What is cob?
Cob is a mixture of sandy-sub soil, clay and straw. It is mixed by crushing the particles together by either dancing on it or using the head of a digger. Historically cob might have been mixed by farm animals who would walk up and down on the sand, clay and straw. The sandy sub-soil must be sharp and ideally contain angular stones and gravel – this will make it stronger. About 75% of cob is made up of this sandy aggregate. Any type of clay can be used, but be careful not to use silt which can sometimes appear like clay.
How long does cob last?
The oldest cob house still standing is 10,000 years old. Cob is strong, durable and cob houses should stand forever as long as their roof is maintained and the property is looked after properly. In the UK we ensure our cob houses last for hundreds of years by incorporating a few basic design features that make them suitable for our climate. These include: a gravel foundation to stop the capillary action of water, a 50cm stone or brick stem wall to keep the cob off the ground, a roof that overhangs by about 50cm and a lime render on the external walls.
Why is cob so good?
Cob is the most sustainable form of building there is. The materials for your cob walls are usually excavated from your foundation trench and on-site. This means there is no manufacture or transportation of materials. Many so called ‘eco homes’ claim to be green because they are cheap to run once built but the materials used to create them, usually have a massive carbon footprint. In contrast cob is genuinely as ‘eco’ as you can get, as it has almost zero embodied energy. Since cob is made of the earth it is also entirely recyclable and non-polluting.
Cob is affordable – your cob walls cost you nothing! As long as you have some land to build on, anyone can afford to build cob walls. Our cob expert Kate Edwards was originally taught by Ianto Evans who built his own cob house in the USA for only a few hundred dollars. And she built her own cob-bale home for about a tenth of the cost of a conventional home.
Cob houses are breathable and healthy to live in. There is no damp in a cob house.
Cob houses require almost no heating. On our courses we teach you how to design your cob house on passive solar principles. We show you how to use cob on the south facing walls to make the most of its excellent thermal mass, and straw bales on the north walls to provide excellent insulation. The Edwards cob-bale home is heated by a single cob fireplace as we used these passive solar principles in our build.
Cob houses are beautiful. When you create a cob house you literally sculpt it. Cob allows you to easily create curves, and carve shelves and features into your walls. Of course cob houses don’t have to be curvy though, they can be made with straight walls and right angles for that modern look.
How easy is it to build with cob?
On our earth building workshops we encourage anyone to join us – you definitely do not need any previous building experience to build with cob. Cob is so easy to work with – it is a lot more forgiving and less precise than working with bricks. Of course you have to learn to keep your walls exactly vertical, but if you find your wall is slightly out you simply shave off some cob with an old saw or add a bit more cob. Fitting windows and doors is so easy too – you can add more cob around the frame if your gap is too big, or chip bits away from the cob if you hole is too small! On our cob house building courses we show you how easy it is to fit your windows, doors and even roof.
How does cob compare to other earth building techniques?
Kate’s background as an artist makes cob the perfect sculptural medium to work with and so cobbing is her preferred method. However there are other earth building techniques which are made of a similar mix of sand, clay and straw and these include clay-lump (mud bricks), wattle and daub (used for internal walls usually), rammed earth (you need to be good a carpentry to make your wooden shuttering first), adobe (another form of mud bricks), and rammed chalk. We prefer cob as it is the most accessible form of earth building there is – due to its simplicity it allows more people to build their own eco homes.
See how to build your own cob home with this BBC film of our sustainable building courses here in Norfolk, UK.