The worker, 56, was sat in the driver’s seat when it became separated from the machine, causing him to fall beneath the machine as it reversed, a court heard.
A Health and Safety Executive investigation into the incident on August 13 2012 at Ranch House Farm in Bath Road, Wiltshire, found that the roadroller was in a poor condition and the operator was untrained.
The driver’s seat was connected to the top of a water tank which sheared off from the body of the machine.
The safety critical defects included that the seat safety cut out switch that would have immediately halted the machine had been defeated and the emergency stop button was missing.
His employer Chippenham-based Globareward was fined £10,000, with costs of £30,750, after pleading guilty to safety offences at Swindon Crown Court.
Company director Paul Andrews was sentenced to two months in prison, suspended for two years. He was also ordered to do 200 hours community service.