St Pancras Overall Roof

St Pancras station was designed by William Henry Barlow. The approaching line to the station crossed the Regent's Canal at height allowing the line reasonable gradients; this resulted in the level of the line at St Pancras to be 12 to 17 ft (3.7 to 5.2 m) above the ground level.

The lower area was used for freight, in particular beer from Burton and as a result the undercroft was built with columns and girders, maximising space, set out to the same plans as used as those used for beer warehouses, and with a basic unit of length of that of a beer barrel.

To avoid the foundations of the roof interfering with the space beneath, and to simplify the design, and minimise cost, it was decided to construct a single span roof, with cross ties for the arch at the station level. The arch was sprung directly from the station level, with no piers as seen here. The arches ribs have a web depth of 6 ft (1.8 m), mostly open ironwork. The span width, from wall to wall is 245 ft 6 in (74.83 m), with a rib every 29 ft 4 in (8.94 m). The arch has a slightly pointed design. The Butterley Company was contracted to construct the arches. The total cost of the 24 rib roof and glazing was over £53,000, of which over half was for the main ribs. The cost of the gable end was a further £8,500.

The single-span overall roof was the largest such structure in the world at the time of its completion.

The materials used were wrought iron framework of lattice design, with glass covering the middle half and timber (inside)/slate (outside) covering the outer quarters. The two end screens were glazed in a vertical rectangular grid pattern with decorative timber cladding around the edge and wrought iron finials around the outer edge. It was 689 feet (210.01 m) long, 240 feet (73.15 m) wide, and 100 feet (30.48 m) high at the apex above the tracks.

[Information from Wikipedia]