Devon Earth Building FAQ
1 Q. How are earth walls constructed?
A. There are three principal building methods, all of which are based upon traditional construction techniques. Because DEBA is primarily concerned with the technique of earth building called "cob" which was commonly used in the past in the South West of England, this website concerns itself largely with this technique. For the sake of comparison the other two principal earth building techniques are also described below, but detailed information on these must be sought elsewhere.
1. Cob, in which subsoil containing ideally approximately equal proportions of clay and silt, sand and fine gravel is thoroughly mixed with water and straw to a fairly stiff but malleable consistency, then built up off a masonry plinth, or underpin course, in successive layers about 600 mm high, allowing each one to dry out before the next one is laid, until eaves level is reached. Walls are normally 450 to 600 mm thick. The Cob Building tradition in Devon is described in the leaflet The Cob Buildings of Devon - [ see reading list ] on this site.
2. Rammed earth, or pisť de terre , is formed by compacting moist subsoil inside temporary formwork (shuttering). Loose, damp (rather than wet) soil, containing no straw, is placed in layers 100 to 150 mm deep and compacted using either manual or pneumatically powered rammers. When the soil has been adequately compacted the formwork is removed, usually straight away. Construction can proceed continuously, without waiting for successive lifts to dry out. Wall thickness varies, typically, from 300 to 450 mm.
3. Adobe or clay lump is an earth/straw mix similar to that suitable for cob construction which is placed into timber or steel moulds to form bricks or blocks which, when dried out, can be used to construct solid or cavity walls. Normal bricklaying techniques are employed but with the blocks being laid in either an earth or composite earth/lime mortar.
2 Q. Can you mend cob?
A. A. Certainly. Various techniques are set out in the DHBT leaflet The Cob Buildings of Devon 2 and in Earth Building by Laurence Keefe – see the reading list on this site.
3 Q. How much does it cost to build in cob?
A. The cost is very variable and depends largely on the labour price, as the materials are inexpensive, especially if they can be sourced on site. If they have to be brought in from elsewhere, then the cost rises proportionately.
4 Q. How do I know if my soil is suitable for cob building? Can I get my soil tested to see if it is suitable?
A. If you live in an area where there are existing cob buildings and your soil looks similar to that used in these, then it is likely that it will be suitable. You can always experiment to see how well it works. If however you want to embark on an ambitious project then you should get your soil tested - the following links are suggested www.buildsomethingbeautiful.com or email David Clark at Plymouth University: firstname.lastname@example.org
5 Q. I have a plot of land -what would I need to do to be able to build upon it? What sort of problems are likely to arise when Planning Permission and Building Regulations approval is being sought for a new earth-walled building?
A. With regard to Planning Permission, earth-walled buildings would be treated in exactly the same way as those with walls of masonry or timber-framed construction. Any objections raised would normally be concerned only with the appropriateness of location, design and finishes, not with construction materials and methods. Compliance with the Building Regulations can be achieved by supplying sufficient data and adequate constructional details to enable the Building Control Officer to make an informed assessment of the proposed building (over the past 15 years, numerous earth-walled buildings have been built in Britain with full Building Regulations approval). Compliance with Building Regulation Approved Documents is not necessarily mandatory if another method can be shown to be as effective in achieving the desired end. The recently revised requirements relating the thermal performance of external walls (Approved Document L1) can cause problems. However, there are various ways in which compliance can be achieved, by reducing the 'k' value (thermal conductivity) of earth walls and/or by enhancing the insulation of other building elements. Similarly UK Building Regulation also require that:" The walls, floors and roof of the building shall resist the passage of moisture to the inside of the building" and this may be able to be achieved by allowing effective evaporation of ground moisture from the plinth. The various ways in which compliance with Building Regulations can be achieved are fully described in Earth Building by Laurence Keefe-see the reading list on this site
6 Q. Where are cob buildings found?
A. Cob buildings are found throughout the world except perhaps in the Americas. In the UK the strongest tradition of cob building is in the South West, particularly in Devon where many thousands of cob houses and farm buildings survive dating from the 14th century to the later 19th century. Cumberland also had an ancient tradition of cob building. Throughout the rest of the UK cob buildings are less common but are also to be found alongside other vernacular building techniques.
7 Q. How much straw do you put in a cob mix?
A. This cannot be specified precisely as it depends on circumstances. Typically it will be 2.0 to 3.0% by dry weight according to nature of the soil used. This is roughly one [small] bale of straw to a cubic metre of soil.
8 Q. Do you use shuttering to build in cob?
A. Shuttering can be used to build cob but it is not necessary and makes the construction process more complicated.
9 Q. Do you add cow dung to cob?
A. No! This is a common but untrue belief.
10 Q. Do you know of any practical courses in the UK on cob building?
A. Various people and organisations
hold courses. In Devon the following contacts are suggested:
Kevin McCabe - Tel: 01404 814270 www.buildsomethingbeautiful.com
J & J Sharpe - Tel: 01805 603587 www.jjsharpe.co.uk
Mike Wye - Tel: 01409 281644 www.buildingconservation.com
Abey Smallcombe - Tel: 01647 281282 www.abeysmallcombe.com
Yarner Trust - Tel: 01288 331692 www.yarnertrust.co.uk
Centre For Earthen Architecture, Plymouth School of Architecture - Tel: 01752 233630 www.tech.plym.ac.uk/soa/arch/earth.htm
Devon Rural Skills Trust - Tel: 01803 615634
Centre For Alternative Technology -Tel: 01654 702400. www.cat.org.uk
11 Q. What books should I read on cob building?
See the reading list on this site. There is a much more extensive reading list in Earth Building  by Laurence Keefe.
12 Q. What are the environmental benefits of using raw (unfired) earth as a building material?
A. The construction process consumes virtually no energy and produces no environmental pollution (zero CO2 emissions). The material is eminently recyclable and is thermally efficient. It can often be dug from the building site eliminating transportation.
13 Q. Are earth walls really durable, and as structurally sound as those of conventional masonry construction?
A. Yes. Provided they are properly maintained, and protected from the effects of rising and penetrating dampness, well-built earth walls can last for hundreds of years (there are, in south west England and Cumbria, cob buildings that are known to date from the 14th and 15th centuries).
14 Q. What are the appropriate external and internal finishes for cob buildings?
A. This topic is covered fully in the "Appropriate Plasters and Renders etc" publication which can be found on this website. The fundamental principle is to achieve vapour permeability in finishes on earth buildings to avoid saturation. DEBA for this reason advises strongly against the use of renders and plasters which incorporate Portland cement as this heavily reduces or prevents such permeability. Similarly the use of traditional finishes such as limewash or distemper is to be preferred.
15 Q. Would a damp-proof course need to be incorporated in a new earth wall? Are damp-proof courses appropriate in existing cob walls?
A. Traditionally, earth walls relied on the fact that, being of "breathable" construction, rising dampness was drawn out by evaporation through external wall faces. However, compliance with Approved Document C of the UK Building Regulations would probably require the installation of a dpc in a new masonry plinth (underpin course). Chemical injection could be appropriate in the very rare occasion where there is a solid brick plinth [although there is doubt about its efficacy even in these] under a cob wall but it is not appropriate for stone plinths in which voids make the injection application patchy and ineffective. Moreover specialist dpc installers also require the plinth to be rendered with a 3:1 sand:cement mix after they have carried out the injection and this drives the rising dampness of the failed areas into the base of the cob. Modern tanking systems can also cause damp to rise into the cob if the plinth is low.
16 Q. Do earth walls need to be protected from the effects of weathering and erosion?
A. The essential protection for earth walls is provided by the building’s roof and there is no absolute necessity to protect the walls. In the past, cob farm buildings in particular were often not rendered externally. However, other than in cases where, for example, buildings are of only one storey and have deeply over-hanging eaves together with a high masonry plinth (in excess of 600 mm), some form of protective wall covering is normally recommended. It is essential to use renders, plasters and paints that are vapour-permeable. Advice concerning appropriate finishes for earth walls can be found in DEBA leaflet "Appropriate Plasters, Renders etc" on this website.
17 Q. Are cob buildings more prone to structural failure than other types of traditional construction?
A. Cob buildings are not more prone to structural failure unless they are misunderstood and in consequence not looked after properly. Because they are susceptible to the effects of excessive moisture, it is essential to ensure that correct finishes are used, as when cement rich renders are applied, rising dampness can be driven into the base of the cob, especially where the plinth is low and sheer failures may then occur as the cob becomes saturated and loses its coherence. Again, when gutters and downpipes are not maintained and water flushes down cement-rich renders, penetrating cracks, moisture is trapped and failures occur.
18 Q. What are the thermal/insulation properties of cob?
A. Cob is well know for being a very comfortable material in which to live; in part this is due to its excellent thermal properties, being a combination of high Thermal Resistance and Thermal Storage and good humidity regulation.
However, from a building regulation point of view (in the UK) only Thermal Resistance is taken into account. An 860mm thick cob wall, lime plastered and rendered, bringing the total thickness to 900mm, [which is much thicker than a traditional cob wall] will give a thermal resistance of 0.45w/m/k. In practice in a new build situation it is usually necessary to put extra insulation in the plinth to compensate for this perceived inadequate value.