21", used as formwork, has 21 millimetre deep V-shaped pits. Echo Clapping hands or snapping one's fingers whilst standing next to perpendicular sheets of corrugated iron (for example, in a fence) will produce a high-pitched echo with a rapidly falling pitch. This is due to a sequence of echoes from adjacent corrugations. If sound is travelling at and the corrugated iron has a wavelength (pitch) of this will produce an echo with a maximum wavelength of that order, which corresponds to a frequency of 4500 Hz or so (approximately the C above top A on a standard piano). The first part of the echo will have a much higher pitch because the sound impulses from iron nearly opposite the clapper will arrive almost simultaneously. Corrosion Although galvanising inhibits the corrosion of steel, rusting is inevitable, especially in marine areas – where the salt water encourages rust – and areas where the local rainfall is acidic. Corroded corrugated steel roofs can last for many years, particularly if the sheets are protected by a layer of paint. See also Chattel house Metal roof Nissen hut Quonset hut Theorema Egregium, for more information on why corrugation increases strength Tin tabernacle References External links Corrugated Metal Roofing & Paneling Heritage Roofing in Victoria, Australia (PDF, 181 kB) Steels Building materials Corrugation Roofing materials