St. Leger Homes admitted breaching health and safety regulations following the accident in Moorends, Doncaster, which left the employee unable to work for seven months.
The company, based on White Rose Way, in Doncaster, appeared at Sheffield Magistrates' Court, where it pleaded guilty to contravening regulations about working at height. As well as the hefty fine, it was ordered to pay £1,086 in costs.
The court heard how two workers had been replacing a flat roof at a single-storey outhouse on September 14 last year when a joist failed, sending one of them plummeting to the floor of the property.
An investigation by the Health and Safety Executive (HSE) found the company had failed to ensure the work was properly planned, adequately supervised and carried out in a safe manner.
Lakeside Doncaster: Emergency services launch search after man gets into difficulty in water
Lakeside Doncaster: Online tributes pour in for man who's body was found in South Yorkshire lake
Major Doncaster bridge will be closed for a whole month for essential maintenance
Lakeside Doncaster: Tragic end to huge rescue operation as body is found following search of lake
Scawthorpe fire: 'Avoid the area' warning as five crews tackle large wildfire during Doncaster heatwave
HSE inspector Stuart Whitesmith said: "This incident could so easily have been avoided by simply carrying out correct control measures and safe working practices.
"Those in control of work at height have a responsibility to ensure that it is properly planned and appropriately supervised to ensure that it is carried out in a safe manner."