Contents at a Glance..
Cob is one of many methods for building with raw earth. It surpasses related techniques such as adobe, rammed earth, pise and compressed earthbricks, both in ease of construction and freedom of design. You can add-on, cut-out, or reshape anytime, even after the cob is dry.
Working with cob is a sensory and aesthetic experience similar to sculpting with clay. It is easy to learn and inexpensive to build with. Because there are no forms, ramming, cement or rectilinear bricks, cob lends itself to organic shapes - curved walls, arches and vaults. Earthen houses are cool in summer, warm in winter. Unconsolidated, hand-formed earth is probably the world's commonest building material. Cob has been used even in the harsh climates of coastal Britain, as far north as the latitude of the Aleutians. Thousands of comfortable and picturesque cob homes in England have been continuously occupied for many centuries and now command very high market values.
With the soaring price of timber and increasing interest in natural and environmentally-safe building practices, cob is enjoying a renaissance.
Once the basics are understood, cob building is amazingly simple. In just a few days, you can learn how to select materials, prepare a mix, form cobs (ie, lumps of cob) and build a simple wall.
Cob workshops should include other information that you will need: site-selection, foundations, windows and doors, attachment of wood and other materials, detail work and finishing.
Cob is highly resistant to weathering. Because of its porous nature, it withstands long periods of rain without weakening. However, too much exposure is best avoided by the "boots and cap" strategy: the building needs wide roof eaves to protect the walls and an impervious foundation. In windy areas, a stucco plaster or whitewash is traditionally used on exterior cob surfaces but we haven't found this necessary at our western Oregon sites.
No building system is earthquake-proof under every seismic condition, but cob buildings have several important advantages over the other earthen buildings. The curved, tapering walls are inherently more stable than straight ones; if they were to fail, they would tend to fall outward rather than inward. Since a cob building is one integral unit reinforced with straw, like a single adobe block, it has no weak, straight-line mortar joints. Carefully constructed earthen buildings in Latin America have withstood earthquakes of enormous force, while nearby concrete and wooden buildings collapsed.
Cob walls one to two feet thick provide immense thermal mass and adequate insulation, ideal for passive-solar construction. On clear winter days, the cob absorbs energy from the sun, then re-emits it in the form of heat during the night, resulting in an extremely even indoor temperature,
As it is fireproof, cob can be used for building ovens, stoves and chimneys. One of our favourite designs is a cob bench or bed, heated by the flue of a wood-burning stove.
The rate of building depends on the weather and the size of your workforce, but racing to build a structure would be missing the point and halving the fun. Unlike the conventional, modern building-practice with its frenetic pace, power tools, and scope for errors and accidents, cob-making is a peaceful, meditative and rhythmic exercise. In dry weather we build up to a foot (30 cm) of height per day. It is necessary to allow each course of fresh cob to consolidate before applying the next one. Building cob is faster and easier with a crew, so it lends itself to community projects, building parties and workshops.
The basic ingredients of cob are soil, sand and straw. Under normal conditions, the topsoil removed from the footprint of the building is enough to supply all construction needs. Clay soils require the addition of more sand than do silty soils, but the overall quantities of sand and straw needed for a modest cob building are surprisingly small.
Cob is one of the cheapest building materials imaginable. The owner-builder can take great satisfaction in supplying the labour, building little by little in leisure time, or inviting friends to share the excitement of hand-sculpting a whole house. With inventiveness and forethought, the costs of other components (doors, windows, roof, floors, etc) can also be extensively reduced. Our Cob Cottage Company works primarily with salvaged and recycled materials and hand-worked timber. Using local materials such as poles, bamboo, native stone, and cedar shakes, our second cob cottage was completed for about $500.
Most houses are designed to alienate people from Nature. Cob designs can emphasise the character and climate of their site. Through careful placement of openings, focus on views, integrating outdoor spaces and encouraging wildlife, we try to help the residents of our buildings be more aware of the natural world.
Wonderful features have been built into our own home, including a wood-heated cob bench, a cob oven, a heart-shaped window and arched niches in the walls.
There are no laws against building with earth, but there are always local standards and conditions that must be adhered to. Anyone embarking on a cob building-project will need to check their local regulations.
For more info on cob and cob building courses, see the Alternative Directory listing under Builders - Earth, Mudbrick, Adobe, Cob.