The Trumax Building operative, who was under 18 years old at the time, was working on the first floor of a house being built on a development site in West Butterwick, Scunthorpe, when the accident happened on 18 May 2018.
Sheffield Magistrates' Court was told that he was using a trowel to remove excess mortar from the back of breeze blocks, when he stepped backwards and fell through the unprotected stairway opening on to the concrete floor.
He sustained two fractured ribs and contusions.
An investigation by the Health and Safety Executive (HSE) found the company had failed to install a proprietary guardrail system or traditional scaffold guardrails around the perimeter of the opening.
Trumax Building pleaded guilty to breaching reg 6(3) of the Work at Height Regulations. It was fined £10,000 and ordered to pay costs of more than £800.
HSE inspector Stuart Whitesmith said: "This incident could so easily have been avoided by simply carrying out correct control measures and safe working practices, particularly to protect inexperienced workers.
"In this case someone suffered serious injuries which could been easily avoided by installing guardrails around the opening, to prevent workers falling from height."
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