Jamie Lee Duddin, 18, of Corbridge, and another Scotts of Whittington employee were carrying out repairs to the roof lights of the barn at Heddon Low Farm, Heddon on the Wall, Northumberland, when the incident occurred on July 23 2010.
Newcastle Crown Court heard during a two and a half week trial that the teenager was rushed to hospital but died two days later.
Duddin was standing on a roof panel made of fibre cement board when it collapsed. He fell through the roof landing on a concrete floor six metres below.
A joint investigation by Northumbria Police and the Health and Safety Executive found that the work had not been properly planned.
It also identified that inadequate equipment had been provided and that the workers were not properly trained or supervised.
Scotts of Whittington Ltd was fined £65,000 after pleading guilty to safety breaches.
Company directors Alfred Wood, 65, of Great Whittington and his son, Christopher Wood, 41, of Great Whittington also pleaded guilty to safety breaches and were fined £13,000 and £2,000 respectively.
In addition the defendants were ordered to pay combined court costs of £19,000.
HSE Inspector Martin Smith, said: “This was a tragedy that could and should have been avoided. Instead a young man has lost his life as a result of collective failures including confusing instructions on how the work should be safely carried out, a lack of supervision and a failure to properly plan the work and provide adequate equipment.
“Alfred Wood was also prosecuted for a similar, but non-fatal incident, in 1998 when he was a partner in another company – so he in particular has no excuse for any failures to follow guidance on working at height.”