John Quinn, 35 and Joshua Perry, 21, were developing a vehicle spray booth in Perth when it collapsed on 22 January 2014.
They accessed the roof in order to fit an extractor fan.
Mr Quinn sustained fractures to both his heels which resulted in a four week stay in hospital and multiple operations.
It is understood he has also been diagnosed with post-traumatic stress disorder (PTSD).
Mr Perry suffered multiple fractures to his spine and ankle along with soft tissue damage to other parts of his body including nerve damage to his right leg and foot.
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He had to wear a full body spinal cast for 16 weeks and required surgery along with bone grafts.
A Health and Safety Executive (HSE) investigation found that although Clive Thompson were aware of the fragile roof, no safe system of work was put in place.
The company did not provide employees with sufficient risk assessment for the work they were due to carry out.
The Lichfield-based firm also failed to visit the site before carrying out work, therefore was unaware of any potential site hazards. No training for working on fragile roofs was provided either.
The company pleaded guilty to breaching Section 2 of the Health and Safety at Work etc Act 1974.
HSE Inspector Norman Buchanan said: "This was an entirely avoidable incident. Falling from height is one of the most common reasons for injuries and fatalities at work.
"Clive Thompson Installations Ltd should have been aware of the risks and the precautions that needed to be taken before starting the work.
"By failing to properly plan the work and provide sufficient training , two workers have suffered serious injuries which have had a profound effect on their lives."
(LM)
