There are stone structures that have stood for centuries… even millennia. But with new uses being found for stone and new stones from around the world being used it makes sense to let science play its part in decisions, says geologist and surveyor Barry Hunt.

From time immemorial natural stone has been used successfully to build the world’s most spectacular architecture and create its most impressive interiors, as well as much more prosaic structures and environments.

For much of history its use has been a matter of taking local resources that may or may not have been properly suited to the tasks in hand – although they often fulfilled the roles they were given for centuries, sometimes millennia.

Then industrialisation brought increased mobility from waterways, trains and, eventually, roads. Those natural stones with the best all round performance were identified and transported far and wide to be used in many different ways in a variety of environments.

The natural stone industry blossomed during the 18th and 19th centuries but was struck down during the 20th century by a change in design aesthetic and competition from brick, concrete, glass, steel and other manufactured products that seemed to offer simpler, more predictable and less expensive construction methods – utilitarian materials of the brave new world emerging from the great human conflicts of the era.

The hiatus in the natural stone industry left it rooted in out-dated practices, too small to attract the investment needed to embrace the new modernism. For a time, it appeared that natural stone had had its day, with only a handful of quarries still working in the UK producing material to maintain the nation’s great monuments, stately homes and governmental and municipal icons of power.

Then the 1980s sowed the seeds of a resurgence in the natural stone industry as people rediscovered an appreciation of natural materials and the quality finishes stone offered. At the same time, scorn was being heaped on the crumbling concrete edifices that seemingly blighted our landscapes at every turn.

New technologies were developed and old ones were updated to allow natural stone to begin to re-establish itself as the most desirable finish to our homes, offices and public spaces.

It was at this point that I started my career in natural stone investigation. Old problems were being re-discovered and new ones were appearing as new uses of stone were developed.

It has taken me almost 30 years to realise that natural stone is an innocent party in construction. Use of stone can be ruined by inappropriate selection, design, installation, maintenance and all manner of other processes because of our patent lack of understanding of the materials.

The great majority of issues arising with the use of natural stone in construction involve water. We must consider how water will interact with the stone.

Stone is porous to a greater or lesser degree, depending on the stone, and design considerations must include the shedding of water from stone surfaces. Water is the universal solvent. Left unchecked it can lead to decay, either directly through dissolution and frost attack or indirectly through the transportation of salts and other media.

Externally, we need to provide breaks to take the brunt of the weather and shade façades from the onslaught of the atmosphere. There are many examples showing how stone protected below copings, flashings, string courses and other projections can remain in pristine condition for hundreds of years.

Plinths need to prevent moisture rising from the ground, which is where slate and igneous rocks (often granite) come into their own. Their low permeability helps give them high resistance to weathering.

With paving and external floors we need to design effective run-off and to prevent relentless moisture migration up through the stone from the ground below. This can be achieved by creating free-draining bedding or incorporating moisture barriers.

Internally, moisture is often given insufficient attention and the time constraints imposed by construction programmes resulting in stone being laid on top of wet screeds has probably caused more problems, particularly staining and discolouration, than any other single cause.

More recently, the growing demand for wet rooms has created new challenges in water management, while swimming pools present potentially the greatest challenge as a result of the cocktails of chemicals added to the water. Stone Federation Great Britain has published guides on these uses of stone that can help you avoid repeating mistakes that have been made in the past.

Challenging the grey matter

Most stone use involves some form of binder or adhesive, commonly a form of cement. Until the 19th century this was typically lime (calcium oxide) that was hydrated (or slaked, as it is called) for use. Slaked lime interacts with the atmosphere and gradually converts to stable calcium carbonate. In antiquity, the ancients added volcanic ash, ground up ceramics and other materials (known as pozzolans) that were found to react with the slaked lime and create calcium silicates that resulted in stronger mortars.

Developments in the UK in the 18th and 19th centuries resulted in the formulation of Portland cement, which sets more rapidly than lime mortars and achieves greater strength. With the development of concrete and improved brick masonry, Portland cement almost caused the destruction of the natural stone industry. It certainly led to the decline of the lime industry and, unfortunately, became the cementing material of choice when the stone industry began to recover. I say ‘unfortunately’ because it can discolour and accelerate the deterioration of stone, as we shall see.

In the 1920s the Building Research Station (now BRE) ran experiments with Portland cements and their interaction with natural stone and found that many sedimentary stones, notably limestones, frequently discolour in the presence of Portland cement.

This is due to natural organic materials and minerals such as pyrite being leached or attacked by the alkalis in the cement. The lesson was that if moisture could not be avoided, it should not be alkali laden. It is a lesson the construction industry has apparently found difficult to learn. The dark colour of the cement could also be a problem as it could be seen through relatively thin, lighter coloured and more crystalline stones, darkening the look of them.

Another problem is that some proprietary adhesives contain high alumina cement that reacts with Portland cements, compromising the adhesive.

Finally, many Portland cements can be stronger than the stones they are jointing, leading to enhanced stone decay because any movement stresses the stone rather than being accommodated by the joints between the stones, as it is with a weaker lime mortar.

Using lime mortar or white Portland cement can avoid such issues.

The mechanics of fixings

A large column capital was installed on a major London building in around 1800. It had been intricately carved in Portland stone, the building material of choice for the City of London ever since the reconstruction began after the Great Fire of 1666. In the column, a vent was discovered cutting across the top of the capital. Rather than rejecting this large piece of masonry, an iron cramp was installed across the vent to provide a degree of security. It was in a sheltered location and the solution remained effective for two centuries. Then a leak through cracked leadwork flashing caused the iron cramp to corrode.

Being trained in industrial roped access (abseiling), which enables me to get close to problems on external building envelopes quickly, efficiently and cost-effectively, I was asked to check the capital. I ended up with my legs wrapped around the column shaft holding on to a large piece of stone that had finally decided to liberate itself.

Iron is a major problem with historical ashlar façades, whether used as fixings or as steel members supporting large façade sections. The propensity of the problem affecting the many great buildings along London’s Regent Street has led to the problem becoming known as ‘Regent Street Disease’.

Again, moisture is the overriding factor. These days we have stainless fixings and treatments that prevent such problems, but with older buildings good observation and preventative maintenance are key to longevity.

The new modernism in construction brought American style skyscrapers to the UK and with it the use of thinner stone panels used as cladding.

Various fixings have been used to support and restrain these panels in many different ways, but all of them place stresses on the stone in one way or another. At the same time, the stone has to resist induced stresses from wind loading and impact. And on tall buildings there are large temperature differentials that can lead to more than 100 freeze-thaw cycles a year.

Cladding systems need to be carefully designed and just as carefully installed to cope with such conditions.

Domestic bliss

The most common exposure to stone for the average householder these days is a granite worktop in the kitchen. Householders often spend tens of thousands of pounds on kitchens and for many the worktops will be the most expensive stone they will ever buy.

Perfection of finish is paramount here. The stone will be scrutinised daily at close quarters. It will be touched as well as looked at under probably quite harsh lighting. Colour consistency, stain and scratch resistance and other issues demand that the highest quality stone is used.

Most people are delighted with their worktops and remain so well into the future as long as they take note of the care and maintenance advice they should be offered when the worktops are installed. The same is true of any product.

When it comes to floors, screeds that the stone is laid on are a particular bugbear in domestic settings where they are typically thinner than they would be in commercial properties and are often not reinforced. This leaves them prone to curling and shrinking, which can be exacerbated by incorrectly commissioned underfloor heating. As ever, problems are often the result of pressure to complete quickly, resulting in stone finishes being installed before the screed is ready.

Thin stone is no match for the tensional stresses screeds can exert on it, so it will crack if the screed moves. This will often identify where movement joints should have been incorporated but were not.

Designing by numbers

If you are a builder, developer or designer, how do you justify involving stone in your project? You crib from the NBS, ask for the stone’s CE certificate and away you go! Or you can just ask the specialist stone contractor to sort it all out for you.

For a large project, you might employ an engineer who will tell you to test every third piece of stone. And when you have done that you will do more tests because the results showed variations – such is the nature of stone.

There are now in place various British and European Standards and Codes of Practice that deal with the performance of stone for a variety of uses, notably cladding, paving, setts, kerbs and tiling. But don’t forget that these are minimum requirements and there are many instances when the Standards do not provide an answer because they lag behind innovation.

This is where trying to understand stone helps. Testing is a necessity to be able to do that but should not be carried out just for the purposes of meeting legal requirements.

Working out which tests are appropriate is not always straight forward and even then the data have to interpreted. Most durability tests are misleading, if not pointless. A granite with almost no moisture absorption and high strength can resist frost and simply does not need to be frost tested.

The Shri Swaminarayan Mandir in Neasden, the first traditional Hindu construction of its type in Europe, is one of the finest examples of natural stone use in the UK, as far as I am concerned.

There are 5,000tonnes of natural stone in the project fixed without the use of steel (because ferrous metals disrupt the karma). The structure relies for its integrity on the quality of the stone in compression.

The external stone is from Vratza in Bulgaria. The quality and consistency of it has been achieved as a result of two members of the religious mission that built it checking each block in the quarry before it was shipped out to be cut and carved.

There can be issues with Vratza limestone but I had the good fortune to survey the quarry and show the mission the qualities of stone needed – as well as what they needed to avoid.

Very little testing was involved. Why? Because the testing of every three blocks (or whatever) is a box ticking exercise that is not necessarily going to identify the few individual pieces of stone that can cause the problems. Geological skills can do that but are not sufficiently used in an industry that should be crying out for them. It appears that most people would rather have a number from a test result.

Keeping up appearances

Stone is often treated in the same way as an expensive car: initially, people are scared to use it and wait with bated breath for that first scratch or dink. But stone, like a car, needs to be used (although not abused).

Floors will scratch, dirt will run across surfaces, finishes will change. You can fight that – a whole industry supplying treatments and cleaners has sprung up to do so – or you can accept it.

It is still my contention that if you select the right stone for a given use it will not require treatment. Nevertheless, I accept stones are chosen largely on aesthetics, so for interiors, at least, if a stone can be treated to give it the required performance, why not?

What about cleaning?

External façades present the greatest challenges. There are many good systems for cleaning them but they are often used only after many years of neglect. Maintenance is often initiated too late – perhaps only when a piece of masonry has become detached.

This is an attitude that needs to change. The solution is simple: spend half an hour or so once a month casting an eye over the building to look for developing water issues and other simple features that could be precursors to damage.

Simple preventative actions such as unblocking gutters and drains and pulling out saplings that have seeded themselves on a building can significantly reduce future repair costs. If there is something more worrisome, call in an expert.

The costs of regular surveying and early identification of problems are insignificant compared with the cost of correcting failures. In an ideal world, insurers would reduce their premiums for larger buildings that have a plan of regular survey in place, with the difference in price more than covering the cost of the survey.

The final reckoning

It is all too easy to blame the stone when things go wrong with an installation. This is because natural stone is the finish and often we do not know anything is wrong until the problem has broken through or in some other way affected that finish.

We take stone from the ground, process it, test it, make a design and specify it. We build with it and (hopefully) maintain it. Stone only reacts badly if we have got some part of that process wrong. It can never be the stone’s fault. It can only be our fault because we have misunderstood it or abused it. And even then stone is exceptionally forgiving. n

Barry Hunt is a regular contributor to Natural Stone Specialist magazine. He is a chartered geologist, surveyor and scientist. He has been awarded the designation of European Geologist and is a Corporate Building, Conservation & 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 various professional committees, including the Technical Committee of Stone Federation Great Britain (SFGB). He is one of the authors of SFGB 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 establishing his own consultancy, IBIS, in 2001, 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.