The building material Britain almost lost

For centuries, roofers around Stamford worked with a remarkable material that depended on winter itself to make it usable. Collyweston slate was quarried from underground beds, exposed to frost and then split by hand into thin roofing stones. Then, in the 1960s, the industry effectively disappeared.

Today, Collyweston slate is being produced again, but its story is a fascinating example of how a construction material can vanish when the supply chain, technology and the skills that support it disappear.

Historic England describes Collyweston slate as an important part of the character of the towns and villages around Stamford in Lincolnshire. Production stopped in the 1960s, leaving owners of historic buildings increasingly dependent on reclaimed slates from other roofs or concrete replicas when repairs were required. The material had become a victim of its own peculiar manufacturing process.

A roof material made by winter

Collyweston slate is not slate in quite the way most people imagine it. The stone is a limestone that occurs in thin beds underground. Traditionally, it was quarried in December and exposed to the winter weather.

Then the frost did some of the work. Water penetrated the stone and froze, causing it to split along its natural bedding planes. The resulting pieces could then be worked into roofing slates. It was an extraordinarily low-tech process that depended on a very specific combination of geology, climate and craftsmanship and that became a problem.

As the industry declined during the twentieth century, production eventually ceased altogether in the 1960s. Historic England notes that increasingly scarce suitable material led to reclaimed Collyweston slates being taken from other roofs for repairs, while concrete replicas were also used.

There was a particular irony in the disappearance. The technique depended on natural frost, yet changing weather conditions made reliable production increasingly difficult. Historic England's research found that fewer cold winter frosts had become a major obstacle to reviving traditional production. A material that had been produced using the winter climate for centuries suddenly needed modern technology to reproduce the same effect.

As a result, Historic England became involved in research to find a way of recreating the frost-splitting process artificially. Trials were carried out with Sheffield Hallam University, eventually leading to the installation of an industrial freezer at Apethorpe Palace. Experiments continued for almost two years before a reliable method was established.

The process was then taken a step further. A Collyweston slater and miner with more than 60 years of experience was brought in to train contractors in producing the material. The technique subsequently became commercially viable and an old mine was reopened.

This is where the story becomes bigger than a roof tile. The material could be recreated, but the knowledge required to make and use it also had to be recovered.

You cannot manufacture the skill

This is one of the recurring problems with traditional construction. A material can disappear because nobody produces it anymore, but even when production is restored, there may be nobody left who understands how it should be quarried, prepared, laid and repaired.

Historic England's wider research into traditional building materials makes the same point. The availability of appropriate materials is closely connected to the survival of traditional industries and craft skills. Its current guidance says that authentic traditional materials can help retain the character of historic buildings while supporting the specialist industries and skills needed to maintain them.

That creates a difficult cycle. Fewer buildings use the material. Demand falls. Suppliers disappear. Skills decline and repairs become harder and more expensive. Alternative materials take their place and eventually, the original material can become almost impossible to obtain.

This is why Britain has lost more than one material. Collyweston slate is an unusually good example because its production has been revived, but it is part of a much wider story.

Historic England's current research identifies traditional materials that are now difficult or impossible to obtain, including particular building stones, thatching materials and regionally specific products. It notes that many historic quarries have closed, making suitable replacement stone difficult to source for conservation work.

Thatching provides another warning. Historic England says that before the mid-twentieth century, thatching materials were largely sourced locally. Since the Second World War, increased movement of materials and labour, together with greater standardisation, has diluted many regional traditions. Water reed and timber spars are now often imported, while traditional thatching skills are also being lost. In other words, construction did not just become more industrialised. It has became less local.

So, does that matter to modern construction? It matters particularly when repairing historic buildings. Traditional buildings were constructed around materials with very different physical properties from many modern products. Historic England points out that materials such as earth, stone, brick and timber can absorb, store and release moisture and heat, helping regulate conditions within the building. Replace those materials with something that behaves differently and the consequences can extend beyond appearance.

The wrong repair material can trap moisture, accelerate deterioration or alter the way the building performs. That is why Historic England's current guidance stresses compatibility when choosing materials for historic building repairs.

What happens when the old material disappears?

The Collyweston story offers an encouraging answer. Sometimes a lost material can be brought back, but doing so may require archaeology, scientific research, modern technology and crucially, people who still understand the original craft. That may be the real lesson from Collyweston slate.

Construction technology does not always move forwards by replacing old materials with new ones. Sometimes progress means finding a way to keep something valuable from disappearing altogether and somewhere beneath the roofs around Stamford is a reminder that a building material can be much more than a product. It can carry the geology, climate, skills and history of an entire place.

FAQs about Britain's forgotten building materials

1. What is Collyweston slate?

Collyweston slate is a distinctive roofing material made from thin beds of limestone found around Collyweston and Stamford in Lincolnshire and Northamptonshire. It has been used on local buildings for centuries.

2. Why is Collyweston slate unusual?

Unlike conventional slate, Collyweston slate is produced by splitting limestone along its natural bedding planes. Traditionally, frost was used to help split the quarried stone into thin roofing pieces.

3. How did frost help make Collyweston slate?

Quarried stone was traditionally exposed to winter weather. Water entered the stone and froze, helping it separate along natural planes. Skilled workers could then split and dress the material into roofing slates.

4. When did Collyweston slate production decline?

Traditional production declined during the twentieth century and effectively ceased during the 1960s. The loss of suitable stone, changing construction practices and declining specialist skills all contributed to its disappearance.

5. Why was it difficult to replace Collyweston slate?

Its appearance and physical characteristics are distinctive, while the traditional production process required specialist knowledge. Reclaimed slates became an important source for repairing historic roofs, with modern replicas also used where necessary.

6. Has Collyweston slate production been revived?

Yes. Research supported by Historic England explored ways of reproducing the traditional frost-splitting process artificially. This eventually involved using an industrial freezer to recreate the conditions traditionally provided by winter weather.

7. Why are traditional building materials important?

They can be important for maintaining the character and performance of historic buildings. Traditional materials often behave differently from modern products, particularly in how they handle moisture, heat and movement.

8. Can modern materials damage historic buildings?

They can if they are incompatible with the existing construction. Historic England warns that inappropriate materials can alter moisture movement and contribute to deterioration, which is why material compatibility is important when repairing historic buildings.

9. What happens when traditional construction skills disappear?

Repairs can become more difficult because finding the correct material is only part of the problem. Specialist knowledge may also be required to quarry, prepare, install and repair it correctly.

10. Is Collyweston slate the only traditional material Britain has lost?

No. Historic England has identified numerous traditional materials and techniques that have become difficult or impossible to source, including some regional building stones and traditional thatching materials.

11. Why have traditional building materials disappeared?

There is rarely one reason. Changes in quarrying and manufacturing, reduced demand, industrialisation, regulation, availability of alternative products and the loss of specialist skills can all contribute to the disappearance of traditional materials.

12. What can modern construction learn from Collyweston slate?

The story demonstrates the importance of understanding local materials, traditional skills and the environmental conditions in which buildings were constructed. It also shows that preserving construction knowledge can be as important as preserving the materials themselves.