Gas discharge occurred at a storage tank facility. The subsidence on one side of a floating roof (approx. 10° incline) caused a momentary release of fumes (gasoline and thiols). In a span of 8 hours, the roof completely collapsed. Any release of gases could be avoided by application of a foam cover, and the tank was then emptied. The event was detected from the odours produced. Fire-fighters set up a wide safety perimeter, covered the foam liquid and proceeded with analyses that turned up nothing abnormal. This incident caused nausea among a few of the individuals on or near the site.