We attach a frightening video of the damage which cement pointing causes to the sandstone of a building.
Cement mortar pointing does not allow stone to “breathe” so it stays damp as moisture can only escape through the stone rather than through the pointing and this damages the stone. http://www.underoneroof.scot/articles/986
To the untrained eye, the sandstone of this traditional west-end tenement appeared sound. Quick tap (see the video) and the sandstone de-laminates, that is sections shear off and fall. This is caused by our old enemy – cement pointing.
Lime mortars play an important role in traditional buildings, helping them to ‘breathe’ by dispersing the moisture absorbed during periods of rainfall. Ensuring that ashlar repointing is carried out using appropriate materials and techniques will reduce the chances of moisture penetrating the masonry, and the associated negative impacts such as masonry deterioration, damp interiors and rotting timbers, that can otherwise arise.