Inside a pyrotechnic facility, traces of black powder ignited during maintenance work on the roof of a blackpowder production building. The roof tiles were composed of plates with an asphalt lap joint. To replace a defective joint, a technician from a subcontracted firm removed the tile, sprinkled the working zone with water and lit a blowtorch to reseal it. The blowtorch flame then triggered combustion of black powder dust that had accumulated in the overlap zone between 2 plates; combustion spread to other dusts accumulated in the roof boarding extending from the plate overlap. This roof boarding had previously been washed with water from inside the building. Since the interstice located at the level of the 2-plate overlap was inaccessible to washing water, the black powder had not been neutralised.

The technician was not injured; the building’s roof was partially damaged. The operator inspected all overlap joints of this type on all production buildings; other joints would be developed using silicone mastic by means of a cold bonding process.