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Cob Building

By: Chris Nickson - Updated: 17 Jan 2017 | comments*Discuss
 
Cob Clay Sand Straw Overhang Foundation

Making buildings with dirt is an idea that's been around almost as long as man has been on earth. We've all done it -as kids most of us built little things with mud. Cob building, a tradition from Cumbria and Southwest England, is like that, but on a bigger scale. It was used for centuries, dying out in the 1800s until interest in sustainable housing sparked a revival. Kevin McCabe made waves when he built the first new English cob house in 70 years in 1994 (and with four bedrooms and two storeys, it wasn't small). Another new building, Cobtun House in Worcestershire, won an award in 2005 and sold for a staggering £745,000. But cob buildings can even be made on a shoestring budget; an Oregon man built a liveable cob house for under £500!

The Traditional Building Technique

The traditional material for English cob was soil (clay-based) mixed with water and straw, sometimes with crushed flint or sand added. People shovelled or stamped the mixture together, after which a cob fork was used to ladle it onto a stone foundation, before workmen on the walls trod it into place. It was quite possible to lay a course or "lift" of cob between 150mm and 900mm high (but usually averaging 450 mm) in a single day. After it had dried - which could take up to a fortnight - the next lift would be added. The walls would be trimmed to plumb and straight as they rose and made between 500mm and 900mm thick. The builders would either leave openings for windows and doors, adding stone lintels as they went, or carve them out later. It was a community effort, with men working one day a week to build a house in a season.

Modern Construction

Unsurprisingly, perhaps, modern cob techniques remain much the same. The biggest development has been Oregon cob, where people mix the material into mud loaves, then add them individually to the wall before treading them in. This method means houses can have walls that are stronger and thinner (generally 300-500mm thick on load bearing walls, as little as 100mm on others).

Materials

You can make your own cob, even if you don't live in an area with a heavy clay content in the soil. Just mix soil, clay, sand and straw to a consistency like dough, and start your wall.

Construction

You're going to find the process very labour-intensive. The good part is that it's very environmentally-friendly - all done by hand. In fact, you can have the walls take on any shape you wish, something you'll find in many adventurous modern cob homes. The only thing you really have to remember is to lay the material in courses, and let each one dry thoroughly before laying the next, and make sure they taper slightly as they rise. You can embed windows and doors as you build, or simply cut holes later.

Cob is excellent for load bearing, meaning you can easily make a two-storey house, and it has very good insulation for both heat and noise. During the day it absorbs heat outside, so it's cool inside, but at night radiates that heat into the interior.

Foundation

You need to start with a foundation that's wider than your wall will be - 300mm wider is recommended - and deep enough for the load. Usually you'll build a cob house on a stone plinth or a concrete base, raised off the ground about 600 mm - for obvious reasons it can't come in contact with the ground. There are any number of foundation options you can use, like a rubble trench, earth bags, or even rammed tyres.

You'll also need a good roof overhang to protect the cob, at least 200mm. If you find any vertical cracks, use cob or even clay tiles to fix it before moisture can penetrate.

Rendering and Moisture

Cob has to "breathe" - to dry out naturally after becoming wet. It used to be that the exterior walls were either left bare or lime rendered (which is expensive these days). Excessive moisture can give you a problem, but you also need a balance, as the material needs some moisture (3-5% is considered good - much higher than that and you might have rising damp).

U-Value

Cob generally exceeds the minimum u-values for a house. It rises higher if you put plaster inside and render the exterior. You can also use stone facing on the base wall as insulation.

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My builder has put 5 new oak lintels in place however the oak lintels separated therefore leaving approx7/8mm gap between each beam .I can't clamp back together . How can I fill the gaps to make it look like one beam? Thanking you Dancin ders
Dancinders - 17-Jan-17 @ 8:51 PM
Hi I have a medieval coband stone hall house with a cross passage. Over the. Over the passage we're 3.Large rotten oak lintels.My cob builder has put 5 new oak lintels in place however the oak lintels separated therefore leaving approx7/8mm gap between each beam .I can't clamp back together . How can I fill the gaps to make it look like one beam? Thanking you Dancin ders
Dancinders - 17-Jan-17 @ 8:50 PM
Hi, my partner and I have been wanting to build our own cob home for quite a while now. We've recently had our first child and we want to raise our children in our own cob house, preferably in the countryside. We're interested in living off the land as it's something we started previously - growing our own food, farming animals on a small scale. What we're trying to find out is if there are any communities that we can join where building our own cob house would be a possibility? We've been part of a community before and we loved it, but this time it's really important to us that we can build our own living space. We've looked into buying land, but it's just not a possibility in the near future, so we're really just trying to see what our options are. If anybody could help us with this, we would be very grateful, thank you!
Jacinta - 23-Nov-16 @ 10:04 AM
WE MAKE A BIO FOAM,CASTOR BASED FOAM IN MERSEYSIDE LIVERPOOL IT WOULD COMPLIMENT THE COBB HOUSE, I NOTICED ON TV THAT BOARD WAS USED SPRAY FOAM INSULATION HAS HIGH INSULATION VALUE AND WOULD FOLLOW LINES OF COBB, 00151 653 8753 if you require further information
BIO FOAM - 1-May-14 @ 2:31 PM
Hi I am about to undertake a dissertation on: The Environmental Benefits of Cob Compared To Traditional Block/Brick Buildings, the title still needs a little work I know but you can get the jist, my question to you all really is whether anyone knows where I may obtain actual u-value data or if anyone can suggest a good reference book I may purchase ? Thanks in advance
Luke - 6-Nov-13 @ 5:37 PM
Some pics would be goog, of the process
Sarge - 25-Jan-13 @ 2:16 PM
Thank you for the good reference to cob building. I am about to build a cob dwelling but have discovered that the clay content of the subsoil is below the proportionrecommended in modern times. Although, in the vicinity, there are a lot of old(in excess of 100 years) cob buildings, which probably share a similar sub soil,building control are keen that we conform to the modern scientific approach. I have been experimenting with adding puddling clay to the subsoil and have yet to find a satisfactory method. Please, does anyone have a similar experience or ideas to suggest a way forwards? Thanks in advance. Harpo
Four Winds Harpo - 28-Oct-12 @ 4:58 PM
Cob building has been around for centuries because it’s a good and simple idea. As you say, it’s labour intensive, but that actually gives you a much greater involvement in the house, and you can change the design a little as you go if you choose. The houses might not last forever, but well-built it was outlast the builder.
diane - 10-Oct-12 @ 12:43 PM
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