Natural Stone in Modern External Wall Construction
Natural stone has been used in the construction of walls for thousands of years. However, the introduction of new construction methods and cheaper building materials has meant that the use of stone in new buildings has declined significantly. Natural stone was once often used in a load-bearing context, but it has been replaced by concrete blockwork, timber frame and metal frame construction. As a result, building with natural stone has become a more specialist area of expertise.
In this post, we will look at some of the key things to consider when it comes to specifying, designing and detailing new external walls built with natural stone.
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The use of natural stone to face external walls can be a durable, sustainable and authentic alternative to using man-made masonry or other modern cladding materials.
The Window On Wild Lindisfarne visitor building designed by Icosis Architects, is an example of a modern building faced in natural stone
Benefits of Natural Stone
In more recent years, there has been some resurgence of natural stone being used in modern facade design. Three key reasons for this are outlined below.
Durability: natural stone can be exceptionally durable and buildings constructed from it can last for hundreds of years if they’re designed and detailed well. Many stones can withstand harsh weather conditions and their longevity can often translate into a lower life cycle cost.
Authenticity: stone is a naturally occurring material and each piece has its distinct characteristics, which adds individuality to the project. It also makes it one of the most beautiful and timeless materials to work with.
Reduced environmental impact: working with natural stone is relatively non-toxic and relatively little energy is needed for its manufacture or maintenance compared to most other man-made building materials. If the chosen stone is quarried locally in the UK, the transport emissions can also be relatively low. Natural stone can also be reused and repurposed beyond its first use.
Stone is a robust and diverse material which can last for hundreds of years if detailed well
Selecting a Stone
There are a great number of different stones to choose from, and they each have their own characteristics. Selecting the right stone for the application is key. Below are some of the factors to consider when selecting natural stone for use within an external wall:
- Existing context – a stone may be chosen to match or complement the materials used on adjacent buildings. Traditionally, building stone was often chosen for its local availability. This often gives a place a distinctive local character
- Colours and finishes – aesthetic preferences can dictate the type of stone to be used. The design may require a stone of a certain colour or finish to achieve the desired look. Ideally it is recommended to visit the quarry to see the product in situ and to agree samples
- Sizes – the size of stones and depths of courses can have a big effect on the way a wall looks. The size of blocks available can vary according to the stone type and the quarry chosen. The thickness of the stone should be specified to suit the height of the building and the laying style / structural requirements
- Formality – more formal laying styles are often used for urban facades, while less formal random rubble are often used in rural settings
- Local availability – using stone that’s sourced close to the site is a great way to lower the project’s carbon footprint. There are a vast range of British stones available to choose from
- Environmental conditions – different types of stone react differently to water absorption, atmospheric pollution, frost, and salt action. It is extremely important to consider the physical properties of the stone and ensure that it will be suitable for the environmental conditions that it will be exposed to. Incorporating engineering bricks or weatherbed limestone below the damp proof course is recommended to enhance durability
- Buildability – typically, the stone specified should be manageable in size for hand placement. The health and safety of those constructing the project should be considered where larger, heavier stones required
It is recommended to discuss the stone selection and finishes required with the stone supplier at an early stage in the design process as this will influence the detailing of the project. The expertise of the stone supplier will be key to the successful delivery of the design and so it is important to spend time finding a company that best suits the project. The Stone Federation Great Britain website can provide a good starting point when finding a British stone supplier. They provide information, advice and guidance in sourcing an appropriate material and a reliable service.
Stone is a natural material formed over millions of years. Limestone is visible in distinctive layers in the above photo. It is a sedimentary rock formed by the accumulation of shells and skeletons of marine animals on the sea floor.
Stone Walling Types
Traditionally, stone walls were solid and very thick (often around 600mm or more in thickness). The thickness of the wall was what prevented moisture from transferring into the building from the outside. These thick walls would also usually carry the structural loads of the building.
The thick walls of this traditional stone walled house are noticeable at the window reveals
Today, stone walls are designed and constructed in a similar way to other modern masonry walls (such as brickwork and blockwork walls). They usually have a load bearing backing structure of masonry, timber frame or metal frame and incorporate insulation to achieve the required thermal performance. The walls also include a cavity behind the stone, primarily to prevent the transfer of moisture to the inside (now that the walls aren’t so thick). Some types of stone are more permeable than others and rainwater might penetrate the outer leaf. As such, provision needs to be made to enable any moisture to drain out of the cavity.
Diagrams highlighting the structural backing walls of two example wall to roof junction details faced with stone. These are two of the many details available on the Detail Library
Diagram highlighting the numerous different ways of constructing an external wall faced in natural stone
Stone walling can generally be catagorised into one of two types according to its laying style – ashlar or rubble.
Ashlar stone walls are the most formal in appearance and are formed from blocks of a uniform size and finish which are laid in courses.
Diagram and photograph showing an example of ashlar stone walling
The appearance of rubble stone walls depends on the exact laying style chosen, but generally they are less formal and more random in appearance than ashlar walls. They are formed from blocks of varying uniformity, which may or may not be arranged in courses.
Diagram showing example laying styles of rubble stone
A random rubble uncoursed stone wall where the stones are all shapes and sizes
Detailing Considerations
Whilst there can be similarities between detailing a stone faced facade and detailing facades faced with more common types of masonry (such as brickwork and blockwork), there are also some key differences. Below are some key factors to consider.
- Staining – some types of stone can be more prone to soiling and staining. Careful stone selection and robust detailing to protect from the weather are key to ensuring a long lasting stain-free facade. Stone can also be stained when it reacts to certain other materials, so it is important to avoid the use of these (such as corrosive metals).
- Water penetration – stone that is subjected to long term dampness is at risk of decay. Good design of details to control and shed water from the surface effectively are critical.
- Quality of construction – specifying the services of an experienced stone mason will be key to ensuring that the stone walls are built safely and that the desired aesthetic is achieved.
- Mortar – the mortar used with the stone should be less hard and more permeable than the stone itself. This is to discourage prolonged moisture retention in the stone which could cause decay. A lime mortar is often recommended, but the stone supplier should be consulted for project specific guidance.
- Movement joints – a structural engineer with experience of working with stone should specify the requirement for movement joints.
Above, an ashlar stone wall has stained due to a concentration of moisture around a metal fixing
Detailing Stone in New External Walls
The use of natural stone to face external walls can be a durable, sustainable and authentic alternative to using man-made masonry or other modern cladding materials. Whilst natural stone can be expensive and require specialist labour, it has a number of benefits and can last for a very long time which reduces its relative whole lifecycle cost.
Careful detailing and consideration of managing moisture is essential to ensure that the stone will look its best and stand the test of time.
Check out the Detail Library for examples of key construction details for stone external walls. Our recent details show examples of stone walls with blockwork and timber backing walls.
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Resources
Related Reading
References and Useful Links
https://www.labcwarranty.co.uk/technical-blog/natural-stone-outer-leaf-in-cavity-construction
BS5250:2021 Management of moisture in buildings
Approved Document Part C
https://www.gov.uk/government/collections/approved-documents
Author
Written by Emma Thackstone. Emma is an architect. At the Detail Library, Emma helps the Detail Library with drawing new details and carrying out technical research.
Image Credits
All photographs by Emma Thackstone