A fire broke out at 5:00 p.m. in an air exhaust duct of a hot rolling mill at a lower-tier Seveso metallurgy plant. The rolling mill had been shut down for preventive maintenance. The flames set off the sprinklers above the duct. The fire was put out by the onsite firefighters and public firefighters. Stripping work was conducted and a search for remaining hot spots was conducted. Multiple hot spots were extinguished inside the ventilation duct as well as in an air inlet duct. No injuries were reported. Damage was limited to a hole in the roof which required the replacement of around 50 m² of roofing materials. The fire was caused by welding work performed on a protective cover of a squirrel-cage motor. A hot particle, drawn into the ventilation duct when it was turned back on, ignited the creosote deposits inside the duct. The corrective actions planned by the operator consist in periodically cleaning the air ducts, identifying the hazardous portions of these ducts, and the possible fitting of fire dampers inside the main air exhaust duct and at the front of the squirrel-cage motors with a thermal fuse inside the main duct.