Cob Construction

Cob is nothing but the earthen construction, known by different names in different cultures. The word 'COB' meaning 'a lump or loaf' in old English, refers to the mud building system common in old Britain and former English colonies like Australia and New Zealand, which uses no form-work, no wooden supports and no bricks.

Cob is made from sand, straw and clay based subsoil mixed with water to produce moist and malleable building material. It is quite similar to adobe but here higher straw percentage is mixed in. Instead of creating the uniform blocks, cob is normally applied by hand in large gobs where it can tossed by one person to another during the building process. Traditionally cob was mixed by stamping the ingredients together bare feet, but it can also be combined by hand or mechanically by using the backhoe for mixing. This stiff mud is then built up into walls while still wet in courses. Each of the course was sewn into the prior one while both are still pliable. As the bottom of the wall becomes firm and dry, more cob is added on top until desired height is reached taking along the windows, doors, elect. conduits, plumbing, cabinet anchors etc.
This gives a strong, hard and monolithic earthen wall reinforced with interwoven high tensile strength straw fibers.
Advantages :-
1. Durable, resulting in buildings that will last for 100s of years.
2. weather resistant even in harsh climate of Britain coastal line.
3. more resistant to earthquakes as no mortar joints.
4. walls don't burn, rot or get eaten by insects
5. enormous thermal storage capacity, making it ideally suited to passive solar designs.
6. extreme fluidity of forms, cob can be shaped into expressive sculptural forms.
7. Made entirely form natural and recyclable materials, consuming virtually no energy and producing no pollution in manufacture.

Disadvantages :-
1. Extremely thick walls, impractical for smaller plots or tight corners.
2. slow to build with since each layer of 450-600 millimeters takes 3 to 4 days to dry.
3. Variability of applicable soil types requires slightly different ratios of mix ingredients.
4. Cob is susceptible to water damage.
5. Must be built on a masonry foundation to protect against rising damp, and gutters, drainpipes and an overhang roof must be properly maintained to prevent excessive moisture penetrating the walls.