Patrick Moran, 48 from Stretford, had been working on a loft conversion in Chorlton when the incident happened on 21 May 2013.
Quaintbrook Properties Ltd was prosecuted by the HSE after an investigation found he had been asked to help install partition walls without any floorboards in place.
Trafford Magistrates’ Court heard that the company had been hired to carry out loft conversions at two neighbouring properties on York Road.
The firm had installed new floor joists but wanted the partition walls to be fitted before the floorboards, so they could be easily lifted up in the future.
The job sheet instructed the two joiners to use boards on the site as temporary flooring but when they arrived they could only find three pieces of wet plywood outside, which were unusable.
They had to use some old loft boards to create temporary walkways and kept moving them to reach different areas of the loft.
As the joiners were installing the wooden frame for a partition wall, one of them slipped and his leg went between the floor joists and through the plasterboard ceiling up to his hip.
He managed to pull himself up onto the joints but suffered severe internal bruising and continues to suffer from pins and needles to his left hand, caused by possible nerve damage.
Quaintbrook Properties Ltd, of Chorlton, was fined £5,000 and ordered to pay £5,518 in prosecution costs after pleading guilty to a safety breach.
Speaking after the hearing, HSE Inspector Laura Moran said: “The joiner was badly injured in the incident but it could easily have been much worse if he had fallen all the way into the room below.
“Quaintbrook Properties was responsible for his safety but it failed to take this responsibility seriously.
“The company did not give its employees any information about health and safety, or carry out a proper assessment of the risks ahead of the work starting.
“As well as the missing floorboards, there was also a large opening for the new staircase without any markings or protective measures around it.
“Following the incident, the company arranged for temporary flooring to be installed across the whole area. If this had been in place at the time of the incident then the joiner’s injuries could have been avoided.”