Most pre-nineteenth-century buildings used lime mortars and plaster. Materials used to repair or replace original masonry should have similar
properties so as not to disrupt the balance of interaction within the building. Cement pointing is particularly detrimental if applied to soft stone or bricks. It is hard, nonresilient
and comparatively non-absorbent. Nor does it respond to the variations in the atmosphere to the same extent as the surrounding stone or brickwork. Hard pointing
can cause rapid weathering of the softer stone or brick. Many causes of stone decay have been traced to the use of impervious mortar with a
porous stone. In such cases saturation and evaporation are confined to the stone whereas the process should be distributed evenly over
stone and pointing. Where a particularly soft stone is employed then the mortar should sacrifice itself for the stone.
Is The Work Necessary?
If the joints are only weathered back to a rough finish or if the mortar appears soft compared to modern mortars then repointing may be unnecessary.
If the joints do not allow rain to penetrate or to rest in them it may be best to leave them alone. However, if they have weathered back by more than the joint thickness,
or are partly or wholly open, then they probably require repointing. It is not necessary to point every open joint in a wall. The mortar, although weathered back, may still be sound. Unless recessed more than 12mm there should be no need to fill the joints.
