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.
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).
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.
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.
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).
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.