Luton Crown Court heard that on 5 September 2016, an engineer was testing a sprinkler system for leaks at a site in Hemel Hempstead.
He climbed onto an internal roof and was inspecting the leak from an extension ladder.
The ladder slipped away from him and he fell almost three metres into the gap between the internal roof and the external wall.
The worker suffered severe blood loss which required a transfusion. He needed 14 stitches to his head and also sustained a fractured vertebrae and suffered soft tissue damage.
An HSE investigation found that reasonably practicable measures had not been taken to prevent a fall from the internal roof for both the engineer and other contractors working on the roof.
The investigation found that Modus Workspace Limited, the principal contractor, had failed to discharge its duty to ensure those not in their employment were not exposed to risks, in particular that of falling from height.
Modus Workspace Limited of Greencoat Place, London was found guilty of safety breaches after a five-week trial and was fined £1.1m and ordered to pay costs of £68,116.18.
After the sentencing, HSE inspector John Berezansky, said: “This case highlights the importance of taking reasonably practicable measures when planning and managing the risks regarding work at height within the construction industry.
“Falls from height remain one of the most common causes of work-related fatalities and injuries in this country and the risks and control measures associated with working at height are well known.
“The engineer’s injuries were life changing and he could have easily been killed. This serious incident and devastation could have been avoided if basic safety measures had been put in place.”