Mud, Mud, Glorious Cob
Estimates suggest around 33% of the world's population live in houses constructed of unbaked earth. Earth buildings are found throughout Europe, from Scandinavia and Russia to the Mediterranean. In Britain this tradition is represented by earth buildings in the South West, and predominantly in Devon. Where the county's clayey sub-soil was mixed with straw to form what is commonly known as cob.
Cob buildings which are typified by thatched cottages with thick walls and rounded corners are dotted throughout the Devon countryside and represent the survival of a method of building based upon the energy efficient use of locally available renewable and re-cyclable materials. A material which provides thermal qualities equal to, or better than, most conventional houses. But as with many houses built using traditional techniques the use of modern materials changes the equilibrium between the building and the environment in which it exists. The result, a slow deterioration in the number of cob buildings.
To address this issue the Devon Earth Building Association have forged ahead in a bid to educate people on the correct way to maintain their cob properties. The aim of this article is to provide a taster to the kind of detailed help and information which can be provided by the association.
Cob, What is it
The soils of Devon are believed to be among the best in Britain for earth construction. There are two reasons for this. Firstly the soil in the region contain clays which are fairly course and therefore does not expand and contract excessively. Secondly the soils are normally well graded giving a even spread of coarse gravel through to fine sands. When the clay is well distributed throughout the soil its forms a coating around the silts, sands and gravels, effectively bonding them together.
When old cob walls are examined straw is found. The straw helps to bind the material into workable clumps for builders to use. The other function of the straw is to distribute shrinkage cracks throughout the wall during the drying process. This helps to reduce large cracks in the material thus reducing the likelihood of future structural failure.
A good well graded clayey sub soil containing plenty of straw required no further ingredients for it to be effective. Although after the 1850s lime was some times added to the mixture to achieve a faster set.
Cob walls are built off a stone plinth, which can vary in height from around 450mm above ground level up to first joist level in some domestic buildings. Once the stone plinth was constructed soil from the locality was broken down into a fine tilt, with large stones being removed during the process. The tilt is then spread out in a bed some 10cm in depth on a hard, pre-wetted surface on top of a thin layer of straw. Water is then added and a second, thicker layer of straw is spread on top. The straw is then trodden into the soil by either the builders, cows or oxen. Leading to the clay in the soil being evenly distributed and rendering the mixture suitable for building.
The mixture is then lifted with a small fork with a slightly curved wooden handle onto the plinth where it is laid in layers and thoroughly trodden in by workers on the wall. The material always overlapped the stone plinth by around 10cm, with surplus being removed before commencing the next layer.
From the 1820 onwards the technique was altered slightly to enable builders to construct town houses and terraced cottages where some degree of symmetry was required. The new technique involved the use of timber shuttering. When this technique was employed the straw and soil was poured between the shuttering which represented the internal and external wall faces and trodden in.
Dampness in Cob Walls
The major cause of failure in cob buildings is excess moisture. When too much moisture is absorbed into a cob wall, the clay particles which bind the wall together are forced apart, leading to the wall becoming plastic in nature and finally failing. The precise point at which cob walls fail due to moisture content is variable and depends on the soil type and clay content of the wall. For example cob wall constructed with sandy soil deficient in clay have been known to fail at relatively low moisture levels.
Because cob walls are built on stone plinths, rising damp should never be a problem as long as external land drainage is adequate. Particular attention needs to be paid to the build up of soil surrounding the stone plinth. Overtime this can build up leading to water penetration. A French drain surround the house and simply reducing the soil level to well below the internal floor level should prevent such damp in the future.
Because of cob's coarse structure the capillary movement of moisture is very restricted. Tests have show the vertical movement of moisture in an unrendered cob wall will not normally exceed 15 to 25cm. Because of this a chemical damp proof into the base of a cob wall may well be a complete waste of money, providing the cob wall is covered in a lime render which allows any moisture to evaporate before it becomes a problem
Early cob walls were not originally rendered with lime plaster resulting in moisture evaporating quickly from the exposed cob surface. But in time it generally became accepted to render cob with lime plaster which helped to protect the material from erosion and also ensured any moisture within the wall was allowed to dry out.
However, the modern day trend has moved from lime plaster to cement with householders attempting to square up their properties with the use of angle beads and metal lathe. In their quest for modernisation householders are destroying the physical properties of their homes which help to maintain their physical soundness. Cement-rich impermeable render increases the weight of cob walls by up to 10%. But more importantly because the cement is inflexible cracks appear allowing rain water to penetrate the wall. Which in turn gives rise to dampness because the water is unable to evaporate from the cob. The most usual way in which these problems manifest themselves is when the cement render becomes detached from the wall in areas. Once, detached from the cob, water penetrates the cement render and runs down the inside of the wall soaking the foot of the wall.
Causes of Decay and Structural Failure in Cob Buildings
As with all buildings, evidence of cracks, can appear alarming. But in many instances they can be remedied without turning to extensive rebuilding or reinforcement.
Movement, cracking or decaying cob walls can result from the following:
- Over time structural timbers such as ties across trusses are removed or decay giving rise to wall spread.
- Failure to maintain thatch can result in rain water leaking into the wall head resulting in the decay of roof timbers and the soaking of cob wall leading to reduced load bearing capacity.
- If the external ground level has risen above the stone plinth moisture may enter the side of the cob wall.
Repair of Cob Buildings
Cob is represented by a homogenous material rather than thousands of individual units such as bricks held together by mortar. Therefore structural repairs and reinstatement are different.
Following advice from the Society for the Protection of Ancient Buildings most conservation builders aim to repair and consolidate problem areas of a property without causing physical disruption. This ensures the long term stability and optimum performance of the building.
But with cob a problem exists. When attempting to repair cob with new cob the drying process results in shrinkage. This in turn leads to an unhappy repair because the material fails to bond with the existing material. Recent work suggests these problems can be overcome and cob can be used in conjunction with lime and non-ferrous metal supports in cases where structural movement has not been arrested.
Materials to avoid when repairing cob are cement, cement and cement. By using this material to repair your home you are likely to make any structural problems worse as well as increase the likelihood of damp.
Cob houses form an important part of our heritage, with their gently rounded corners and undulating surfaces held in high esteem by house hunters. Don't damage or remove these features, invest in a quality cob builder to undertake work on your home. You will be rewarded with a home which maintains its unique character and the 'look' which house hunters dream off.
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