The concrete filler-joist floor was a form of fireproof flooring developed in the second half of the 19th century that came to be used quite extensively in industrial and commercial buildings. Iron, later steel, joists embedded in concrete provided a crude form of reinforcing. This form of flooring came to be adopted in Lancashire cotton spinning mills from the late 1870s, but there has been some confusion over the issue, which this paper seeks to clarify. The Bolton architect J.J. Bradshaw was the first known user. Some mill architects followed this lead, but others preferred forms of brick-arch flooring. Filler-joist floors ceased to be used, both generally and in Lancashire cotton mills, after around 1909 as other forms of reinforced flooring became available. Spinning mill construction moved towards the freestanding steel frame, although reinforced concrete framing was not adopted in Lancashire. Lancashire architects have been seen as conservative. However, it is argued that this was not necessarily a bad thing, and that they were willing to use new methods where these were seen as advantageous.