We all know that mortared joints are part of the appearance and character of a stone or brick built wall and they also help to keep the building dry.
The two main reasons why are by preventing rainwater finding its way into the inside part of the wall and by allowing moisture already in the wall to expand during drier conditions. In reality we need walls and pointing work to be able to breath during these changes. If a hard mortar is used then the wall is unable to breath and causes damage such as moving stone etc Lime pointing in Leeds is the best practise for historic stone built walling and old brick walls.
In summary we know that natural lime mortars allow old walls to breathe. The make up of natural limes allow moisture movement and moisture dissipation which enables old building to be restored to their former glorys. Their ability to absorb moisture and re release that moisture enables them to breathe. The problem we have with the wrong mortars that once the moisture gets in it can get out and over time it expands and needs to get out. One of the most common causes of damp walls is using a cement based product. Cement based mortars do have their uses but on old stone work this is not the place.