Mr Nikitas Coulson, 40, from Middlesbrough, broke his arm after falling three and a half metres from a flat roof to the ground below. He needed surgery to insert a plate into his arm following the incident at the former Sheffield College in April last year.
The Health and Safety Executive (HSE) prosecuted both Mr Coulson's employer, Lilquest Asbestos Management, and the principal contractor for the project, JF Finnegan Ltd, who were responsible for access to the roof.
Sheffield magistrates heard that immediately after the incident, the remaining 10 rooflights on the flat roof were boarded over, making them safe. The court heard this should have been done prior to any work starting on the roof.
Magistrates fined JF Finnegan, of Ecclesall Road, £15,000 with £5,179.90 costs for failing to provide suitable protection for any fragile surfaces on the roof under the Work at Height Regulations 2005.
Lilquest, of Hoyland Road, was fined £3,000 with £2,000 costs for the same offence. Both companies pleaded guilty.
HSE inspector Dave Bradley said: "This was an obvious and known risk, and one that should have been dealt with before any work went ahead. Falls from height are the most common cause of fatal injuries in the workplace and are also responsible for many serious injuries.
"The control measures needed to avert these risks are cheap and simple and widely-known throughout the construction industry.
"Mr Coulson has been unable to work since this happened so the impact on him has been significant. The HSE will not hesitate to use the enforcement powers we have to protect people from injury at work."