Cob is an old technique that offers the potential to create very sculptural wall shapes. Many of the old cob cottages in the South Island have survived from the last century.
Broadgreen House in Nelson is a historic double storey cob building open to the public
For cob construction, straw and often small gravel is mixed into a sandy soil. The stiff mixture gets formed into lumps or cobs, which are then thrown on to the wall and stamped or worked into the previous layer. The rough surface is later trimmed up, and usually rendered to give a smooth surface. The result is often a softly undulating surface, which can follow whatever shape you choose to build into the wall.
Cob builds extra thick or curved walls easily, and it is common for them to taper inwards towards the top.
This technique is very shrinkage sensitive and a mixture has to be found that minimises shrinkage. Because of the comparative lack of modern experience in New Zealand with cob, it is included in the Earth Building Standards only as an informative section, so careful analysis of materials and design is required.