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“Building with cob fosters a rejection of global homogenisation, monoculture, mass-manufacturing, top-down solutions and high tech approaches. This age old technique can be used in a truly fresh way to encourage regionality, the local, the specific, the appropriate, the low-tech, the simple. It can be connected to a larger movement that is going on in the world right now, that embraces home grown and local production…..” From Weismann and Bryce Building With Cob (2006: 7)
Cob, a mixture of clay, straw and sand, is a natural building material used in Devon, Cornwall and other areas of with clay-rich subsoils Thatched cob cottages are synonymous with the rural Devon landscape in particular and many cob buildings survive in the region, some of which pre-date Queen Elizabeth I.
People have made their homes from locally available materials such as stone, timber and earth for thousands of years. Buildings and other structures made from earth are common in many parts of the world, including Islamic Mosques in Mali, the Adobe brick buildings of New Mexico and parts of the Great Wall of China. Where earth building is most common though is in the vernacular. Around 30% of the world’s population, in particular in developing countries, live in earth buildings.
In Devon and other regions where earth building is traditional, there is a renewed interest in cob in part because people have begun to realise the drawbacks of using modern materials in the conversion and repair of ancient buildings, and the construction of new ones. Beyond domestic houses and barns, cob lends itself particularly well to the construction of garden walls and seats, ovens and sculptures. The clay rich properties of cob mean it is a material perfect for forming rounded curves and niches and expressing the creative processes that can be involved in ecologically friendly and ethical building.
Basically, cob is a mixture of clay-rich subsoil, sand and straw. This means that if the geological conditions are right, it is possible to excavate local subsoil for use as a building material. Traditionally, cows were used to tread together the materials comprising mass cob, however more recently it has been the wheels of a tractor that have done much of the hard work. Once mixed, the cob is forked onto the wall, then trodden and beaten in by hand and foot. Although mixing and building with mass cob can be a valuable communal experience, it is time consuming and labour intensive. Cob blocks are an economical and efficient alternative to using mass cob. For both large and small scale building and repair work, cob blocks are cheaper both to produce and to build with. These can be sourced from small scale local suppliers in both Devon and Cornwall.
Cob is an enormously robust yet flexible building material, but it must be shielded from water ingress. Cob buildings demand foundation plinths or ‘boots’, traditionally made from stonework, and a ‘hat’; a timber roof traditionally covered with thatch, slate, tin or tile, designed to throw rainwater out beyond the footprint of the cob wall. Construction of stone plinths and cob block walls demands the use of lime mortars . These remain durable when set, yet allow movement and settling within cob and stonework whilst maintaining the strength, stability and the breathability of walls. As lime is sympathetic to cob’s flexible and porous nature lime plasters and renders are used to protect cob walls from the elements, whilst also allowing them to breathe. Applying cement based mortars to cob prevents the flow of moisture through walls which can result in structural damage and lead to problems with damp. Find out more about lime plasters and renders.
Look at more pictures of this Jack in the Green cob barn conversion in Devon.
The environmental impact of cob is negligible. Compared to the large scale production and transport costs of modern materials such as concrete blocks and cement, cob and cob blocks are exceptionally sustainable. Digging your own cob requires only a digger and a dumper/tractor and trailer for a day and buying in cob blocks supports small local businesses. As a material, cob is infinitely recyclable in that it can be re-mixed and used again. Any cob demolished as part of our site work is retained and recycled within the new build and used in repairs to the existing building. The use of lime as plaster and mortar also means cob blocks, stones and bricks can be reused, as lime does not stick to building materials in the same way as sand and cement mortars.
Please feel free get in touch if you need advice (or a price) and/or check out our ‘suppliers and links’ list for other useful websites, books and organisations.
Because cob is a traditional material, its application outside is dependent on seasonal conditions. Whilst inside work can take place all year round, outside activities are best between frosts (April-September/October) and in the dry…
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Page last updated by HE 07.02.10.