Building Using COB

Cob is an ancient earth building technique which uses a combination of earth, straw, sand and water. These ingredients can be mixed together by hand, using tarps, or by machinery. The mix is formed into lumps or "cobs" which are pressed together to form the walls of a building, rather like building a giant clay pot..

When left to dry the cob sets to become extremely hard and durable, although in wet areas like Britain cob walls need protection from driving rain which will eventually erode them. There are thousands of cob houses still inhabited in England today mainly in Devon and Cornwall. It is estimated that 30% of the world's population live in earthen homes.

A simple technique to learn, safe, non toxic and lots of fun to build with, you can use cob to create almost any structure - from the simple to the fantastic. Cob structures can be finished using different mixes of the same materials, to produce beautiful paints, plasters, floors and sculptural features.

Cob building uses the world's most abundant and sustainable building materials, has very low embodied energy, and its cost is minimal. Virtually all the materials used in the cob building process are bio degradable or can be recycled, making it one of the most environmentally friendly ways of building.

Today there are many cultures who still recognise the advantages of building with earth and draw upon the natural landscape to create their own architecture which is both practical and sustainable. Architecture which reflects the rich variety of human culture, imagination and personal identity.

Why build with earth?

Save - A third of the world's land mass is suitable for building with earth which means the majority of the building material can be found on or near the site. This reduces the cost of transporting and processing the bulk of materials used in construction. It also saves exploiting new resources by incorporating recycled materials into the design. Cob is load bearing and needs no framework. A house made of earth typically uses 60% less lumber than a stud frame building.

Increase - By careful consideration of the site, passive solar design and the integration of indoor and outdoor spaces, cob building increases the awareness of natural systems, our impact on them and how we can build in balance with these systems.

Reduce - Cob building reduces the need for specialist skilled labour and expensive equipment. The construction techniques we use are easy to learn and reduces the amount of money required to build structures. Typically in America a house might cost $200,000. but with these techniques people have built their own homes for less than £1000. The flexibility and fluidity of cob building allows for curvilinear shapes which enclose space most efficiently reducing the size of building required.

Improve - Cob building improves the quality of the environment, the materials used for building as well as finishes such as paints and plasters are non-toxic and non-polluting

As well as the important aesthetic of form, colour and texture and an automatic blending with the local environment (as long as your sub-soil is locally sourced). Cob has several other important characteristics, firstly walls are usually very thick, typically 2-3 feet and this, as well as being very good insulation against heat and cold is a massive heat store. So there is a fly-wheel effect on thermal movement

In other words, cool nights and warm days are evened out as well as when doors and windows are left open resulting in a change and cooling of the air in a building. As soon as they are closed the building warms up again through the heat stored in the thermal mass of the walls. This means that typically a cob house will use approximately 20% less energy to heat compared with a typical modern house meeting the same building regulation insulation requirements.

Another important property often not appreciated is the humidity store of earth walls, they can easily absorb moisture from the atmosphere and release it again when the air dries out thus resulting in an overall higher background humidity than a typical new building. This is much more comfortable and healthier to live with not drying out airways resulting in less susceptibility to throat infections such as colds

There has been much research to endorse this, particularly by “CRATerre-EAG” at the school of architecture of Grenoble in France, and professor Germont Minke of Kassel university in Germany. Most of the new building projects in these countries have been done in either rammed earth (a form of building not native to the UK using a drier mix of earth and shuttering, resulting in a quicker drying time but much less sculptural characteristics), or in adobe (earth bricks). Minke has also experimented with “sausages” of an earth mix on the inside of a highly insulated wall simply to get the benefits of some thermal mass inside the insulation and mainly the humidity store.

The disadvantages of using cob

The fact that it is mixed wet means in a thick wall it will take some months (depending on site conditions usually 6-9 months) after building, for the walls to finish shrinking. Most of the shrinkage will occur in the height of the wall, this means in practice waiting around a week of good weather between each lift (a lift being two feet or so in height) before it is sufficiently dry and strong to take the next lift

Therefore three months is ample time, allowing for a typical English summer to build a two-story building. Although the roof structure can go on a week after the wall plate height is reached it’s best to wait about six months before fitting windows and door frames, otherwise there is a danger of the reveals either side of the opening shrinking and then the lintel crushing the window frame. This means it is usually fifteen months from start to finishing a cob house

Although this is longer than a typical new estate house, it’s not actually especially long for any bespoke one off design building. It’s also worth noting if any green oak is used in the structure for lintels or roof structure then this will continue to shrink for much longer than the cob (about one year per inch thickness of timber).

Another disadvantage is that usually site space is at a premium these days and the simple fact that to pass the latest “part-L” insulation performance requirements for building regulations, the exterior walls will probably need to be three feet thick. This means the whole building will take up a significantly larger footprint and also need a larger roof area all around for the same internal space. However for me as long as the site is not really tiny this is a small price to pay.

Is cob more expensive?

No. The actual cob work with the aid of modern machinery, namely a JCB3CX is suprisingly cheap, it will depend on site accessibility and how much adjustment is required to the onsite sub-soil but usually the cost of the cob itself including labour and materials will be no more than £200 per cubic meter. In practice this means between £15,000 - £20,000 for a typical three bedroomed house.

There are wider footings to consider and a stone plinth which brings the price up a bit and of course if you have many curves it makes most building processes (except the cob itself) more time consuming. In fact the walls of a new house are usually less than 25% of the cost of the whole building. In the case of cob, it’s usually about the same i.e. half of this 25% to put in footings DPC and build a well insulated stone-faced plinth to a height of around three feet and the cob work up to roof height about the same cost again.

Repairing Cob

Cob is a remarkably durable material provided it is treated correctly. In practice this means keeping it dry. Unfortunately as ordinary portland cement has taken over from lime in the building trade largely during the second half of the 20th century builders and home owners have often made the mistake of thinking cement being very strong and waterproof will protect the cob well.

Unfortunately the reverse is true, although it will indeed keep off driving rain well it is very brittle and often cracks due to movement in the much more flexible substrate. These cracks then let in water which is trapped by the impermeable render thus rather than evaporating away when the rain stops this water builds up in each subsequent storm usually sinking to the bottom of the wall where in extreme cases it can eventually lead to total failure of the cob i.e. collapse. This in a wall that might have sat quite happily for over four hundred years prior to the addition of a cement render.

The answer is to stick to either no render at all or a lime putty based render which have been used litreally for thousands of years. The same applies internally, either lime or even better earth plasters work best. Most of my work repairing cob buildings over the last twenty years has been to strip off cement render, carry out repairs necessary, sometimes replacing existing repairs in a more compatible medium namely cob blocks and an earth / lime mortar. The render or plaster can then be replaced with lime or earth mixes, before being decorated in lime wash.

It is often also necessary to carry out roof structure repairs as roofs often spread forcing the top of walls out causing cracks in the corners of buildings. Another common problem is exterior ground levels being too high, the best answer is simply to lower the level to below the interior floor level and make sure any surface water can easily get away. One thing that can never be recommend is to damp proof inside a cob building.

This is again mistaken logic, what must be done is to let the water out with breathable lime or earth plasters inside the building and again lime wash finish is best although there are other breathable finishes available. The net result is a dry wall and a comfortable, healthy, breathing home. A cob house in a good state of repair should never be damp (another popular misconception). In practice it is not possible to get cob too dry and there is no problem (other than the initial shrinkage of the building drying out if it has been damp) with central heating a cob building.