Daniel Thorley, 25, from Llantrisant, was working on the roof of a new three-storey home in Dinas Powys when he fell.

He suffered spinal injuries, needed significant rehabilitation and was unable to work for more than a year.

Cardiff Magistrates’ Court heard that Thorley had been building a block wall on the timber-framed house when it started to rain and a colleague asked him to lend a hand to waterproof the flat roof.

A window was in the process of being fitted to the roof at the time.

As he was laying-out and fixing the polythene to the roof Thorley took a step backwards and fell down the hole to the ground floor, landing on a concrete slab.

HSE found there was nothing over the hole where the window was to be fitted to protect workers from falls and no safety measures underneath to mitigate falls.

The work had not been properly planned by contractor Blackflair Ltd and insufficient measures had been put in place to reduce the risk of workers falling through the roof.

Blackflair, of Four Elms Road, Cardiff, pleaded guilty to safety breaches and were fined £7,000 and ordered to pay £1,137 in costs.

In his sentencing remarks, District Judge Bodfan Jenkins, said: “This was an accident waiting to happen and in the circumstances caused serious injuries. The lack of a risk assessment and supervision were the underlying causes”

HSE Inspector Liam Osborne, speaking after the hearing, said: “Mr Thorley was badly injured and has suffered severe pain for a prolonged time, but he could have been killed.

“The risks could have been much reduced if measures such as guardrails had been installed around the hole or a temporary crash deck structure placed underneath. Airbags or beanbags commonly used in the construction industry could have been placed to cushion any fall.”