointing cement mortar joints, there is always a case for making the mortar relatively flexible, by adding lime to the cement mix, in order that it can accommodate natural building movements and allow moisture a degree of permeability. The joint should be cleaned of dust and moistened before the replacement mortar is pushed into the joint. The new mortar is finished by running with a trowel or jointer to deliver a flush, concave or weather-struck surface.