Dudley Magistrates' Court was told that in April 2016 James Durrans and Sons enlisted the services of Kingswinford Engineering Co to carry out the repair works at its premises in Bilston, West Midlands.
Workers had climbed on to the roof from a "man cage" lifted into position by a forklift truck. The cage fell short of the roof level by about 0.9 m and there was also a gap between the cage and the roof, which workers needed to climb across.
The roof itself was wet and slippery, and there were no barriers in place to prevent workers falling.
An investigation by the Health and Safety Executive (HSE) identified failings relating to how the work -- specifically access to the roof -- was planned, managed and monitored.
It said that James Durrans and Sons and Kingswinford Engineering had failed to risk-assess the task and implement a safe system of work for the roof access. Instead, each company had assumed that the other had systems in place to protect workers from the risks of working at height.
James Durrans and Sons pleaded guilty to breaching s 3(1) of the Health and Safety at Work Act and has been fined £100,000 and ordered to pay costs of £20,000.
Kingswinford Engineering Co admitted breaching s 2(1) of the same act and has been fined £18,000 plus £9,000 costs.
After the hearing HSE inspector Edward Fryer said: "This incident highlights the need for contractors to be managed properly. Both the contractors and those engaging them must assess the risks of the site and the specific work to ensure it can be done safely. In this case, no risk assessment was carried out and arrangements made to access the roof put workers at significant risk of falling from height."