An ancient building technique that utilises basic and readily available materials, earthen construction has been a cornerstone of architectural heritage worldwide since pre-historic times. With origins in the United Kingdom dating back to at least the 13th century, earth as a construction material has been favoured on this island for a considerable period of time. Enjoying a surge of popularity in the latter 18th and early 19th centuries, in part a response to the Brick Tax introduced to fund the defence of British colonies in North America, thousands of earthen buildings can still be found today across much of the southwest and East Anglia. Due to the nature of their construction and our unfavourable climate, unfortunately there are limited examples of earthen structures predating this period in the UK.
Earthen construction takes many forms but often found here in the southwest is a form of mass earth walling, commonly referred to as cob. However, names will vary depending on location – this type of construction may be referred to as ‘clob’ in Berkshire, ‘clom’ in Wales, and ‘witchert’ in Hampshire. As we are based in the southwest, we will stick with ‘cob’ for the purposes of this post.
Materials –
While the variations can sound confusing, the principle is essentially the same. In essence, cob construction is an earth mixture which is built in lifts over a masonry plinth. The cob itself is a very basic product comprising of materials you might find in a typical, rural setting:
- Subsoil – some is more suitable that others for construction. Ideally, your subsoil for construction should contain 1/3 each of fines (clay and silt), sand and stone/gravel. Clay will shrink and expand as it releases and absorbs water, so finding the right balance here will be key.
- Straw – straw will act as a binder and mitigate against shrinkage, much like hair or fibres function in a lime plaster. If good quality straw is not available, more rudimentary materials such as twigs or heather can also be utilised.
- Water – the key addition which will ensure the cob is of the optimum consistency for mixing and building.
Mixing –
Once you have gathered your raw materials, you will set about combining them to a workable consistency. Traditionally, the materials would have been mixed by the ‘machinery’ of the time – namely livestock. Deposit your soil and straw in a pen, lead in a few of your cattle herd and leave for a day or two. It is perhaps unsurprising why dung is often found in historic cob mixes. While this method is certainly still feasible, you might opt for a slightly less organic option.
Mixing by hand is a feasible but laborious process, and you will need to be reasonably fit if the cob is to be ready in good time. This process comprises both treading through the mixture in a study pair of wellingtons, and turning it over with a fork while adding water to achieve the desired consistency.
If you need to produce large quantities of cob for a substantial project or new build, the use of modern machinery can be sought such as a backhoe loader or tractor with bucket. The process is no different to mixing by hand but you will find it quicker and easier on the back and legs!
Once mixing is complete, the end product should be a pliable, sticky yet workable material that can be moulded and shaped as desired. Now you are ready to build.
Building –
The stability and longevity of a cob structure is in large part based on achieving a balance of moisture. That means keeping the material reasonably dry but not sealing it off from the elements completely – all cob structures should have a good set of ‘hat’ and ‘boots’. Sufficient drainage will also need to be considered so to minimise the risk of ground movement and subsidence.
You should avoid building cob from the ground up, instead a masonry plinth should be constructed to provide a buffer between the earth walling and the damp ground – the plinth will serve as your ‘boots’. The masonry should be the desired width of your cob wall, likely between 400-600mm. While porous masonry is still susceptible to dampness defects caused by sitting in a wet field, assuming it has been constructed with permeable materials such as lime mortar, it will be resilient and very much functional.
Once your plinth is constructed you are ready to build your first lift. This is effectively a two person job – one person to fork the cob mixture onto the plinth, and a second person to compact the cob into place by treading it down. The cob must also be beaten from the sides to ensure it keeps its shape, rather than splaying outwards. As the lift progresses, it will need to be tidied up to create a uniform, flat face. This is known as paring and can be achieved by standing atop the cob and trimming any overhanging material or overly lumpy sections with a spade.
This process continues until you have reached the desired height. Timber formers for doors and windows should be incorporated as you build as this will be much easier and safer than cutting in openings once the walls are built to full height. Consider also the positioning of timber lintels over doors and larger window openings. At this time, cupboards and niches can also be introduced to the inner face of the walls as required, as well as timber insets which will be used as a firm fixing for door and window frames, shelves and so on.
Next comes your ‘hat’. This refers to the roof covering, design and function of which will be critical if rainwater is to be thrown clear of the cob walls. Thatch is a traditional covering with pairs very well with cob as an organic material. Equally, simpler and cheaper options such a corrugated sheet roofing can be used. Efforts should be made to ensure cob roofs are noted excessively heavy so to avoid overloading the structure and causing issues such as roof spread.
Final Touches –
A cob wall with a good hat and boots does not necessarily need to be rendered. However, if you do wish for a rendered finish then choice of material will be critical. One of the main reasons that cob buildings fail is the use of inappropriate materials, such as cement render. You will no doubt have seen images on the news of earthen structures in a state of partial collapse, often the result of the introduction of cement renders which inhibit the free passage of moisture through the cob, and trap excessive amounts of moisture within the wall, leading to saturation of the material.
If the cob is to be rendered, it is important to use a non-hydraulic lime with adequate capillarity to cater for the daily wetting and drying cycles the wall will be subject to in this country. It is when this process is hindered by impermeable materials that we see an imbalance of moisture within the cob, and subsequent failure. To achieve a strong bond, the lime should be harled on (essentially thrown at the wall) in reasonably thin coats. The cob can be trowelled smooth if desired, but the larger surface area achieved through a roughcast finish will help the wall to dry out faster as effectively you have a greater evaporation front. To decorate, a limewash should be used for full breathability and protection.
The Future of Cob –
Although the widespread popularity cob enjoyed some 200 years ago has made way for modern construction methods, there has been a resurgence in the use of cob for contemporary properties in recent years. Well known examples include Cob House in Ottery St Mary, The CobBauge House in Fakenham and Keppel Gate in East Devon.
The use of cob in the modern day has been driven by a call for more sustainable and eco-conscious building projects, with the environment at the heart of such construction methods. Earthen construction is perhaps the most environmentally friendly means of construction at our disposal. Cob can boast low embodied energy, strong thermal performance and full recyclability, as well as providing a healthy indoor environment through effective moisture regulation.
It is promising and encouraging to see traditional earthen construction re-enter an industry which is not known for its environmental sympathy. For those who enjoy getting stuck in, cob construction is approachable on many levels, from two storey homes (subject to planning permission and building regs!) to pizza ovens. Why not give it a try?
If you are planning your own cob build, or need to undertake repairs to a cob structure, we recommend purchasing this book by Jane Schofield & Jill Smallcombe, which comes highly recommended: