Cologne Cathedral - iron roof construction
The roof structure of Cologne Cathedral impresses not only by its sheer size but also by the modernity of its construction. At first glance, the original roof structure of the 19th century, erected over the vaulted ceilings of apse and chancels between April and June 1860, is reminiscent of a modern, post-war creation, this due not least to the orange protective paint coating. Built just under three decades before the Eiffel Tower was erected in Paris, the roof structure of Cologne Cathedral represents one of the most significant mementos of early ironwork construction.The weight of the entire roof structure, including nave, apse and the two chancel arms, amounts to about 180 tons to which you must add the weight of the lead cladding. You can then gain some impression of the total loads being exerted on foundations and walls constructed in the Middle Ages. However, the main aisle walls of the cathedral would also have been strong enough to cope easily with a roof structure made of solid oak beams.
The roof structure based on Zwirner's original concept and drafted by his then assistant and eventual successor Voigtel envisaged a construction comprising 40 monumental half-timbered spans. These were assembled from rolled girders and their cast base plates were anchored in the walls down either side of the main aisle (Hochschiff). A latticework of differing thicknesses of intermediate spars and beams with additional slim diagonally arranged round rods holds the structure together. Tension rods running at right angles to the main aisle lend strength to the roof construction. The majority of individual rolled profiles and castings are riveted together, although some are also secured with bolts.
The ironwork in the cathedral's roof structure withstood the heavy bombing of World War II almost unscathed - including 14 direct hits and innumerable fire bombs. This provides some retrospective corroboration for the futuristic decisions taken by Zwirner.
Following its completion, the roof structure received two coats of asphalt-based paint to protect it against corrosion, although this proved inadequate. Indeed, serious damage was only averted because the process used to manufacture the ironwork ensured that virtually no rust occurred. Between 1968 and 1992, painters from the cathedral workshops removed this asphalt-based paint with wire brushes in a series of stages, then applied two coats of a lead-based paint to which the orange top coat seen today was applied.
