Stonework was used by the Romans with brick as a regular building material. Since then, it has been used to convey wealth, status and beauty. As a natural material, stone will be one of three basic forms – igneous (molten magma that has cooled and solidified – eg granite); metamorphic (rocks pressured or heated – eg marble or slate); or sedementary (layers deposited over time – limestones and sandstones).
When constructed, stonework is either dressed with narrow jointing and smooth finish (an ashlar stone); or laid in more random fashion with uneven course sizes. It is important with stone to ensure that it is bedded correctly. The layers of the stone should normally run horizontally as they would have done when it was formed. This makes it more resistant to the layers breaking off.
The porosity of stonework affects its ability to endure. Often stone will deteriorate through the effects of salts. these can be deposited on the surface of the stone or can crystallize in the stone’s pores which can lead to damage. Acid in the rain will also cause problems particularly in limestone. Frost attack will also damage surfaces where water ingress has been possible – surfaces exposed to consitent weathering are particularly prone.
Mistakes with stonework often involve applying a strong cement based pointing. Although it is strong, it is less permeable than, for example, more lime based mortars and it can force water to evaporate through the stone. This can accelerate damage.
It is important with stonework to ensure that diagnosis of issues is accurate and that repairs are appropriate. If properly maintained, however, it can be an enduring material that adorns some of our finest buildings.