Natural fibres, grasses, leaves
Considering that various living creatures build shelters out of leaves, grasses and natural fibres, these materials were perhaps the earliest building materials of mankind, where caves or other natural dwellings were not available.
These materials are available in continuous supply in all but the most arid regions. In some places, they constitute the only useful construction material available, in others they are used together with a variety of additional materials.
The common features of these vegetable (cellulose based) materials are their renewability and their low compressive strengths, impact resistance and durability. Single fibres, grasses or leaves are usually too weak to support their own weight, but in larger quantities, when twisted, interwoven, bundled or compressed, can be used for various structural and nonstructural applications in building construction.
• Natural fibres (such as sisal, hemp, elephant grass, coir) as reinforcements in soil constructions or fibre concrete and other composite elements (eg fibre boards).
• Natural fibres, twisted to ropes, to tie building elements together or to produce tensile structural members, especially in roof construction.
• Straw for thatch roofs or for making particle boards. In an industrial process, compressed straw slabs (Stramit) are produced by heat and pressure, without any binders, but with paper on both sides.
• Reeds, bundled or tied together as boards, or split and woven as mats, for various uses as columns, beams, wall cladding, sun screens, or roofing material, ores substructure for wattle and daub constructions.
• Leaves, mainly palm leaves, for thatch roofs or for making mats and woven panels for floors, walls and roofs.
Production and installation of Raphia palm leaf tiles, Ghana
• Usually locally available abundant, cheap (or even no-cost), quickly renewable materials (which can also be grown in the backyard).
• Traditional techniques (in most cases), easily comprehended and implemented by local people.
• Thatch roofing, if properly implemented, can be perfectly waterproof and possesses good thermal and acoustical properties.
• Reed constructions have high tensile strengths, good strength-weigh ratio, hence usually good earthquake performance. In case of collapse, their light weight causes less damage and injuries than most other materials.
• Compressed straw slabs have high dimensional stability, and resistance to impact and splitting, are not easily ignitable, and (if kept dry) are not attacked by biological agents. The slabs are used like timber boards.
• In most cases, low life expectancy, about 2 to 5 years, though with good constructions and maintenance useful service lives of 50 or more years are achievable (in the case of reed thatching).
• Vulnerability to biological agents (attraction and nesting of insects, rodents, birds, and development of fungi and rot).
• Risk of fire, either originating within the building or spread through flaming or glowing fragments carried by wind.
• Tendency to absorb moisture, thus becoming heavy, accelerating deterioration and creating unsanitary conditions.
• Low resistance to destruction by hurricanes.
• Deformation and gradual destruction due to impact, structural stresses and fluctuations in temperature and humidity.
• Low acceptance due to general view that these materials are inferior, used only for "poor people's houses.
• Impregnation of materials against biological hazards and fire, either by pretreatment or surface application, similar to bamboo and timber preservation. (Caution: these are costly, and easily washed out by rain, contaminating surroundings and drinking water collected from roofs. Moreover, fire resistant treatments may promote mould growth, leading to rapid decay.)
• Wide roof overhangs and roof pitches of at least 45° help to protect exposed surfaces and drain off rainwater quickly.
• Reduction of fire risk on thatch roofs by application of a coat of stabilized soil on the exterior surface to prevent ignition by wind-borne fragments, and restrict air-flow through the thatch in the event of fire.
• Maintenance of dry conditions and good ventilation to avoid attack by biological agents. In many traditional dwellings, smoke is developed inside the houses to prevent rot and nesting of insects.
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