Cob in Cornwall - The Cob Specialist
Leslie Cornell Snr - the most trusted Cob Wall Specialist in Cornwall - has over 20 years of experience with cob building techniques that includes repairs, builds, lime rendering and pointing. Les Cornell Senior is based in St. Austell, Cornwall and performs cob wall surveys and is trusted and well known for his extensive knowledge and experience with historic buildings, their maintenance and renovation.
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Cob in Cornwall

A History of Cob in Cornwall

Recent estimates suggest that over a quarter of the worlds’ population live in dwellings constructed of earth. Earth buildings can be found across Europe from Russia to the Med, while there are also many countries in the Middle and Far East that boast an unprecedented amount of structures and homes constructed from un-baked earth. In the UK, this tradition is represented by an abundance of earth-built structures in Cornwall and this building medium is what we, in the West Country, call cob.

cob-in-cornwall-map

Cob buildings, which were typically thatched, can be found across Cornwall and are generally recognisable by their thick, lime-washed walls, small windows and doors and ‘iced-cake-like’ chimneys. Proportionately, most of these buildings are white, however there are a few that continue with localised tradition and adopt a pink hue (hogs blood, mixed with lime-wash, was used as an additional water repellence to the external walling). Constructing with cob was [is] also seasonal and would traditionally have taken place from the arrival to migration of swallows. Of course, this makes perfect sense because swallows build their nests with a form of cob during the dry, summer months. An old saying goes ‘given a gud ‘at n boots an it’ll last forever’.

So…….cob in construction represents historic building methods using locally sourced and renewable/recyclable materials like no other building medium. If we consider the Cornish cottage holistically, where else can you build a dwelling from the contents of a meadow or field that guarantees energy efficiency and epitomises sustainability? Remove labour costs…and it’s free to boot!

cob-in-cornwall-1A reflective percentage of Cornwall’s sub-ground soils are considered to be some of the best in the UK for earth construction. Principally, there are two reasons why……firstly, they contain clay (between 10 and 25%) and secondly, they comprise of well-graded aggregates from coarse stones/gravel to fine silts. The combination of these elements ensures a building medium that balances initial consolidation with ultimate strength. Simplified, cohesion is achieved by molecular attraction which bind together the surfaces of the clay particles and forms a coating around the aggregates and silts, bonding them together. There is one more important ingredient that mustn’t be disregarded and that is straw. Most commonly in Cornwall, barley straw was [is] used because of its non-brittleness and readiness of availability. Again, there are two fundamental reasons why straw is used to make cob. Primarily, when straw is introduced at the mixing stages, it functions as a conduit for the clay, silts etc to be evenly distributed, whilst binding the mixture for ease of usability. Secondly, the integrated presence of the straw will ensure any shrinkage cracks are distributed (through the drying process) preventing any large cracking to develop and which could, in turn, ultimately lead to structural compromise or failure.

A typical (but not definitive) ratio of components to achieve a usable and effective cob mixture are; 4 parts sub-soil, 3 parts straw and 1 part water (all by volume). Undoubtedly, our forebears would have experimented with what was available and many more probably ‘had a nose’ for what was good practice.

Once a suitable soil had been located (more often than not in the vicinity of the proposed construction) excavation, using hand tools, would begin. Suitable soils are commonly found some 400mm – 600mm below the top or upper layers of the local topography and are generally easily accessible.

Making cob

These clayey seams vary in dimension and depth across the County although some raw material would have almost certainly been collected from the surfacing of tilled or ploughed fields. Interestingly, there are a substantial number of cob buildings in Cornwall that are sited into valley or hill sides and this reinforces my theorem that the soils, aggregates and stones that were dug out created a level-plane on which to construct. Excavated soils would be used immediately or in some instances where the soil is too wet or saturated, dispersed in such a way to encourage or accelerate natural drying-out prior to use.

How to build with cob

The soil would then be distributed over a layer of wetted straw. Large stones (over 65mm in diameter) are removed. There are several reasons as to why larger stones were not favoured. Principally, the incorporation of large stones into the walling increased the likely-hood of peripheral shrinkage [of the cob] around them resulting in ‘pockets’ within the cob mass. Too many of these can create weak points within the wall and compromise its structural integrity. In addition, when the time comes to paring back the elevation surfacing, protruding [large] stones offer little in the way of desirability to achieving a uniform, flat finish.

How is cob made

A second layer of straw is introduced atop of the soil and water is added. If too little water is added the necessary distribution of clay will be difficult to achieve and the cob will be challenging to compact when it is placed atop of the wall. Excessive amounts of water will reduce the cob to an unworkable plasticity and will prove impossible to build with.

Working with cob

Thorough treading of the mix (traditionally by labourers, builders and/or cattle) is vital because it ensures an even distribution of the clay and reduces the material to a consistency suitable for building. (There is no other way to successfully make cob other than by compaction. Rotational mixing methods will render the ‘cob’ utterly useless for purpose). If animals were adopted for the mixing process, then inevitably dung would be incorporated into the material. Whilst there is little evidence to suggest that this additional ingredient was essential it was certainly unavoidable.

Each treading is followed by turning the material over, usually with a 3-pronged, short-handled fork. An effective material is realised after several turns of the mixture. A good, workable mixture has been identified to be incredibly successful if the newly mixed cob has been allowed to stand for 24 hours. Whilst there is no readily available evidence to corroborate this practise, I’m sure the builders of old were more than familiar with this technique.

What is a cob wall made of

Cob walling was always constructed from a stone plinth….although [I hear] there have been written commentaries suggesting that some cob walls were built straight off of the ground. (Needless to say, that there may not be any surviving examples of this fashion). A stone plinth height can vary from 450mm to first-floor level or higher. There was never a vogue locally (or regionally), plinth height was determined by availability, affordability and/or ease of conveyance of materials. Whatever the height of the plinth, its primary purpose is to ensure that the base of the cob wall is suitably distanced from ground-level in order to prevent inevitable saturation and certain collapse. Of all the cob walls I have seen in Cornwall, there may be one or two that indicate a likeness to each other, however, there is a widespread consistency of variation in their cob to stone proportions and gathered intelligence reveals that cob has been used either profusely, sparingly or equally.

Cob construction

Early stone plinths had a common binder consisting of sieved clayey soils, fine aggregates and slaked lime mixed together to form a ‘mortar’. This mortar was used in the same way to that of how contemporary building mortars are used today when constructing with stone, brick or block, however its porosity permits evaporation of moisture from within the wall. As the mortars are more permeable than the materials they bond, old, solid walls are often dependent upon evaporation of moisture from the joints to remain acceptably dry. Correspondingly, the thickness of the walling, commonly 450mm (approx.), undoubtedly would have been relied upon to achieve adequately damp-free conditions internally. Noticeably, solid wall construction has no cavity unlike modern construction, therefore the need to ensure moisture evaporation is wholly dependent on not just its binding mortars but the permeability of internal and external claddings.

Early stone plinths had a common binder consisting of sieved clayey soils, fine aggregates and slaked lime mixed together to form a ‘mortar’. This mortar was used in the same way to that of how contemporary building mortars are used today when constructing with stone, brick or block, however its porosity permits evaporation of moisture from within the wall. As the mortars are more permeable than the materials they bond, old, solid walls are often dependent upon evaporation of moisture from the joints to remain acceptably dry. Correspondingly, the thickness of the walling, commonly 450mm (approx.), undoubtedly would have been relied upon to achieve adequately damp-free conditions internally. Noticeably, solid wall construction has no cavity unlike modern construction, therefore the need to ensure moisture evaporation is wholly dependent on not just its binding mortars but the permeability of internal and external claddings.

How to build a cob wall

Traditionally, a 3-pronged, short-handled fork would have been used to transfer the newly mixed cob to the [plinth] wall head. Builders and labourers would fork the mixture on to the wall in layers approximately 100mm – 150mm thick, along the walls’ entirety and/or up to an opening like a doorway or window.

Cob house construction

Proposed openings within the cob walling would have either been shuttered or cut out after the wall was complete. (Many moons ago I tried this very idea….cutting out the window opening using an adze and a bar……a day and a half it took me!!)

How to build with cob

Treading the cob on the wall was carried out by the builders, townsfolk and/or labourers who would ensure the cob was compacted and well-heeled. Repeated layering can achieve a ‘lift’ height of up to 500mm per day and each lift would be introduced diagonally, in effect spiralling upward until the required height is reached. Constructing in this manner, ensured there were no vertical joining to the previously laid cob and thereby eliminating the potential of vertical shrinkage and separation.

Cob wall construction

Each lift would be pared back the following day using an adze or flat-bladed tool to achieve a flat, uniform surface and the spoil would be re-used in the next mixing process.

Externally, the finished cob always overlapped the edge of the stone plinth by 75mm and respectively was pared back flush with the stonework internally, establishing an overall wall thickness of 600mm (approx.)

Cob home builders

The [external] ‘overhang’ enjoys much warranted adulation, it’s pleasing to the eye and typifies the vernacular of a traditional cob cottage, however, our predecessors were not so much influenced by aesthetic but more by practicality, durability and function. Creating an overhang serves a purpose, chiefly the external elevations are exposed to the elements, in particular the rigours of a Cornish south-westerly, and are therefore susceptible to erosion (the majority of cob walls in Cornwall in need of repair and/or no longer exist are south, west or south-west facing). Whilst a good overhang may deflect rainfall away from the base of the plinth (arguable), its primary function relies on the increased surface area it presents…a greater surface area encourages accumulated moisture to evaporate more rapidly, essentially when there are drying winds and/or sunshine. Subsidiary to this central function, and what mustn’t be overlooked, is the sacrificial role that this ‘additional’ cob plays. If we understand that the most important part of a cob wall is its base and that this can be subjected to high loads from not just the cob above but the roof structure and possibly the internal flooring, potential failing/s to this area could lead to partial wall malfunction at best or localised/complete wall collapse. Whilst a degree of erosion is expected, on balance sacrificing the cob surfacing is considerably less of a bother.

Cob structures

Historically, cob walls had no additional weather protection other than the overhang of thatch (typically half a metre or so). Whilst this afforded some alleviation from the Cornish climate, inevitably wind-driven rain would have, over time, instigated the onset of erosion to the cob wall surfacing and especially to the exposed corners of a cob building. Lime-wash somewhat improved resistance to the elements with several coatings applied annually. Regular applications delivered an increase of layering of up to 5mm in thickness and were particularly effective. With modern-living offering scarce release, this once ritualised task has taken a back seat and is being replaced with a more desirable (perhaps) and dependable lime render.

Leslie Cornell Senior