What is Cob?
It’s a mixture of clay, sandy aggregate and straw.
Cob is usually made from the subsoil dug up for the foundations of the house. The subsoil is beneath the top soil. Top soil contains organic matter which will break down and is not suitable for building with. Not all subsoil is suitable too much clay causes cracks and too little and it won’t hold together. There are tests that can be done and it’s also good to attend a course to get a feel for the material. For cob it’s difficult to beat the hands on experience. We would also recommend (advice we didn’t take ourselves!) building small structures to start with such as an oven or coop.
Won’t it wash away?
Because of its porous nature, cob can with stand long periods of rain, however, too much moisture is best avoided by a good ‘hat and boots’ that is an overhanging roof and build on a foundation of stone. Cob house are plastered in (or should be) with breathable renders such as lime.
Don’t cob house look like hobbit Houses?
The answer is they can look like anything you want them too. Cob is very versatile, lending itself well to curves and circular designs but it can be built to look exactly like any standard house.
Is it difficult to obtain planning?
We didn’t encounter too many issues with our planning. Any difficulties were not cob related but the same issues that can affect any build such as site lines. Dealing with waste etc… The Mudhouse is quite a standard design in that it has no external curves or green roof. For designs more out of the ordinary you may have to fight your case. Each site has its own challenges its best to have all your research done speak to other natural builders and people who have built in your area to foresee any challenges you may have.
Are Cob houses cheaper to build?
Yes and No, yes in that the material itself is cheap and by building yourself there are savings on labour costs. There can be additional costs related to it, for example at the Mudhouse we had to get a structural engineer to sign off on our house, for regs our house has additional insulation as the Cob does not have a ‘value’ on any of the systems used by building control.
How tall can you build with cob?
There are cob building standing nine stories high with no issues.
How long does a cob house last?
Cob houses are still standing from the 1400’s in England, many are still lived in. There is debate on the oldest cob building in the world. The oldest known cob house is 10,000 years old (Source: https://www.heritagecobandlime.com/why-ancient-cob-will-always-have-a-place-in-our-modern-world)
To learn more about cobbing, get some hands on experience and meet other future cob builders we have two Introduction to cob courses this summer.