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Why Use Lime Mortar?

Traditional building construction in this local area is based on the use of relatively soft and porous materials such as stone, brick, timber and cob, together with a lime based mortar for bedding and plastering. These buildings usually have solid walls with no cavity, and are often built on insubstantial foundations. They are therefore liable to settlement and movement associated with seasonal changes in ground conditions. Lime mortar is softer and weaker than the stone or brick which it bonds and is therefore able to accommodate slight movements caused by settlement or temperature changes without significant cracking. Also, it is permeable and allows evaporation of rising and penetrating damp from within the wall. It is this permeability, or 'breathing', which helps to keep the building dry inside without a damp proof course or chemical treatments.

What Is Wrong With Cement Mortar?

Modern cement pointing is very different from lime mortar. It is hard and brittle, much less porous and sometimes completely water-proof. Its use on traditional masonry is damaging in several ways.

Cement pointing is harder than soft brick or stone and is too rigid to accommodate settlement or movement in the wall. When movement occurs, the edges of the stone or brick are forced against the hard mortar spoiling the masonry and cracking the mortar itself.

Further damage is caused by rainwater seeping into the cracks in the pointing and around the edges of the stones. Because the mortar is not permeable, this moisture cannot evaporate from the mortar joints once rain stops. Instead it is forced to evaporate through the face of the brick or stone and soluble salts present in the water crystalize in the surface layers of the masonry leading to crumbling and decay. This is something so severe that the entire face of the stone is lost and the hard cement pointing is left standing proud. Further rainwater is trapped and the decay continues. The concentration of trapped water in the masonry also increases its susceptibility to frost damage in winter.

In contrast, soft lime mortar allows moisture movement and, being more porous than the masonry, encourages evaporation and salt deposition in the mortar joints. Thus it is the mortar which decays and not the stone or brick. It is much easier and cheaper to repoint a wall than to repair or replace damaged brick or stone, and there is less loss of important historic fabric.