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Corrugated galvanised iron, commonly abbreviated CGI, is a building material composed of sheets of hot-dip galvanised mild steel cold-rolled to have a linear corrugated pattern in them. The corrugations increase the bending strength of the sheet in the direction parallel to the corrugations, but not across them. Normally each sheet is manufactured longest in its strong direction.
CGI is lightweight and easily transported. It was and still is widely used especially in rural and military buildings such as sheds and water tanks.
Pitch and depth
The corrugations are described in terms of pitch (the distance between two crests) and depth (the height from the top of a crest to the bottom of a trough). It is important for the pitch and depth to be quite uniform, in order for the sheets to be easily stackable for transport, and to overlap neatly when making a join. Pitches have ranged from 25 mm (1 inch) to 125 mm (5 inches). It was once common for CGI used for vertical walls to have a shorter pitch and depth than roofing CGI. This shorter pitched material was sometimes called "rippled" instead of "corrugated". However nowadays, nearly all CGI produced has the same pitch of 3 inches (76 mm).
By eliminating a frame, nails and their holes are also eliminated, so that the corrugated iron remains waterproof. The sheets are usually galvanised to prevent the iron from rusting.
History
CGI was invented in the 1840s, and was originally made (as the name suggests) from wrought iron. It proved to be light, strong, corrosion-resistant, and easily transported, and particularly lent itself to prefabricated structures and improvisation by semi-skilled workers. It soon became a common construction material in rural areas in the United States and Australia and later India, and in Australia also became (and remains) the most common roofing material and is even used in urban areas (in which application it is usually painted) but not as commonly as in rural areas. For roofing purposes, the sheets are laid somewhat like tiles, with a lateral overlap of two or three corrugations, and a vertical overlap of about 150 mm, to provide for waterproofing. CGI is also a common construction material for industrial buildings throughout the world.
Wrought iron CGI was gradually replaced by mild steel from around the 1890s, and iron CGI is no longer obtainable - however, the common name has not been changed. Galvanised sheets with simple corrugations are also being gradually displaced by zincalume or powder-coated sheets with complex profiles. However CGI remains common.
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See also
Iron | Metals | Construction
"Corrugated galvanised iron"