Lee Byrne, aged 29 and from Ince, was working on a project to replace the roof on the sports hall at Loreto High School in Chorlton when the incident happened on 9 November 2011.
His employer, K Pendlebury & Sons Ltd, was prosecuted by the Health & Safety Executive (HSE) for safety breaches.
Trafford Magistrates’ Court heard that workers had removed old steel beams under part of the roof so that new beams could be installed. However, this meant the corrugated tin panels on part of the structure were left unsupported.
Mr Byrne was walking over the roof to his colleagues to get their lunch order when the panels under his feet gave way. He fell 10m, hitting a section of scaffolding on his way down to the ground.
He suffered a fractured pelvis, broken fingers and his right arm and elbow were smashed to pieces. He has had to have an artificial elbow fitted and has so far been unable to return to work due to the extent of his injuries.
The court was told there was no barrier around the fragile section of the roof, and the scaffolding had only been erected under parts of the roof rather than covering the full width.
K Pendlebury & Sons Ltd, of Ormskirk Road in Pemberton, pleaded guilty to a breach of the Work at Height Regulations 2005 after it failed to ensure workers were prevented from standing on fragile parts of the roof. It was fined £10,000 and ordered to pay costs of £3,539.
Speaking after the hearing, HSE inspector Matt Greenly said: "The injuries the employee has suffered will affect him for the rest of his life but he could easily have been killed if he hadn’t hit the scaffolding on the way to the ground.
"The company had been removing a series of supporting steel beams but no barriers were put up to prevent access to the fragile roof panels despite the company recognising before the incident that barriers would be needed. There should also have been scaffolding under the whole of the roof to catch anyone who fell.
"This was a big project that should have been carefully planned but sadly the company’s failings have led to an employee being badly injured."