R J Scaffolding had not trained its employee, the Health and Safety Executive (HSE) said, nor had it given him appropriate equipment.
It also found that the supervisor overseeing the work was not competent.
The worker was in an induced coma for two weeks after the fall on 2 June 2016. Bristol Magistrates' Court was told that he sustained five skull fractures and lost the sight in his right eye.
R J Scaffolding pleaded guilty to breaching reg 2(1) of the Health and Safety at Work Act. It has been fined £26,000 and ordered to pay costs of £1,658.
"If industry-recognised safe systems of erecting scaffold had been in place prior to the incident, the life changing injuries sustained by the employee could have been prevented," HSE inspector Ian Whittles said after the hearing.
