St Pancras' roof was, by the standards of the day, astonishingly economical.
'In iron roofs as usually constructed, the depth of the principal is about one fifth of the span, ' Midland Railway chief engineer William Barlow told the ICE in 1870, 'but here, by adopting one arch extending across the station, the height from the tie beneath the rails to the crown of the arch became the effective depth of the truss.
'This height being about two fifths of the span, all the horizontal strains arising from the dead weight of the roof, its covering, and the accumulations of snow, etc, would be about the same in the arch of 240 feet span with a depth of truss of 100 feet, as in an ordinary truss of 120 feet span with a depth of 24 feet.
'Excepting, therefore, such additions as might be necessary for retaining the form and figure of the arch, the actual sectional area at the crown, and for about two thirds of the entire arch, did not require to be greater than in an ordinary truss of 120 feet span.' The 'upper chord' of the main truss is 6 feet deep and 16 inches wide.
'There were several other advantages belonging to the arch, ' Barlow noted, 'one being that, as the weight of the roof was carried at the floor line, and did not rest on the tops of the walls, there was no necessity to make the side walls thicker. For not only was the weight on the tops of the walls avoided, but also the raking motion from the expansion and contraction of an ordinary roof, which, though it may be mitigated, is not prevented by the use of roller frames at the feet of the principals.'