Cob construction technique involves the direct placement of a mix of moist soil and straw to form a wall.The word cob is derived from the Old English word for ’loaf’, and similar techniques are known by different names in different cultures, such as chineh in Farsi and pakhsa in Uzbek. A similar layered technique has been described in Oman and Iran.
Cob almost always contains short straw or grass added to the earth mixture to provide resistance to shrinkage cracking, and improved strength.
Cob is usually constructed by a team of two people, one working on the wall, and the other at the base, shovelling the cob mix to the head of the wall. A cob wall is constructed in layers 400–600mm high; wet cob is forked into position and compacted at the head of the wall. Compaction takes place either by treading the mixture, or by using long-handled, flat-footed tampers. Where the compacted cob falls outside the line of the wall, it may be shaved from the face to provide a vertical face. Construction progresses with each layer of cob allowed to dry slightly before the wall can be stood on and the next layer placed. Walls are typically between 0.5m and 1m thick, and may taper as the wall rises.
Openings are formed either by adding ‘blank formers’ to the wall, around which cob is placed, or by adding lintels at the correct height during construction and then cutting the openings out after the cob wall is finished.
Because of the free-form nature of cob, it is very simple to construct structures that curve both on plan and on elevation. For this reason, cob is often used as an artistic material for sculpture or temporary structures, and is very simply decorated either by inserting objects into the wall or by creating relief patterns.