On our profiled metal roof the rust always seems to start at the sheet ends and along the side laps, then work its way in. The flat areas of the sheets are still clean. Is this something I need to deal with now or can it wait?
That is textbook cut-edge corrosion and it is worth acting on rather than leaving. When sheets are cut to length the steel core is exposed at the edge, and with no coating protecting it that bare steel rusts and creeps back under the factory coating along the laps. Left alone it undermines the coating, lifts the laps and eventually leads to perforation, and UK weather with its wet, salt-laden air only speeds it up. Caught early it is straightforward to treat by cleaning back the edges and applying an edge treatment or coating system to seal the exposed steel. The guide to cut-edge corrosion explains the mechanism, and a survey will tell you how far it has already travelled.
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