The Health and Safety Executive (HSE) said Richard Perry, 32, was working with a colleague covering roof lights with blackout vinyl at Whiteghyll Plastics Ltd, in Bradford, when he fell 5.5m (18ft).
The HSE said the work Mr Perry was doing was an attempt to block out sunlight to reduce the heat within the factory, which makes signage and displays.
A spokeswoman said he fell through a fragile roof light to the fabrications department below in June 2014.
Whiteghyll Plastics Ltd of City Road Bradford pleaded guilty to breaching Section 2(1) of the Health and Safety at Work Act 1974 and was fined £120,000 and ordered to pay £37,655 costs by Judge Jonathan Rose at Bradford Crown Court.
The HSE said the company failed to adequately supervise Mr Perry and another employee and there were several missed opportunities by a number of managers to stop them from working on the roof.
Mr Perry’s wife, Samantha, said: “Not only did my boys lose their daddy but I lost my husband, my best friend and my soulmate.
“Nobody should die at work and leave behind a young family and wife. He had his whole life ahead of him.”
HSE inspector Andrea Jones said: “Two employees were on the roof for some time with no precautions in place to prevent falling through fragile roof material or off the open edge of the roof.
“This accident would not have happened if these two employees had been appropriately supervised by management.
“Falls from height, particularly from roofs, is the highest cause of fatal accidents.”