Soaring from 18m above ground level at its eaves to 30m at mid-span, St Pancras station was hailed as a marvel of its age. Its uncluttered interior gave the Midland Railway Company a terminus that adapted to the advent of new railway technologies, while the roof's robustness meant little was needed in terms of maintenance.
The 4.8m pan roof was composed of tied lattice trusses, made up of riveted, wrought plate and channel iron, fabricated and erected by contractor Butterley. They were positioned at 8.9m centres over the station's 213m total length.
Truss purlins and diagonal bracing stiffened the structure longitudinally; three curved rafters were riveted to the purlins between the main arches.
Cladding consisted of slate over timber boarding on the roof's lower quarters. So that the station interior would be flooded with light, the topmost half of the roof was glazed with a 'ridge and furrow' design giving the station a shimmering, corrugated skyline.
During the blitz, Luftwaffe bombers scored a direct hit on St Pancras, blasting off much of the roof's cladding and destroying a pair of the intermediate rafters. The station was patched up, with slate replacing the roof's glazed portion.
Aside from this intervention, though, 'the roof had been almost totally neglected for maybe 100 years', notes Werner Hennies, field architect with Rail Link Engineering (RLE), which is directing the major overhaul of the station now under way.
In 2007, St Pancras Station will be reborn as the terminus for the Channel Tunnel Rail Link.
As part of a £385M contract which has included construction of a new steel and glass station extension, a huge remodelling operation is under way to strengthen the station floor for Eurostar trains and partially open up the undercroft to daylight - the floor support girders used to tie the roof's main arches are being rendered redundant, with new reinforced concrete floor slabs taking over the role.
At the same time, the roof's ironwork is being given a major overhaul and repaint - and it is being entirely reclad. The aim is to restore St Pancras to its high Victorian splendour at the same time as equipping it for life at the leading edge of 21st century railway operation.
Hennies observes that, structurally, St Pancras is an efficient building, measuring up well to modern standards. And first impressions of the glazing work, now half complete, are that RLE has overlaid state of the art cladding on its iron skeleton, fusing the old with the new.
But, Hennies is quick to point out, St Pancras is Grade I listed and every detail of the current refurbishment is tightly vetted by English Heritage (NCE 28 April).
'We took possession of the site in 2003, and found we'd inherited a heap of rust. The ironwork was terribly corroded - inspecting the roof was like being in a rust shower, ' Hennies recalls. This initially caused alarm - the section of some members was severely eaten away - and Arup, one of the members of RLE, was tasked with analysing the roof to reassure that there was enough strength in it to cope with anticipated dead and live loads.
The weight of the wrought iron alone is approximately 90kg/m 2 in plan; new glazing weighs in at 30kg/m 2 and the slate at 50kg/m 2. The station is being designed for a 120 year life and will have to cope with all kinds of potential weather extremes during that time.
Though some localised repairs have been needed in the form of re-plating, to everyone's relief, the structure as a whole came through the analysis with flying colours.
Cladding design was, in a sense, easy, says Hennies.
'English Heritage and Network Rail put old drawings on the table which showed how the shed was originally. Design was all a matter of copying it with new materials.' Slate over board has been replaced with new Welsh slate laid over 'crinkly tin'. Timber framed glazing with lead lined gutters - all originally manufactured on site - is being replaced with a bespoke, prefabricated system built up of aluminium frames and components.
Every detail of the new glazing system had to satisfy English Heritage, which has kept an eagle eye on the dimensions and visual weight of glazing bars, the colours of materials used, and perhaps most stringently, on the interaction between old and new.
'We've not been allowed to drill or cut any part of the original structure, ' says Hennies.
As with the original glazing system, the connection between the glazing and ironwork is being achieved by clamping to the top flanges of the main arches and rafters.
The glazing consists of five principal elements: Specially cast aluminium upstands are clamped down to the arches and rafters by means of stainless steel 'Lindapters' - L-shaped brackets that hook under the top flange and are bolted tight. On to the upstands, gutter sections are fitted, which in turn support glazing panels, forming the 'furrows' in the ridge and furrow glazing pattern.
Where glazing panels meet at each ridge, 650mm above the furrows, they are connected to specially cast aluminium replicas of an original roof detail - to provide ventilation in the station the apex of each ridge was originally left open. Screwed joints to connectors hold the glazing panels firmly in place.
And a fan-shaped profi le above the ridges provides a mounting for semi-circular ridge caps that run, unbroken, from one side to of the circumference of the station vault to the other.
'Aside from the clamped connections the skin is self supporting, and closely resembles what was there before, ' Hennies says The highly repetitive nature of the glazing system being installed belies some of the difficulties being encountered as work advances. Hennies' team had to design in a high degree of flexibility to cope with imperfections in the ironwork's geometry.
Reports from 1870 record that the deflection of the main arches once the massive timber staging was struck was, on average, 4.8mm, with a maximum recorded deflection of 6.3mm. Little additional deflection had taken place over time. However, RLE found that many of the main arches and rafters were not perfectly vertical or parallel.
To accommodate this, the connectors between upstands and I-beams had to allow a relatively high degree of longitudinal flexibility.
And it was found that the bomb-damaged rafters had been hurriedly replaced with straight sections of iron.
Packing has been required to accommodate the deviations of these members from the smooth curvature of their neighbours without pulling the cladding out of shape.
There is no firm completion date fixed as yet, but Hennies - and English Heritage - is happy with the speed at which his part of the huge CTRL project is racing ahead.
Sometime in 2007, no doubt, he and the rest of his team will raise a bottle to St Pancras' next century.
Work is being carried out with the aid of a vast suspended scaffold. Decking laid immediately beneath the ironwork has enabled unfettered access and dramatically reduced the risks of working at height. It has also ensured that workers at platform level will not be hit by any falling tools, materials or debris. The roof's ability to support the scaffold has also proved beyond doubt the original structure can cope with heavy loading.