Bristol Crown Court heard how work was taking place at the former Cadburys Somerdale factory near Bristol on 9 November 2011.
James Stacey, 31, was operating a mini digger when he accidentally drove the machine out of a fourth floor opening.
He had been using the opening as part of a 'drop zone' to drop large fiberglass tanks that had been cut in half to the ground below.
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The opening was not properly protected to prevent the digger falling from the building and the tank became stuck on the machine, pulling the vehicle with it as it was dropped.
Mr Stacey died from his injuries.
The building contractor, now in liquidation, pleaded guilty to Section 3(1) of the Health and Safety at Work etc Act 1974.
Site Supervisor Paul Ben Priestley, 54, was also charged under Section 7 of the Health and Safety at Work etc Act 1974 after failing to take reasonable care.
He denied the charges but was sentenced on 21 May 2015.
Mr Priestley was given an 18 month custodial sentence suspended for two years and fined £7,500.
(LM)
