Bristol Crown Court heard how in August 2014 the 25-year-old employee of N C Tucker Limited of Wedmore, fell five metres through a fragile roof light at a farm during roofing work.
It was his first roofing job, having only worked for the company for four weeks prior to the incident.
He suffered brain injury, a fractured spine and broken ribs.
N C Tucker Limited, of Wedmore, was fined a total of £26,000, and ordered to pay £3,738 in costs after pleading guilty to a safety offence.
Speaking after the hearing HSE Inspector Sue Adsett said: “The young man who fell has had life changing injuries which have severely affected him and his family.
“The use of nets to protect against falls through fragile roof lights is well- established in the industry, if nets had been in place this accident would have been prevented”