Sheffield Magistrates' Court was told that the Bland Scaffolding trainee was standing on a loading bay, passing roof tiles to a colleague who was working on the scaffold platform.
He caught his foot in a gap between the platform and the bay and fell backwards.
He sustained a fractured cheekbone, broken wrist, rib injuries and a deep cut above his eye that needed 13 stiches.
The Health and Safety Executive (HSE) found there was no intermediate guard rail or toe board on the loading bay edge protection.
Bland Scaffolding pleaded guilty to breaching reg 8(a) of the Work at Height Regulations for failing to use a guard rail, toe board or barrier on the working platform.
It was fined £100,000 plus £920 costs.
"A fall from this height could have easily been fatal," said HSE inspector Trisha Elvy.