Corroded laps and fixings on profiled metal
The industrial roofs around Ingate Place and the market fail at the laps and the fixings rather than through the sheet. Water tracks under an end lap, the fixings corrode from the underside where nothing is visible, and the sheet itself is often still structurally sound. That distinction decides the whole job, because a roof failing at its details can be waterproofed while a roof that has genuinely corroded through has to be re-sheeted.