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Cob is made from subsoil (sand and clay), water and a fibrous organic material, typically straw. For a strong cob mixture you need around fifteen to twenty-five per cent clay, seventy-five to eighty-five per cent sand and one part straw. Always test samples and different ratios for strength before building, as natural materials are different from place to place. If you can’t use subsoil from your property, try calling local landscapers and building sites or look on community websites for free subsoil. Cob is also extremely fire-resistant.
Mudbricks are made by mixing subsoil and water, and sculpting the mix into brick-shaped forms which are left to dry naturally. Sometimes straw and other fibres are added to reduce the shrinkage in the drying process. Mud bricks are extremely fire-resistant.
Oat, rye, wheat and rice straw are commonly used materials for making straw bales, but not hay (which can germinate and breakdown in the walls). Use previously compressed bales (building grade) rather than regular straw bales
Light earth (also known as light straw clay or insulating straw clay) is not structural but used as infill, and is best for retrofitting walls or cavities due to its great balance of insulating and thermal mass properties. It is made from loose straw, lightly coated in a clay slip/slurry, which is tamped into formwork between
a post and beam structure. Once dry (two to four months), the walls are rendered with an earth or lime render.
Hempcrete is made by combining water, hemp fibre and a lime-based binder. It is both fire- and pest-resistant. Similar to light earth, it’s an infill material: you use formwork to pack the material into
sections, usually a load-bearing timber frame.
Rammed earth is a mixture of gravel, clay, sand, cement and sometimes lime or waterproofing additives. It is most easily machine-compacted into removable formwork, resulting in a water-resistant, load-bearing, long-lasting wall.
Sustainable timber is a renewable material that absorbs carbon from the atmosphere while growing, and stores it for the life of the building. Australia has many native hardwoods that are great for building. Timber can be used sawn or left in the round.