Historically, cob and lime have been used as building materials for thousands of years. Incredibly versatile and natural materials, you can still see many examples of its use throughout the UK and all over the world. Recent years have seen a resurgence in cob and lime building due to its low environmental impact; as a natural product, it is considered highly eco-friendly. As well as this, there are several other benefits of its use in new and old buildings.
What is Cob and Lime?
Cob is made from a mixture of sandy sub-soil (which is the layer of soil just under the surface), clay and straw. Traditionally, the mixture was stamped on as a way of combining the materials, and often livestock was used to assist. These days a digger is commonly used for crushing the elements together. Once compacted, the mixture is left to dry for 24 hours before it would then be built into walls upon a stone plinth. The purpose of the plinth was to elevate the base of the structure from the ground and, therefore, moisture.
Lime is an ancient and natural form of render and mortar that works in a similar way to its modern counterparts. Lime mortar is a sticky substance and is used to coat the outside of the building to provide protection from the elements. You will often see the roofs of traditional cob and lime buildings have a large overhang; this is to give more protection. A limewash is then used on the external walls to provide additional protection, particularly from wind and rain, which over time can cause cob buildings to erode. Lime has many benefits for this type of use as it is a durable material that also allows the building to breathe, which can help to prevent problems such as damp. For this reason, cob and lime have been used together for centuries.
History in Building
The use of mud in building houses dates back centuries and is likely to be one of the first materials ever used in ancient construction. Murals from Ancient Egypt have been discovered that show building with earth was a highly advanced technique by the year 2500BC. Today it is estimated that up to one-third of the world’s population live in earth-built houses.
In England, the term cob dates back to around 1600, and even then, the term was to describe an ancient building material. Classic cob buildings in the UK tend to carry similar characteristics. Walls are generally around 61cm thick, with deep-set windows and doors. These walls have an excellent thermal mass value, meaning the buildings retain heat in the winter and allow the house to cool in the summer. The colour of the exterior would vary depending on the soil used, although most range from creamy white to a pinkish brown.
Cob building became commonplace in the UK during the 15th century, notably in the South West of England and in Wales, where many structures still remain. Colonials also took the technique across the world to New Zealand and North America.
Cob and lime building remained active until the industrial revolution, where fired bricks became mass-produced and more readily available. The art was considered somewhat lost for some 70 years until in 1994 when property businessman Kevin McCabe built a four-bedroom, two-story home using traditional techniques. Since then a number of new cob buildings have been erected. In 2005, a modern cob building won the Royal Institute of British Architects’ Sustainable Building of the Year award. There are now several specialists who design new cob buildings and who can advise on the maintenance and repairs of existing ones. The methods used today only deviate from the techniques of the past very slightly, with the help of machinery to combine the cob mixture.
If you live in a traditional cob building, as structural engineers in Cornwall, Martin Perry Associates have extensive knowledge working with historic building materials. We are able to provide structural reports on cob buildings and those built with lime mortar as well as specify repairs and design additional works and extensions. Please visit our website or call us on 01579 345777.