Barry Hunt authoritatively explores the nature of the stones used for building.
Barry Hunt continues his series of articles exploring the main categories of natural stone used in construction by turning his attention to sandstone, possibly the stone most easily understood by the layperson – simply sand that has been cemented together. But there is so much more it than there appears at first glance.
Sandstone is one of the most widely used stone materials, being both common in many geological sequences and easy to use for construction purposes. In construction, the main advantage that sandstone has over most other stones is the ease with which it can be split along its sedimentary planes using basic hand tools – and the naturally cleft face is highly desired.
Sandstone varies in the nature of the constituent grains, which can be different in size, composition, shape, roughness and degree of dissolution.
The grains might be laid down in variable bands and the quality and type of natural cement is changeable. All of which affects how any given sandstone performs as a dimension stone.
Sandstone is not always the most versatile of natural stones, due principally to the constraints created by the original bedding process. Where sandstone is derived from marine deposition, the beds tend to be relatively thin and these laminations can create weaknesses, although it is these very weaknesses that can be exploited when splitting the stone to use as paving and flooring, and sometimes as roofing.
The quarrying of marine-derived sandstones is often a slower process than with other dimension stones because of interbedded shales and other waste or unusable materials that reduce recovery rates.
The position of splitting planes can dictate the thickness of usable slabs, which then might need to have the lower surface cut or ground to achieve a calibrated thickness, creating even more waste. The finished sandstone product can represent as little as 10% of the sandstone quarried in some locations.
More consistent properties are often achieved with sandstones formed of former desert sands, where bed heights can reach tens of metres and occur across large regions.
The sorting of the sand grains also makes many desert-derived sandstones more consistent in their appearance and properties. Iron oxide is often a major component of the matrix, giving rise to a red colour, and while these stones are typically strong, they are not always sufficiently abrasion resistant to be used for paving.
The American Society for Testing & Materials (ASTM) simplifies sandstone classification on the basis of density with three categories: sandstone, quartzitic sandstone and quartzite. Quartzite is not considered further here.
The classification works reasonably well in that those stones used for pavings and carved details tend to fall into the quartzitic sandstone category and those for block masonry fall mostly into the sandstone category. Few stones that would not be regarded as sandstone by geologists manage to fall into the ASTM classification.
Sandstone probably has the edge over limestone for traditional block stone masonry use, as even the lower quality sandstones do not experience the level of damage in-service that limestones might suffer (although much of the damage experienced in the past by limestones may be related to acidic rain, which is no longer so prevalent).
Possibly the main difference between limestone and sandstone is the lack of a microporous network in sandstones, which means they tend not to hold on to water as long as limestones and thus suffer less from frost action.
Additionally, the total porosity of sandstones tends to be lower. This means less water can be held by the stone and there is less potential for the build up of internal pressure created by water freezing or salts crystallising and resulting in bursting or other damage.
The main disadvantage exhibited by sandstone compared with most other stones is the presence of different types of matrix that can affect the various properties. A small amount of carbonate in the matrix, for example, can dramatically decrease resistance to acidic effects. Many sandstones contain small amounts of clay minerals, which are often inert but can sometimes swell, leading to damage under persistent wetting and drying cycles. In some instances clay-rich inclusions or zones weather rapidly.
Sandstone in use
One of the best examples of the way in which sandstone may be worked and prove durable is Petra, ‘the rose-red city half as old as time’ in the Middle East. We might not know much about the Nabataeans who inhabited it if it were not for the extensive sandstone tombs that remain.
The Anasazi Indian sites in the South-Western United States, where once great cliff-hugging cities flourished, provide another insight into past lives and civilizations. The fact that both sites remain so well preserved is also related to their dry climates and sheltered construction.
We should not forget that Stonehenge, Avebury and many other similar ancient megalithic archaeological sites in the UK were built of Sarsen, a form of sandstone that is an exceptionally hard Tertiary orthoquartzite. There were once outcrops of it over much of southern England.
The ancient megaliths survive both because the Sarsen is strong and durable and because it does not make great building stone, so it was not taken away and re-used in later buildings, which was the fate of many ancient structures.
The Rev Dr William Stukeley (1687–1765), an English antiquarian who pioneered the archaeological investigation of Stonehenge and was one of the founders of field archaeology, noted that Sarsen is ‘moist and dewy in winter, which proves damp and unwholesome and rots the furniture’.
Throughout Northern England and Scotland the wide availability of quality sandstone for traditional masonry has stamped its mark on the character of cities, towns and villages.
There are few metropolitan buildings constructed since the start of the industrial revolution that do not use the local sandstone resources.
Many of these buildings were large and rather austere, reflecting the burgeoning power and wealth of the new empire.
Despite the British weather, these buildings typically remain in good condition, which is testament to the durability of the stones.
The quality of many British sandstones for traditional masonry use is affirmed by the survival of cathedrals, churches and castles, some parts of which can be 1,000 years old.
Of all the places in the UK, Edinburgh could be considered the finest example of building with sandstone.
It is not just the stone used for the grand Regency style architecture of the city, but also the dramatic backdrop of Arthur’s Seat and the Salisbury Crags that complement the extensive use of the local, rather austere Craigleith sandstone.
This sandstone is sometimes locally referred to as ‘liver rock’, which is a term equivalent to ‘freestone’, or a rock that can be worked in any direction.
The original Craigleith quarry face used to be 110m deep, although now only the top few metres are exposed as a Regionally Important Geological Site (RIGS). Rather sadly, the Sainsbury store now inhabiting the quarry has Stainton sandstone from County Durham on its façade.
Possibly the UK’s most famous sandstone is ‘York Stone’, which has an international reputation – even though there is no such thing as ‘York Stone’. It is simply a loose term that was originally applied to some sandstones from West Riding (now South Yorkshire), particularly at Elland Edge.
Today, the term ‘York Stone’ is used to describe a wide range of sandstones, most of them of Carboniferous age (280-345million years ago) from Yorkshire and some of the surrounding counties. Some have even regarded the term as a generic description of paving and have applied it to imports from as far afield as China and India.
As a consequence, there is a degree of variation in what constitutes a ‘York Stone’ on the basis of colour and colour variation, texture, grain size, porosity and strength.
The original York Stones were derived from Coal Measures sequence rocks, although the majority of UK material generally classified as York Stone now comes from the Millstone Grit sequence.
In many UK locations, York Stone works tirelessly, barely noticed, beneath our feet. It used to be said that the streets of London were paved with gold, which was (and remains) true in a way because York Stone is the mostly golden coloured sandstone widely used for external paving.
It is a prime example of a stone that does not perform particularly well relative to other stones in many ways, yet excels in this one particular area.
The reason is that York Stone, like so many sandstones, provides excellent slip resistance because of its granular structure. The grains are knocked out in use so the surface never polishes and a slip resistant surface is continually refreshed.
This is also the reason why the sandstones of the Pennines proved their worth for millstones, hence the geological sequences from which they come is known as millstone grit.
Sandstone is widely available across the globe, but one of the most frequently used resources in recent years is the Kota sandstone from India.
Kota is strong and consistent in a range of green, grey, buff and pink hues with variations that make it an aesthetically attractive material. It is probably the most common paving stone found in DIY outlets and garden centres, not least because it is relatively inexpensive.
There are many quarries in India working the stone, but some supplies can be intermittent due to flooding during the rainy season. The exposed stones in some quarries can soak up flood waters rich in iron and organic matter that can be released as staining after the stone has been laid.
There are currently about 130 quarries in the British Isles producing masonry sandstone commercially for a wide range of uses. Of these, 36 are in Yorkshire.
When viewing a selection of British sandstones, or indeed sandstones from around the world, one thing that is noticeable is the great variety in colour and texture available.
There are reds and pinks of the New Red Sandstone from Locharbriggs, Corsehill and St Bees, Blue Pennant from Wales, greys and browns of other Carboniferous stones such as Talkin Fell and Bolton Woods, and greens of the Cretaceous Sussex sandstone.
For many of these stones there are banded colour variations reflecting bedding differences, while the crystal grains seem to respond to light variation more than other stone types, with increased warmth and deeper hues during sunrise and sunset that can take the breath away.
Fitness for purpose
Unlike many of the building stones available, it is difficult to think of an environment where sandstone would look out of place. This is possibly because sandstones form so much of our own landscape and, just maybe, because of our love of sandy beaches.
One of the major considerations when using sandstone for any form of masonry is the bedding. In most cases it is essential the stone is laid with the bedding plane oriented horizontally (on bed, as masons describe it).
Sandstone should not be face-bedded as whole surfaces may delaminate, and if it is laid vertically on edge it can suffer rapid and uneven denudation. It is this property of sandstones that means some are only used either for paving or relatively short masonry units.
Many sandstone buildings use blocks rarely greater than 400mm in height. Where sandstone is cut with the bedding inverted to obtain longer units (such as columns) splitting seems to be inevitable.
Most sandstones do not suffer strength loss with time. Instead they tend to be denuded gradually. That is why they work so well in block form as there is usually plenty of excess material that can be lost without affecting performance.
By using the stones with the bed, even delamination should not cause a problem as the stone remains in compression.
Great care must be taken when using sandstone for thin stone cladding externally, as the mostly vertical orientation of the slabs will mean that any sedimentary structures (and thus weaknesses) will run across the panel thickness. Only true freestones that exhibit similar flexural strength in all directions can be used in this way. This limits the choice, unless strength deficiencies are countered by increasing the panel thickness or reinforcing the stone.
Of course, at some point thicker panels stop being thin cladding. As a general rule of thumb it is not advisable to use sandstone for external cladding at less than 50mm thick.
Sandstones, along with granites, are preferred for stone paved road surfaces because of their slip resistance. However, they do not have the high strength and abrasion resistance of granites, which is reflected in the lower values sandstones have to achieve to be acceptable for use in the British Standard for trafficked pavements.
Only quartzitic sandstones could hope to achieve the requirements for Class 2 materials for heavily trafficked roads, and only the best sandstones could achieve the lower requirements for Class 1 materials.
The nature of sandstone provides a good surface on to which jointing materials can be keyed strongly, but this does depend on how well the surfaces to be jointed are cleaned. The same is also true for setts, where the joints are potentially the most critical aspect of the construction.
Sandstones are rarely affected by discoloration as they generally comprise the more stable residual components from the weathering of other rocks. Also, the environmental formation conditions seldom incorporate potentially problematic materials such as organic matter and sulphides.
On the other hand, sandstones are more susceptible to staining because of their porosity. Oil can be rapidly absorbed and is nearly impossible to remove without resorting to industrial cleaning procedures. You might be inclined to think some form of protection should be provided, but this can present problems externally as the potential for damage could be increased if the passage of moisture from below (in the case of paving) is retarded.
Sandstones do not generally polish – only those classified as true quartzites are sufficiently dense to take a polish and even then the granular crystalline structure often does not lend itself to polishing. The more natural, unpolished finish of sandstone remains visually unaffected by metal heels and other sharp impacts that can leave scratches and star marking on a polished surface.
Because the maintenance of slip resistance on York Stone paving depends on the persistent gradual surface failure, thick units of stone are traditionally used so that the loss of material does not compromise the breaking load requirements.
Thicker paving stones also mean that if the wearing surface becomes too irregular, the stones can be lifted and turned over to expose a ‘new’ surface.
Barry Hunt is a chartered geologist, a surveyor and scientist. He has been awarded the designation of European Geologist and is a Corporate Building, Conservation and Specialist Surveyor. He is also a Member of the Chartered Institute of Building and a Fellow of the Royal Microscopical Society.
Barry has served on a number of professional committees, including the Technical Committee of Stone Federation Great Britain (SFGB), which provides advice on all stone construction issues. He is one of the authors of the SFGB team that has published codes of practice for the installation of stone floors and internal stone finishes.
Barry gained 14 years’ experience working as a consultant for two renowned civil engineering materials consultancies before, in 2001, establishing his own consultancy, IBIS, specialising in the investigation of construction materials.
The specialist knowledge and services provided by Barry have allowed him to be instrumental in the resolution of problems ranging from blast damaged claddings in London’s West End to advice on the quarrying and extraction of stone from abroad for import to the UK. Other areas of experience include the investigation of all types of building finishes, specialist advice on remedial treatments and the preparation of advice for potential and actual use in litigation or arbitration.
Having worked for consultancies that both undertook in-house laboratory investigation to UKAS requirements, Barry is also able to conduct or oversee a wide range of on-site and laboratory techniques and ensure they are carried out to traceable standards.
One speciality in all investigations is Barry’s hands-on approach. Being trained in industrial roped access (abseiling) allows him to get close to the problems with external building envelopes quickly, efficiently and cost-effectively.
Throughout his working life Barry has published findings from the many investigations he has undertaken. He has also authored chapters for two books on building stone and is currently engaged in other book projects in this field. His most important contribution is considered to be the chapter on the repair and maintenance of stone in the landmark Geological Society publication Stone.
Barry has also written a regular column covering the full spectrum of natural stone use for Natural Stone Specialist magazine.