Any building constructed using timber and lit by naked flames is a fire risk, and cotton mills were especially vulnerable because of the explosive atmosphere caused by cotton fibres in the air. In the most widely used type of fireproof construction, cast-iron columns support cast-iron beams from which spring brick vaults, or jack-arches. These were covered with a layer of ash or sand and flooring of flags, tiles or floorboards. The first buildings erected using this method in England were mills in Shropshire and Derbyshire in the 1790s, followed by the Philips & Lee cotton mill in Salford in 1801-2. In some cases timber was eliminated from the roof structure as well by using lightweight trussses of cast and wrought ironGlossary Term. Until the design of the cast-iron beams had been perfected, structural instability and defective casting techniques were the cause of a number of collapses of fireproof mills. After improvements in the design, fireproof construction was adopted more widely and it continued in use throughout the 19th century. ReinforcedGlossary Term concreteGlossary Term flooring was introduced in a limited way in the 1880s, as were rolled steel beams, but these materials were not widely adopted in Manchester mills until the 20th century. Fireproof construction was expensive and for this reason timber floors continued to be widely used, sometimes with metal or plaster claddingGlossary Term to give a measure of fire resistance. Heavy timber floors without joistsGlossary Term offered some fire protection because large timbers tend to char slowly without losing structural integrity.

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