Cob is the most basic building material you could possibly use. Take some sub-soil containing a mix of clay, soil and sand, add some straw (wheat or barley are most commonly used), mix it together…and you have cob. It can be made weatherproof with an outer rendering, and must have a foundation which keeps it clear of the ground. Add a roof with an overhang of at least 20cms, and you are well on the way to having a cob house.
Why build with cob?
It’s inexpensive, and it’s also very easy. Anyone can build with cob, although it takes time. Each course of cob, the width of the wall and roughly 50cms (20″) tall, must be allowed to dry before the next can be placed on top of it, or the whole lot sags and collapses. Only the weather will decide how long this will take, although a week will usually be enough. When close to dry, cob can be shaped with a sharp blade, mattock, or even by hand. The outside face is usually smoothed with a mallet which adds compression as well as a smoother finish. Walls should taper towards the top. The overall result can be a wonderful, organic form with a personality completely different and far more engaging than anything built with bricks and concrete.
Characteristics of cob
Cob can be built without ‘shuttering’, an example of which is the wooden form into which concrete is then poured. Cob is load-bearing, and there is no need for any kind of internal frame structure. Door and window openings can be added as you go or cut in later, but shrinkage (clay shrinks as it dries) may mean the lintels lose support at either side, so it’s important to make them long enough to prevent this. In the UK cob usually takes between six to nine months to finish shrinking, but a whole cob home needs twelve to fifteen months.
Cob provides extremely good insulation, and a typical home will use up to 20% less energy to heat than a conventional build. It ‘breathes’, so it absorbs and releases moisture. Because of this, concrete should never be used as an outside render. As the cob expands and contracts with the seasons a concrete render will crack, allowing water can get in – but not out again. Instead, it sinks to the foundation and sits there until enough has accumulated to cause the entire wall to collapse. Instead of concrete, a breathable lime wash render should be used on exterior walls instead. For similar reasons, damp proofing should never be added to interior walls as this also traps water in the cob. Well-maintained cob will never be damp.