A guide for building with natural stone
Natural stone is an essential building material in the construction industry, with a place in both traditional and modern architecture. It has an array of uses from architectural stone features, like porticos and balustrades, to building stone used in housing developments. It is often a distinctive feature of the English landscape, with its subtle colour and textural variations that delineate one region from the next.
Building stone can last for centuries as the historic castles, cathedrals and churches in the UK lay testament to. The benefits of this material are immense. It is beautiful, strong and durable but if not correctly installed and cared for, it may not survive more than a couple of harsh winters.
This guide is intended to help with choosing and using building stone to achieve the optimum results.
Choosing the right stone
There are a variety of stone products available, with different characteristics pertaining to where they were sourced from. Choosing your stone will depend upon several factors; performance specification, the purpose or use, the conditions where it will be installed, location and the aesthetics of the setting.
A popular choice is limestone, which is primarily made up of calcium carbonate with small amounts of other minerals including clay. It is a strong rock, with good load bearing capabilities, is reasonably easy to cut and shape and possesses naturally good insulating properties. Limestone is also very versatile and can be used for both interior and exterior applications such as walls, flooring and decorative features.
However, limestone is also a porous rock and can be susceptible to absorbing water and being damaged by the freezing and thawing process.
Great Tew Ironstone (The Marlstone Rockbed) has a distinctive deep orange-brown colour with occasional patches of white mineralisation/marbleing and is commonly found throughout the counties of North Oxfordshire, Northamptonshire through to Rutland. The beautiful darker stone is fine grained with some particles having a deep iron-rich colour with others being paler.
Our natural stone will need adequate protection whilst being stored on site prior to use. It should be kept in a dry condition, protected from rain and frost until needed. During the colder months, it is essential to protect the stone with insulating materials and strong polythene or tarpaulin. Avoid using limestone that has become saturated for any reason during the winter months, particularly when freezing temperatures are expected.
During construction vigilance is required and levels of protection to part-built structures should be upheld, as they are extremely vulnerable to frost damage throughout the winter. Cover the unfinished walls or structure with sacking or hessian, using a minimum of two layers in extreme frost. Then cover with a waterproof layer such as polythene to prevent water ingress.
Building with stone
When constructing with any natural stone, generally one or two faces can be exposed to the elements, so long as it is not subjected to damp from above, behind or below the surface.
Choosing the appropriate mortar mix depends on the type of stone and how it is being used but it is crucial to get it right to ensure the longevity and strength of the structure.
Hydraulic lime mortar is ideal for historic buildings, as it provides flexibility and breathability, allowing the stone to expand and contract without causing damage. The mix typically consists of one part hydraulic lime and two parts sand.
White cement mortar can be more aesthetically pleasing than traditional grey cement and is approximately 33% stronger. However, consider whether the increased strength is appropriate for your project.
A Portland limestone cement mortar mix is a popular choice for most building stone projects. The mixture consists of ten parts clean sand, two or three parts lime and no more than one part Portland cement. This combination provides an appropriate balance of strength and flexibility, ensuring the stone can breathe and dry out properly.
Pointing is another important consideration when building with natural stone. This is the process of filling the joints between stones with mortar. Proper pointing techniques are essential for the longevity and aesthetics of your building stone project.
Flush pointing, also known as wrap pointing, is the most appropriate technique for working with new limestone projects. This method involves filling the joints with mortar and smoothing it flush with the stone surface, avoiding excessive smearing.
Recessed pointing involves filling the joints with mortar and then removing a small amount, creating a recessed joint. While this technique can provide an attractive appearance, it is not recommended for use with certain types of stone.
Building below the Damp Proof Course (DPC)
When using any stone below the damp proof course, suitable measures must be taken to protect the stone from water saturation and potential frost damage and there are several ways that this can be achieved.
Incorporating engineering bricks, natural slate, or PVC membranes as a damp-proof course can help shield the limestone from moisture and water saturation. Implementing an underground drainage system will also help prevent groundwater from rising into the stonework, protecting the limestone from water saturation and frost damage.
Expansion joints are critical for allowing building stone structures to accommodate natural movement and prevent damage due to thermal expansion and contraction. They are usually made from flexible materials such as rubber, neoprene or metal, which can stretch or compress to tolerate movement. The choice of material will depend upon how much movement is anticipated.
Expansion joints run either horizontally or vertically. Horizontal expansion joints should be designed into unreinforced, non-load-bearing walls at a maximum spacing of 12 metres. These joints accommodate movement caused by temperature fluctuations and prevent damage to the stone.
Vertical expansion joints should be placed at regular intervals along the length of a stone structure. The appropriate spacing varies depending on the type of stone used and the specific structural requirements of the build.
In some cases, structural steel may be required to maintain the stability and integrity of a natural stone building. It should meet the standards outlined in BS EN 1996-1-2 (Eurocode 6) or any subsequent British equivalent. It should be resistant to corrosion and allow for thermal movement and expansion.
It is critical to consult a structural engineer, who will provide guidance on the appropriate type, size and placement of the steel components to ensure the stability and durability of the structure.
Cavity wall construction
When constructing cavity walls, proper ventilation is essential to prevent moisture damage and ensure the stone can breathe. Weep holes can often be seen at ground level and above lintels. These small openings have an important purpose, allowing any water that may have accumulated behind the stonework, to escape from the building therefore preventing dampness and allowing it to breathe.
When installing insulation slabs or batts, it is important to maintain a minimum air gap of 25mm between the stone and the insulation, to allow the stone to breathe.
Cavity wall ties are often used to connect the inner and outer leaves of a wall providing stability and structural integrity. Placement of the ties should be carefully considered to take account of location, wind speed and cavity width. Consulting a structural engineer is recommended when selecting appropriate cavity wall ties for your project.
Selecting appropriate course heights and deciding whether to incorporate quoins can have a significant influence over the aesthetics and stability of your building stone project. Course heights refer to the vertical measurement of each row of stones in a wall. Standard course heights for building stone projects include 65mm, 90mm, 115mm and 140mm. Varying course heights throughout the build can add visual interest as well as stability to the structure.
Gradually our natural stone will develop a patina, which is a natural process of ageing through exposure to air, moisture, sunlight, pollutants and chemical reactions. Over time these environmental factors cause the gradual breakdown and erosion of the limestone surface. As well as colour changes, other structural or surface variations such as pitting, erosion or small fissures can occur. You may also see the growth of biological organisms such as mosses, lichens and algae which can colonise on the surface leading to discolouration and a more aged look. This change in appearance is often celebrated, adding character and charm to limestone structures which can be desirable in certain contexts.
Often the reason natural stone fails is because a drainage gap has not been left between the limestone and adjacent hard landscaping so, instead of the rainwater draining away, it becomes absorbed into the limestone wall. Correct landscaping techniques should be employed to protect stone buildings from potential damage caused by water saturation and frost.
Careful consideration must therefore be given to the design of any structure and adjacent landscaping. Protective membranes can be used to separate and protect buildings from soil, bark, grass or other foliage. This additional barrier can help prevent water saturation and potential frost damage.
When building with natural stone, a stone specialist or stone mason will be able to provide the best service, ensuring your stone structure lasts for many years. Contact our expert team for further advice and information regarding our bespoke masonry products, stone products (below) and any aspect of stone care.
- Bath Stone
- Cotswold Gold Limestone
- Cotswold Cream Limestone
- Creeton Limestone