Sandstone is the main type of building stone used in Scotland. Although a durable material, sandstone is also naturally porous, making it prone to erosion if subjected to water and wind over time. Correctly used and maintained, however, stone is a sound, stable building material – as shown by our many prehistoric, medieval, Georgian and Victorian buildings.
Many stone decay problems begin with the failure to maintain buildings. Regular inspection and repair of roofing leadwork, rainwater goods and underground drainage will help to prevent water from entering and saturating stonework. You should also look out for pointing missing between individual stones, as moss, grasses, small bushes and saplings may begin to grow in these gaps, damaging the structure further.
sandstone can erode and decay to quite a depth before causing structural problems that must be fixed
the surface patina that forms over time is very valuable, as it adds a sense of history and a protective layer, and it can also be attractive
Stone cleaning can remove the surface patina and lead to serious erosion. Both physical and chemical cleaning methods have caused severe damage to Scotland’s stone buildings in the past. As a result, stone cleaning is generally reserved only for very heavy soiling.
New cleaning techniques should also be treated with caution, although some are appropriate to use. The safest way to avoid the risk of damage to a sandstone building from any form of cleaning is simply not to clean in the first place.
When maintenance and repairs are needed, you should select a contractor who has the right technical knowledge, good craft skills and the ability to source and work with the right materials.
Sandstone is the main type of building stone used in Scotland. As it’s relatively soft, sandstone can be easier to shape and carve than other stones.
It’s a sedimentary rock, formed of layers, and is much more resistant to weight when the layers are horizontal. Sandstone is prone to erosion if subjected to water and wind over time.
Variation in sandstones
Sandstone from different locations will vary considerably in:
colour – from pale cream through honey yellow to rust red
Composition and durability will affect the performance of the sandstone.
It takes millions of years for sandstone to form:
Existing rocks are worn down
Fragments of the rocks settle in layers
The weight from the top layers puts pressure on the bottom layers, turning them into rock
Scottish sandstone comes mainly from:
the Northern Isles
around the Moray Firth
across the Central Belt
Signs of ageing
Sandstone is a durable building material, but its appearance will change over time. Historic buildings are expected to show signs of ageing, including some soiling.
Aged masonry surfaces often develop a natural patina, which is considered very valuable as it adds a sense of history and endurance to older structures. It also helps to protect the stone from weathering and decay, and may enhance a building’s appearance.
The patina is formed over many years by a combination of:
biological growth – e.g. algae, fungi, lichens, bacteria
soiling – e.g. by airborne particles from soot in the past, diesel today
other processes related to ageing
Stone cleaning may remove the surface patina, and a very old building that is cleaned excessively would look artificial and out of place – especially next to other historic buildings left untouched.
It’s also unrealistic to expect a building to look freshly cleaned for long. Biological growth will quickly re-establish and may even increase on newly cleaned buildings.
Historic masonry should be appreciated for its natural patina, which comes with age and can add to its appearance. It may even protect surfaces from decay.
removes a structure’s unique appearance
may harm the masonry
can put the masonry at greater risk of damage in the future
The safest way to avoid the risk of damage to a sandstone building from any form of cleaning is simply not to clean in the first place.
Cleaning may sometimes be important for safety reasons, e.g. the removal of a wasp nest. Or there may be conservation reasons for cleaning, e.g. to remove soiling that is causing damage. But great care must be taken in considering whether or not to clean sandstone.
Owners must also take into account the effect of cleaning on the street as a whole – particularly in terraces, crescents and squares. Piecemeal cleaning of grouped buildings has caused some of the worst visual effects.
The only relatively safe physical cleaning technique involves manual brushing with a stiff bristle brush (not wire) and clean water. This can be quite effective in removing minor organic growth and loose surface dirt.
Fortunately, the use of highly damaging abrasive techniques – including high and low pressure water washing, dry and wet grit blasting, and mechanical cleaning using rotating discs and brushes – has almost disappeared. But unscrupulous contractors will still use such methods on occasion.
Chemical cleaning methods
Chemical methods involve applying a substance that reacts with the stone and any discolouration that exists on the surface. It is then removed, along with the soiling.
Such techniques have often changed the affected building permanently. Because sandstone is porous, no matter how good the attempts to ‘rinse off’ or neutralise the chemicals, some residue will always be ‘rinsed into’ the stone.
Chemical cleaning may lead to:
irreversible bleaching or staining
unsightly white salt deposits left on the surface
‘pitting’ and loss of detail caused by erosion of certain minerals
faster ‘greening’ of the surface due to algae growth
Most of these changes can also speed up stone decay. Recent research has found numerous cleaned buildings where the rate of decay is 6 to 10 times faster than neighbouring buildings that weren’t cleaned. In the worst cases, 25 to 50% of all the stone in the cleaned walls may now need to be replaced.
New cleaning techniques
These have been in development since around 2000, when new research backed up the belief that cleaning stone had previously caused extensive damage. New techniques include latex poultices, the use of bacteria, and laser treatments. Past experience tells us we must exercise caution with any new approach, however.
A thorough testing programme is a vital part of any planned cleaning project and will help when it comes to writing a detailed specification for contractors.
Most testing involves three basic stages:
Inspection of building defects, surface variations, decoration, stone type, soiling patterns, and the impact of weather and water run-off.
Test cleaning in several areas, which are both unobtrusive and represent the full range of surfaces to be cleaned.
Thorough analysis and reporting of observations and test results, including photographs.
All information recorded during testing should be kept for reference, in case anything goes wrong with the work.
Selecting a stone cleaning contractor
Once cleaning has been decided upon, the next step is to select a suitably experienced contractor. You can ask to see photographs of previous work or to speak to other clients to check that a contractor has the right background.
Stone structures have been built in Scotland for over 5,000 years. When correctly used and maintained, stone is a sound, stable building material – as shown by our many prehistoric, medieval, Georgian and Victorian buildings.
But all materials decay, and some types of stone deteriorate faster than others. Softer sandstones decay faster than harder sandstones, and they in turn decay faster than granite.
Stone is a natural and porous material, which can make it vulnerable to the effects of water. The effects vary, depending upon the type of stone, its position and how much water it’s subjected to.
Water speeds up decay, particularly where:
large volumes of water are involved
the stone is exposed to other contaminants
carved or moulded stones stand out from a wall face
What decay looks like
Stone decay can appear as:
a powdery surface of loose grains or a separating skin (different thicknesses)
a ‘honeycomb’ of holes
open fractures crossing stones at unusual angles or as a ‘starburst’
severe partial destruction of individual stones
a white build-up of salt on the surface
partial failure of projecting mouldings and carvings
deterioration of individual stones so that the mortar stands proud
general blurring and loss of sharpness
tidemarks, water stains and discolourations
Specific causes of decay
Many stone cleaning techniques once popular have since been proven to be very damaging, particularly to sandstone. This because cleaning destroys the surface patina, which is a natural protective layer formed over time. In the worst cases, cleaning has sped up natural stone decay and erosion by 6 to 10 times. This is now adding greatly to the maintenance costs of affected buildings.
The stone cleaning problem has been made worse by the use of ‘plastic repairs’. This is where synthetic mixtures have been spread onto the face of badly damaged stones to try to improve a building’s appearance and performance. But these impermeable materials will trap water behind the patch over time. Often the two materials fail where they were fixed together, and the applied repair may even fall off, creating a safety hazard. Applying synthetic materials is best avoided.
This can occur from a variety of sources, including the laying of winter salt on pavements or roads beside buildings. Salt is soluble and so can be washed into the stone by rain and ground water. As the water evaporates, salt can be left inside the stone. When too much salt builds up in the stone, the salt crystals will exert pressure, causing the stone to break down. The result is a loose powdery surface with a white appearance. To help prevent this, winter salt should be applied well away from the stone courses near the ground.
Unless sandstone has become loose or fractured, it can erode and decay to quite a depth before causing structural problems that will need attention. It can, however, look unsightly.
But deciding where to stop once you’ve begun to replace masonry can be as difficult as making the decision to start the work.
If you’re thinking about replacing masonry, ask yourself:
Would keeping the original stone preserve the building’s integrity and character?
Does the appearance of the building depend on architectural completeness, or on revealing the marks of time?
Is there real concern about safety, collapse or serious failure?
Is the building performing as it was designed to perform?
The selective replacement of eroded stones should generally be all that’s needed. Each wall face should be structurally sound and effective in stopping water from getting into the building. You'll get the maximum benefit from your investment if you focus on replacing eroded stones that originally threw water off the face of the building but no longer do so.
Most 18th and 19th century stone buildings were designed to control the flow of water over their surface. Projecting stones and stone courses are functional, allowing water to drip off the building, rather than run down its face. When such details fail, other problems start to appear. Left unresolved these will result in additional costly repairs.
Some replacement projects have been carried out, at great expense, when there was no real need to do so. Often all that’s required is a brushing down, by hand, of any loose material, followed by the repointing of wall faces with lime mortar. Loose pieces of stone that are a safety concern and which can’t be securely reattached may also need to be removed.
‘Indenting’ is the practice of replacing damaged stones in an ashlar-built sandstone wall. The technique involves replacing badly decayed or damaged blocks with stones of similar size and physical characteristics.
Ashlar masonry, which is made of squared stone blocks, is naturally stronger than rubble masonry. It can withstand a considerable degree of erosion and distress before any indenting work is required. Stones should only be replaced if the decay is affecting the structural stability and function of surrounding stonework.
Best practice is to aim to retain as much original stonework of a building as possible. It’s also recommended that the techniques used in repairs involve the least physical disturbance to the building.
Choosing replacement stone
A proper matching of the properties of the replacement indent to the original stone will produce a more successful and long-lasting result. This means considering the purpose of the particular stone to be replaced as well as its porousness, mineral content and colour.
You should think about:
the function of the individual stone that is to be replaced
whether replacement stone is available from an appropriate quarry
how the new stone will affect the building, its function and appearance
Understanding the natural variation in sandstone from different locations is important when deciding upon replacement stone for the indents. Indenting with an unsuitable stone may cause further damage and decay. A carefully selected stone may also blend in with the look of an aged building.
It’s often difficult to find an exact match to carry out an indent repair due to the limited number of stone quarries currently producing sandstone. You may need to get technical advice to help you source the most similar stone available.
The original construction must be carefully studied and measured to record the exact size of any stones that need to be replaced. New stones should fit exactly in place without altering the size of the surrounding mortar joints. If only a partial indent of a damaged ashlar block is needed, the joint between the two pieces should be as fine as possible.
Surface features on the existing stone should be copied onto the replacement stone, to ensure that the indent repair blends in. But the finished surface of replacement blocks should not be artificially weathered or distressed. Newly indented stones will 'weather down' naturally, over a fairly short period of time, to gain a surface patina similar to the original.
Surface features to be copied include the:
angle of any chisel marks
depth of the grooves
size of any borders around the individual blocks
number of grooves cut into the face of the stone
Care must be taken cutting out the decayed stone to avoid chipping the edges of surrounding stones. The depth of stone removed must be great enough to ensure that the indent repair sits securely in place.
The vertical joints between blocks in original ashlar build are staggered over different rows. Keeping to this original pattern when indenting more than one stone maintains the structural strength of the wall.
Replacement blocks should be set in lime mortar, and any voids behind the indent firmly packed with mortar. The external face of the replacement block should be lined up evenly with the surrounding original face of the building.
You should employ a reputable masonry contractor, who has the craft skills to size, cut, tool and set the indent without damaging the surrounding masonry. Scaffolding should be carefully erected without causing damage to the building.
All can lead to high volumes of water entering stonework, which will lead to decay. To avoid this risk, frequent maintenance checks should be made and any faults repaired as soon as they’re discovered.
You should also look out for pointing missing between individual stones. If left open for long periods, moss, grasses, small bushes and saplings will grow in the voids, damaging the masonry further. Lime mortar should be used to repoint open joints between blocks.
Treatments to avoid
To protect historic masonry, it's best to avoid:
stone cleaning – this should be generally avoided, but if it is thought to be vital, it must be carried out in the least damaging manner
water repellents – their use should be approached with caution as they may have the reverse effect and actually trap in water
oil paint coatings – their use should be avoided, as they are likely to trap in any water that enters the stone
the excessive use of de-icing salts near to buildings – these are absorbed by stone and can lead to severe damage
Where the natural visual effects of ageing, including soiling, are excessive, it may suggest damage is occurring. But intense biological growth – algae, fungi, lichens and/or bacteria – or salt deposits on masonry is generally just a maintenance problem. For example, the repair of damaged or blocked drainage features, or the repointing of stones will resolve many such issues.
the ability to source and work with the right materials
Both the number of fully qualified stonemasons available to undertake good quality work in Scotland and the availability of good matching replacement stones are currently limited. The Engine Shed is helping to address the shortage of traditional skills, and initiatives to encourage access to more old building stone quarries are ongoing. But while shortages remain, it’s important to make choices very carefully.
Consent is required before cleaning is carried out on listed buildings or buildings in conservation areas. Stone cleaning is considered an alteration in Scotland.
Because of the risks involved, cleaning sandstone is unlikely to be permitted except for safety reasons or where the soiling is causing decay.
If you’re planning to make significant repairs to sandstone masonry – including indent repairs – you should also check first whether you must apply for planning permission or any other type of consent.
The survival of original stonework can override the desire for a uniform appearance on buildings of historic significance.
Scotland is a country built with stone. Over the last 5,000 years, Scotland’s traditional masonry buildings have been shaped by the characteristics of sandstone and the other sedimentary and igneous rocks from which they were built.
It’s vital to preserve this unique character that adds to the tapestry of Scotland’s historic environment.
Scotland’s diverse geology is seen in the varied colours and textures of stone used for building – from grey Aberdeen granite to the red sandstone of Dumfriesshire and the white sandstone of Edinburgh’s New Town.
Stone wasn’t hauled great distances before the Industrial Age, and even small towns and villages had their own quarries. We can still see today how the rock found below ground has shaped the look of the local area above.
Development of stone construction
Drystone walling was the first type of stone construction in Scotland. Here stones are placed on top of each other with no mortar. Settlements like Knap of Howar and Skara Brae, built from 3180 BC to 2500 BC, are some of the earliest surviving examples of this technique. Iron Age brochs show a complete mastery of how the stone was chosen, placed and set.
Ashlar masonry – using large, regular-sized, square-edged stone blocks – and lime mortar were brought to Scotland by the Romans. Ashlar wasn’t then much used after their departure, in the 400s, until the 1000s, when the great monastic orders used it to build their grand abbeys. Stone was used to its full potential here, in soaring vaults and in intricately carved decoration.
It was ashlar that was used to create the fine dressed stone façades of great 17th and 18th-century country mansions like Kinross, Duff and Hopetoun. It was also used at this time for rich merchants’ houses in urban areas. Rubble masonry was used on less expensive buildings.
From the mid-1800s, quarrying became more mechanised, making stone cheaper to buy. This made it possible to use ashlar to build the façades of domestic houses, and this approach lasted well into the 1900s.
The Engine Shed is accredited by the Scottish Qualifications Authority (SQA) and National Open College Network (NOCN).
The Engine Shed was supported by a grant from the National Lottery Heritage Fund. Thanks to National Lottery players, we have been able to share knowledge of traditional building materials, develop skills and raise standards in conservation for traditional buildings.