Parts of this building were heated during the winter months, this has been verified by somebody who actually worked there during the 1940s, could this be the reason for the roof i wonder? I would say the floor was installed a good 100 years after the building was first put up, like many parts of the site, there has been several different phases of alterations.
The photo below was taken during the 1984 miners strike, the part of the building in question is to the right and runs up to the wall on the left, if you look you can see a large pipe running up to the roof. Looks to me like this was for a heating system, which again, would explain the arched ceiling.
