Stone has been used as a building material for thousands of years. It has long been recognised as a material of great durability and superior artistic quality, the foremost choice for buildings associated with status, power and religion. The pyramids in Giza, burial chambers in the UK and temples in Malta were all built from stone over 4000 years ago and are still standing. The use of stone in construction has declined over the last hundred years, but it remains an aristocrat of building materials.
Types of Building StoneBuilding stone, also called dimension stone, derives from one of three naturally occurring rock types:
Igneous - Hard and non-porous rock formed from the slow or quick cooling of molten magma. The best example is granite.
Sedimentary - Soft and fairly porous rock formed from deposits of eroded pre-existing rock that settled in layers mostly on sea beds, and became compacted. The best examples are sandstone and limestone.
Metamorphic - Hard and non-porous rock formed from pre-existing rock that has been altered by intense heat or pressure. The best examples are marble and slate.
There are huge variations within each of these rock types, caused by specific mineralogy and geology conditions, and while any stone can be used for building, they each have constraints that make them more or less suitable for different purposes. Granite, sandstone and limestone can all be used for building walls, but slate is only suitable for roofs and floors. Some types of granite can contain mineral salts that cause spalling, where the outer face of stone falls off; slate can contain harmful minerals that break down on exposure to the atmosphere causing stone damage; and sandstone can be too porous and fragile for load-bearing structures. An understanding of how the rock material was formed will reveal how it can be used in a building, what its limitations are, and how it will weather over time.
Dry Stone StackingThe earliest form of stone construction is known as dry stone, or dry stacking. These are freestanding structures such as field walls, bridges and buildings that use irregularly shaped stones carefully selected and placed so that they fit closely together without slipping. Structures are typically wider at the base and taper in as height increases. The weight of the stone pushes inwards to support the structure, and any settling or disturbance makes the structure lock together and become even stronger. Dry stone structures are highly durable and easily repaired. They allow water to drain through them, without causing damage to the stones. They do not require any special tools, only the skill of the craftsman in choosing and placing the stones.
Stone MasonryTraditional stone masonry evolved from dry stone stacking. Stone blocks are laid in rows of even (courses) or uneven (uncoursed) height, and fixed in place with mortar, a cement or lime mixture pasted between the stones. The building stones are normally extracted by surface quarrying, drilled and split using diamond saws or iron wedges, and then shaped and polished according to their requirements. The basic hand tools used to shape stones are chisels, mallet and a metal straight edge, but modern power tools such as angle grinders and compressed air-chisels are often used to save time and money. Stones are either shaped (dressed) into a block, known as ashlar masonry, or left rough and cut irregularly, known as rubble masonry. Mortared stone structures are less durable than dry stone, because water can get trapped between the stones and push them apart.
Traditional stone masonry is rarely used today, because stone is expensive to quarry, cut and transport, and the building process is labour and skill-intensive. Instead, most modern stonework utilises a veneer of stone (thin, flat pieces) glued against a wall of concrete blocks. This is known as veneered stone or stone cladding.
Slipform stone structures are a cross between veneered masonry and traditional masonry. Short forms (around 2 feet tall) are placed on either side of the wall, to serve as a guide for the structure. Stones are placed inside the forms with the flat face out, and concrete is then poured behind the rocks to hold it together. Stone buildings can be constructed quickly and easily with this method.