Earth

Earth has been used in construction in one way or another for thousands of years. And it offers great potential as a sustainable building material for the future.

Its use in traditional construction was common practice in Scotland until relatively recently.

Earth as a building material took many forms:

• turf – blocks were used to form walls, with or without stone
• mudwall – a type of construction formed of earth mixed with straw
• pise – walls were made of rammed earth formed in timber shuttering
• clay and bool – a type of mudwall, with rounded stones set in rows between the earth material
• claywall – stones were added to a mudwall mixture formed in timber shuttering
• puddled clay – used in bridges and harbours as a waterproof layer
• clay mortar – in widespread use as an alternative to lime

Earth structures aren’t always easy to spot: many examples were pointed and rendered with lime as additional weather protection.

In Scotland today, you’re most likely to come across earth in the form of clay mortar, used to build masonry walls and as a surface finish. Lime is usually thought to be the most common mortar material for traditional structures. Yet in many parts of Scotland – especially areas where clay-rich soils are common – clay was frequently used as a mortar in masonry building.

Durability

A significant number of historic earth and clay structures still survive. Cared for with the correct skills and materials, such structures can prove durable and effective – as can clay mortars.

Clay mortars are a significant feature of our traditional built heritage. When correctly identified and maintained, clay mortar buildings can be as robust and resilient as those built with lime mortar.

Clay mortars are also:

• flexible, allowing thermal expansion
• able to handle occasional wetting
• good insulators
• easy to use

Water damage is a risk for clay mortar, so most clay-bonded walls were pointed or harled with lime mortar. This added protection from the weather also hid what the walls were made of. As a result, many clay-bonded structures look much like lime-bonded structures from the outside.

Earth and clay materials can vary in their composition. This is largely due to regional and local differences in local subsoil conditions. Subsoils vary in their ratios of clays and aggregates such as sands, gravels and silts.

Clay based materials are usually soft and malleable when moist. They should be free of organic matter, such as stalks and leaves, unless this has been added on purpose. Turf does feature organic matter, as it comes from topsoil and the roots add strength.

Sometimes lime was added for strength and aggregates, such as coarse sand, to reduce shrinkage during drying or to otherwise alter the working properties of earth materials.

In addition other materials were added, depending on what the clay was being used for:

• chopped straw (wheat, barley, or oat)
• other plant materials (such as flax)
• animal hair – usually added to plasters
• dung

Any replacement materials used for repairs must be physically compatible for use with the original building fabric. This includes any aggregates and additives as well as the earth or clay itself that binds everything together.

Clay mortar mixes

Clay mortars were used in much the same way as lime mortars, although the two mortar types look quite different and their preparation is a very different process.

A clay mortar is usually made entirely of very fine material, with an aggregate of sand or fine gravel as part of the clay or added. Chopped straw or another plant material was sometimes added too. Clay mortar is commonly a reddish or yellow colour and soft and friable (crumbly) in texture.

In contrast, lime mortar is usually cream, off white or pale grey in colour and contains aggregates of various sizes, it is harder and will only crumble under modest pressure. 

Earth construction in Scotland varied regionally according to the materials that were available locally. Below are the main types of earth and clay construction found across the country.

Earth has been used on its own, and with lime, timber, straw and stone, to build structures suited to a wide range of locations and climates.

Such structures aren’t always easy to spot: many examples were faced in brick or stone, or rendered with lime or cement or protect the earth.

Turf

Earth blocks were used to form walls, either on their own or alternating with stone blocks. Turf was often laid in a herringbone pattern for added strength.

Mudwall

Formed of clay-rich earth mixed with straw, mudwall was built in ‘lifts’ (rows) of between 15cm and 55cm tall depending on its exact properties. The sides where then often cut smooth as the mix stiffened up. Elsewhere in Britain, mudwall is known as cob.

Clay and bool

A variation of mudwall construction, where rounded stones unsuitable for other forms of building were set in rows between the earth material. This is seen in many parts of Scotland but especially in north east Aberdeenshire.

Shuttered clay

Raw materials similar to mudwall, clay or earth mixed with straw were formed into walls inside timber shuttering. Brick or stone was sometimes used to make a permanent face to hold in the clay or earth infill. The best stones went on the outside face, to make the wall look like clay-bonded masonry.

Other construction types

Clay was widely used with timber framing to form ‘wattle and daub’ and other kinds of thin walls. It was also used to make plasters and, when mixed with straw, for deafening in timber floors.

In Scotland today, you’re most likely to come across earth in the form of clay mortar, used to build masonry walls and as a surface finish.

It may be hard to identify clay mortar, however. Where clay was used as a bedding material, the wall was often pointed in lime mortar. Later repointing  and rendering might also have been done using lime or inappropriate cement materials.

Clay mortar is often found when opening walls during building works, which may cause unnecessary alarm. Its soft and friable (crumbly) nature doesn’t always mean there’s a problem with a structure. If cared for correctly, and kept dry and left undisturbed, clay mortar can be durable and effective.

How to identify clay mortar

Old clay mortar often appears as a soft material that can be crushed in the hand to make a fine powder. Colour can signal the presence of clay mortar, as clay is often darker than lime. Earth based mortars will also become smoother to the touch when wetted and rubbed between the fingers.

Its texture, reddish or yellow colour and lack of any sizeable aggregates may be enough to let you spot a clay mortar. Another sign to look for is brown or reddish staining on external masonry, which shows that clay is being washed out.

Or you may need to have a sample of mortar analysed to be sure of its composition. Laboratory testing for fine silicates will confirm whether a mortar has any clay content.

How to take a mortar sample

1. Make a small opening in the face of the external or internal wall.
2. Remove a small facing stone.
3. Scoop out some of the mortar from the wall core.

Lime harling

To protect earth structures, the outside face was often finished using a thin lime harling or limewash. It may be best to redo any such lime finish after making repairs to the earth construction beneath. The earth material must be allowed to stiffen up before the lime harling or limewash is applied.

Likewise, clay-bonded masonry was sometimes coated in lime harling. Again, it may be best to reinstate the lime finish after any repairs to clay mortar have taken place.

Cement harling and mortars are impermeable and inflexible, making them prone to cracking. They shouldn't be used on an earth or clay building. If moisture gets behind cement render, for example, it will trap moisture in the earth or clay. Over time, this may speed up the decay of the structure.

If cement harling is found during repair work, you should  check it is not causing problems. If it is it should be removed and replaced with a lime harling.

Lime pointing

Where clay was used as a bedding material, the wall was often pointed in lime mortar. Later repointing might also have been done in lime or using inappropriate cement materials.

For lime pointing to be effective, the clay mortar it is to cover must first be raked out to a sufficient depth. The depth of the lime pointing should be roughly 2.5 times the width of the joint (and usually no less than 40mm).

Lime plaster

For internal areas, it may be suitable to apply a finish coat of clay or lime-based plaster after a repair.

Internal walls were often left with clay joints if they were to be lined with lath and plaster. Clay plaster was also used internally, usually with a limewash or distemper finish.

Water is the main threat to earth and clay structures and clay-bonded buildings. A regular maintenance regime will help you to ensure that water doesn’t enter the building fabric.

Maintenance of earth structures

To prevent decay:

• ensure that the roof is in good condition
• check rainwater goods for blockages and cracks
• maintain the stone plinth on which most earth and clay structures were built to minimise rising damp
• reapply any lime harling and/or limewash finish as necessary
• use only moisture permeable lime or clay plasters internally
• keep the area around the building well drained
• keep vegetation away from walls as roots can quickly cause damage
• avoid hard landscaping and tarmac near walls
• deal with any pests such as mice and rats, which will cause damage especially if decay is already present

Earth buildings, if not looked after properly, can be prone to damp. In particular those built without a stone plinth may suffer from rising damp. A French drain may be the most effective solution to rising damp. Fitting a damp proof course should be avoided: it can act as a barrier and cause moisture to collect in the base of the wall, weakening the structure.

Maintenance of clay mortar

Clay mortar should be covered. It can be quick to show signs of distress if this protection fails. This isn’t a weakness, but an early sign that a building requires attention.

Clay mortar is prone to damage from water saturation and ongoing dampness. Elevations exposed to the prevailing weather are most at risk. Keeping walls dry is a priority for any building – old or new.

High levels of moisture will break down the clay matrix, leading to:

• more friable (crumbly) mortar
• mortar loss within the wall core
• a loss of structural integrity
• movement of masonry

Look out for common causes of damp in walls such as:

• blocked gutters
• raised ground levels
• unsuitable renders
• cracked chimneys
• failed skew copes

Damaged areas of clay-bonded masonry should be repaired with a suitable clay mortar. You should engage a specialist to carry out clay mortar repairs.

Maintaining moisture movement

Breathable materials should be used on both the inside and outside faces of clay-bonded walls to maintain moisture movement.

Approach with caution the use of water repellents, masonry paints and other ‘one-coat cures’ for walls of any type. Such short-term solutions may make masonry less able to manage moisture in the longer term.

Cement render may have been used in recent years to replace weathered lime harling on a clay-bonded building. Where it is in good condition, the render can be left in situ provided all other areas are in good condition. The wall’s health may be able to be maintained from the inside instead, using moisture permeable finishes.

To properly plan repairs to earth and clay structures, you should understand the construction type and the composition of the original materials.

Repairs to the earth structure or clay mortar should be made only once the cause of the underlying problem has been fixed. A condition survey of the structure and its site should be carried out before any repairs are planned.

The survey should record any:

• problems with ground drainage
• defects in roof structure and rainwater goods
• the nature and composition of external coatings
• the form and materials used in the construction

Repairs to earth structures

If the base course of a wall has decayed, repairs will ensure that the earth wall doesn’t settle or distort. Masonry should be re-bedded with a clay mortar or lime mortar.

Soil may build up over time, raising external ground levels higher than the masonry and causing moisture to enter the earth materials. Ground levels should be lowered to below the level of the original base course. The base course should then be repaired and repointed as necessary.

If sections of earth walling must be restored, the replacement materials and methods used should match the original construction as far as possible. Earth block repairs can be a good solution where original techniques can’t be copied or materials are not available.

Patch repairs of earth structures using cement based renders will only make problems as the material is incompatible with the existing fabric and will move and behave differently, leading to cracking. A clay or lime render should be used instead.

Find out more about the use of lime in earth construction.

Clay mortar repairs

Repair mortars, as with all replacement materials, should generally match as closely as possible the original mortar that was used. But the technical requirements of a specific structure may demand a different specification. For small repairs,  lime mortar may be suitable.

A large amount of mortar will be needed to fill voids where clay mortar made up a significant proportion of a wall but has been lost over time. Repair material must be pushed right to the back of the void to make the wall as sound as possible. Pinning stones that have fallen out should be replaced.

Masonry should be dampened before clay mortar is applied. This will help the mortar to stick better to the masonry. After being applied, the clay mortar must be protected from rapid drying as well as rain. Clay mortar solidifies by drying, which takes a longer time than setting (which is how hydraulic limes harden).

If replacing an earth or clay material is required, the new material used should match the original as closely as possible.

An exception to this rule is where the technical requirements of a specific structure demand a different specification of repair materials. You should ask a specialist to advise on the best materials to use for anything other than a simple like-for-like repair.

The original earth building materials would have been sourced close to the construction site. You may still be able to find a local source of such materials. There are also a growing number of commercial suppliers of earth and clay materials, thanks to the rise in popularity of sustainable building techniques.

Checking compatibility

Check that replacement materials are compatible before use.

For example, when forming clay mortars for repairs, the aggregates used should match the type and size of those used originally. If the original mix contained chopped straw or animal hair, the same additives should also be included in the replacement. The chosen replacement material should be tried out on test panels before it is used on a larger scale.

If you’re planning to make significant repairs to an earth building you should check first whether you must apply for planning permission or any other type of consent.

Find out more about listed building consent.

Learn more about conservation area consent.

Using earth and clay materials to form walls was once common practice in Scottish traditional building. Stone and brick largely took over from the mid-19th century, but many earth structures still survive – mostly in Perthshire, Angus and the south-west of Scotland.

Earth construction varied by region according to materials available locally:

• turf building was most common on the west coast
• mudwall was mainly used in Dumfriesshire and the Solway Coast
• clay was used in Perthshire, Angus and other areas where it was readily available due to the local geology
• clay and bool is mostly seen around Spey Bay, Moray

Most earth construction was later covered with render, making it hard to spot. Such structures may only be discovered when the render fails and falls off.

Other signs of earth construction may include a:

• rubble stone base course at the foot of a wall
• pronounced slope to the wall

Clay mortar and masonry

Recent work shows that clay-bonded structures are much more common in Scotland than was previously thought. As it required less processing and was often locally available, clay was cheaper and easier to get hold of than lime.

Heavy clay was dug out of the ground, usually from below the topsoil layer. It was sometimes used directly for bedding masonry and filling the wall core.

Clay mortars were used up until the mid-19th century for buildings of all types – from rubble-built structures such as houses, farm steadings and cottages to the wall core of large houses. Clay mortars only fell out of common use when better transport links made building limes more widely available.

Many medieval and later buildings that survive as ruins are sometimes thought to be dry stone structures (masonry built without mortar). In fact, many were actually built with an earth mortar wall core.

A good example is Mervinslaw Tower, a 16th-century bastle house in the Scottish Borders. At first sight it appears to be dry built, but a closer look reveals that it’s made of clay-bedded rubble, pointed and rendered with lime. The surviving walls are uneven, not as a result of vernacular construction, but because much of the clay mortar has been washed out, causing the masonry to distort.

Clay wall construction is a reviving craft, and shuttered clay – often reinforced with straw – is a popular material for building ‘eco houses’.

Get in touch if you have any questions. We will be happy to help.

• Technical Resources Team
• 0131 668 8951
• technicalresources@hes.scot

Professional institutions

Earth Building UK & Ireland

Publications

Inform guides

An introduction to conservation and maintenance of traditional buildings for complete beginners.

Earth and Clay Construction
Clay Mortars for Masonry Buildings

Technical advice notes

In-depth technical information for building conservation professionals.

06 - Earth Structures and Construction in Scotland

Conference Proceedings

Papers and abstracts from building conservation and archaeology conferences.

Historic Scotland Traditional Building Materials Conference 1997

Other resources

Building Scotland – Celebrating Scotland’s Traditional Building Materials
The Building Conservation Directory