Earth is a material that's been used for centuries, but it's green credentials have caused a recent renaissance
Clean and Green
The eco-credentials of building with unfired earth are pretty impeccable. You don’t need to use energy or chemicals to process it and there are few transport costs – you can dig it up from the garden or the field next door! It’s non-toxic and recyclable, and lasts for hundreds of years.
Earth walls provide fantastic levels of thermal insulation, so you use less power heating the building in winter and it stays cool in summer.
Green issues split the earth building community. Rammed earth can be mixed with small quantities of cement to stabilise it. Hard-line earth builders believe this destroys the green credentials – and the point – of building with earth. So is stabilised rammed earth nothing more than ‘brown concrete’? Bill Swaney of the Stabilised Rammed Earth Company says no. ‘We use up to seven per cent cement and it prevents the walls being water soluble. We also use granulated blast furnace slag – a byproduct of the steel industry – as a stabiliser which is environmentally friendly.’
History and Geography
Earth building isn’t a new-fangled idea. Before cement was invented in the 19th century many buildings with constructed with earth. Stone was expensive and reserved for religious buildings and houses for the gentry, so timber and earth were the only alternatives. The UK has thousands of earth buildings, some over 400 years old. They’re most commonly found in the West Country and East Anglia.
Some people move into an earth building without even realising it. Once rendered, earth homes aren’t immediately recognisable. You might expect a curvy look, but in Georgian cob houses the popular style of the time dictated straight edges. Linda Watson of the Centre for Earthern Architecture at Plymouth University says that the most important thing to remember when moving in is to use materials that breathe. ‘Cement renders and plasters cause a lot of damage to earth walls because they prevent them from breathing.’ This can lead to walls cracking and possibly even collapsing as the trapped moisture causes damage.
Getting Your Hands Dirty
Not every earth house is built in the same way. There are three methods used to build with unfired earth:
Cob – ideal subsoil for cob is made up of equal parts of clay and silt, sand and fine gravel. This is mixed with water and straw to a stiff but malleable consistency and is built up by hand in successive layers about 60cm thick until the required height is reached. Each layer is left to dry before another is added.
Adobe or clay lump – an earth-straw mix like the one used for cob is made up, but then placed in timber or steel moulds to form bricks. These are dried out and then used to construct solid or cavity walls using traditional bricklaying techniques. Instead of cement, the bricks are bonded together with earth or lime mortar.
Rammed earth or pisé de terre– this method uses a drier earth than the other two and is suited to chalkier soils. Loose damp earth, containing no straw, is placed in layers 10-15cm deep between formwork similar to that used for concrete, then compacted manually or using power tools. The formwork can be taken away immediately and there is no drying time.
Where to find it
You don’t have to look farther than under your feet to find a perfectly good material to build with. Traditional bricks are made from clay, which is then fired, but there are other methods that use unfired earth to build everything from full-scale houses to elements such as fireplaces and sculptures.
It is the subsoil, found 20-30cm under a layer of topsoil, which is used for building. But not every type of soil is appropriate to use for building. The simplest test to see if your soil will work is to look around. Are there other earth-built houses in the area? If not, the chances are there is a reason why.
Stony areas are not suited to earth building, but places where there is a high proportion of clay are. Adam Wiseman of earth build specialists Cob in Cornwall describes a simple test anyone can carry out to see if their garden soil will work. ‘Dig down a foot to reach the subsoil and put some into a jam jar with water and detergent. See how long it takes for the clay, grit and water to settle. From this you can see how high the proportion of clay is in your soil.’
There are downsides to building with earth. It’s labour intensive and can be time consuming, especially if you have to wait for sections to dry out in a damp climate. Earth builds can be vulnerable to damp. The saying goes that earth buildings need ‘a good hat and a good pair of boots’. This means solid stone foundations that rise up about a foot out of the ground and a roof with generous eaves protecting it at the top. Many people choose to protect their earth walls, often with a lime render or
sometimes timber cladding.