Spalling: Architectural Stonemasonry Explained

Jun 27, 2024

Spalling is a term used in architectural stonemasonry to describe the process of flaking, chipping, or weathering that occurs in stone structures over time. This phenomenon can be observed in a variety of stone materials, including limestone, sandstone, and granite, among others. Spalling is often the result of environmental factors such as temperature fluctuations, moisture, and chemical reactions, but it can also be caused by mechanical stress or poor construction techniques.

Understanding spalling is crucial for anyone involved in the preservation, restoration, or construction of stone structures. By recognising the signs of spalling and understanding its causes, it is possible to take preventative measures or carry out repairs to maintain the integrity and aesthetic appeal of a stone structure. This article provides an in-depth look at spalling, its causes, prevention methods, and repair techniques.

Understanding Spalling

Spalling is a natural process that occurs when stone is exposed to certain environmental conditions. It is characterised by the breaking off of pieces of stone, often in thin layers. This can result in a rough and uneven surface, and in severe cases, can lead to significant structural damage. Spalling is a common issue in older buildings, particularly those made of porous stone materials that are more susceptible to water damage.

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Although spalling is a natural process, it can be accelerated by a variety of factors. These include exposure to harsh weather conditions, improper installation or maintenance of the stone, and physical damage from impact or pressure. In some cases, spalling can also be caused by the use of inappropriate cleaning methods or chemicals that can damage the stone’s surface.

Signs of Spalling

One of the first signs of spalling is the appearance of small chips or flakes on the surface of the stone. These may initially be small and unnoticeable, but over time, they can grow larger and become more noticeable. In severe cases, large chunks of stone may break off, leaving deep pits or holes in the surface.

Another sign of spalling is the presence of cracks or fissures in the stone. These can be a result of the stone expanding and contracting due to temperature changes, or they can be caused by the pressure of water freezing and thawing within the stone. If left untreated, these cracks can widen and deepen, leading to further spalling and potential structural damage.

Causes of Spalling

There are several factors that can contribute to spalling in stone structures. One of the most common is exposure to water. Porous stones such as limestone and sandstone can absorb water, which can then freeze and expand, causing the stone to crack and chip. This process is known as freeze-thaw spalling.

Chemical reactions can also cause spalling. For example, when limestone is exposed to acidic rain, it can react to form calcium sulphate, which can cause the stone to expand and crack. Similarly, the presence of salts in the stone can lead to salt crystallisation, which can also cause spalling.

Finally, mechanical stress can lead to spalling. This can occur when the stone is subjected to excessive pressure, such as from a heavy load or impact. Over time, this can cause the stone to crack and chip, leading to spalling.

Preventing Spalling

There are several measures that can be taken to prevent spalling in stone structures. One of the most effective is to ensure that the stone is properly installed and maintained. This includes using appropriate materials and techniques during construction, as well as carrying out regular inspections and maintenance to identify and address any issues before they become serious problems.

Another important measure is to protect the stone from water damage. This can be achieved by applying a water-repellent coating to the stone, which can help to prevent water from penetrating into the stone and causing damage. Additionally, it is important to ensure that the stone is properly drained to prevent water from pooling on the surface.

Proper Installation and Maintenance

Proper installation and maintenance are crucial in preventing spalling. This includes using the right type of stone for the specific environment and application, as well as ensuring that the stone is installed correctly. For example, it is important to ensure that the stone is properly supported and that any joints are properly filled to prevent water from seeping in.

Regular maintenance is also important. This includes cleaning the stone regularly to remove any dirt or debris that could potentially damage the stone. However, it is important to use gentle cleaning methods and avoid harsh chemicals that could damage the stone. Regular inspections should also be carried out to identify any signs of spalling or other damage.

Water Repellent Coatings

Applying a water-repellent coating to the stone can be an effective way to prevent spalling. These coatings work by forming a protective barrier on the surface of the stone, which helps to prevent water from penetrating into the stone. There are several types of water-repellent coatings available, including silane and siloxane-based products, which are known for their ability to penetrate deeply into the stone and provide long-lasting protection.

However, it is important to note that while water-repellent coatings can be effective in preventing spalling, they are not a cure-all solution. They should be used in conjunction with other preventative measures, such as proper installation and maintenance, to ensure the long-term health and beauty of the stone structure.

Repairing Spalled Stone

If spalling has already occurred, it is important to carry out repairs as soon as possible to prevent further damage. The specific repair method will depend on the severity of the spalling and the type of stone involved, but may involve filling in the spalled areas with a suitable repair material, or in severe cases, replacing the damaged stone entirely.

Before carrying out any repairs, it is important to first identify and address the underlying cause of the spalling. This may involve improving the drainage of the stone, applying a water-repellent coating, or making changes to the way the stone is maintained or cleaned. By addressing the root cause of the spalling, it is possible to prevent the same issue from occurring again in the future.

Filling in Spalled Areas

One common method of repairing spalled stone is to fill in the damaged areas with a suitable repair material. This can be a specially formulated stone repair mortar, which is designed to match the colour and texture of the original stone. The repair material is applied to the spalled area and then shaped and smoothed to match the surrounding stone.

It’s important to note that this method is typically only suitable for minor to moderate spalling. If the spalling is severe or if the structural integrity of the stone has been compromised, it may be necessary to replace the damaged stone entirely.

Replacing Damaged Stone

In cases where the spalling is severe or the structural integrity of the stone has been compromised, it may be necessary to replace the damaged stone entirely. This involves removing the damaged stone and installing a new piece in its place. The new stone should be of the same type and colour as the original to ensure a seamless appearance.

Replacing a damaged stone is a complex process that requires specialised skills and equipment. Therefore, it is often best left to professionals. However, with the right knowledge and tools, it is possible for a skilled DIYer to carry out this type of repair.


Spalling is a common issue in architectural stonemasonry that can lead to significant aesthetic and structural damage if left untreated. However, by understanding the causes of spalling and taking appropriate preventative measures, it is possible to maintain the integrity and beauty of stone structures for many years to come.

Whether you are a professional stonemason, a building owner, or simply a lover of historic stone architecture, understanding spalling is crucial. By recognising the signs of spalling, taking steps to prevent it, and knowing how to repair it when it does occur, you can help to preserve our architectural heritage for future generations.

Preserve Your Stone Heritage with A F Jones Stonemasons

If you’re facing the challenge of spalling in your stone structures or looking to prevent it from occurring, A F Jones Stonemasons is here to help. With over 160 years of expertise and a blend of traditional craftsmanship and modern technology, we offer comprehensive services from design to installation. Our experienced team is equipped to handle projects of any size, whether it’s a grand architectural endeavour or a small, bespoke commission. Embrace the full potential of stonemasonry and ensure the longevity of your buildings by making an enquiry with us today. Visit our contact page to get started on preserving your architectural heritage.

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