MKM Fabrications was the principal contractor for the project on a factory building at Valley Mills. It subcontracted Clad-It to replace the northern-light roof structure with a composite roof.
Members of the public saw the men working on the roof that had unguarded edges and reported it to the Health and Safety Executive (HSE), which launched an investigation.
It found that fragile skylights had not been covered to prevent MKM and Clad-It employees stepping on them and falling through. The mill workers below were also at risk from being hit by falling tools and debris.
No measures had been taken to stop employees tumbling through a large hole that had been created by the removal of one of the skylights. These dangerous practices continued even when the roof was covered in snow.
The HSE served prohibition notices to halt all activities until safe working practices had been introduced.
MKM pleaded guilty at Manchester Magistrates' Court to breaching ss 2(1) and 3(1) of the Health and Safety at Work Act. Clad-It pleaded guilty at Manchester Crown Court to the same charges and both companies were fined £66,000 with costs of almost £4,000.
HSE inspector Matt Greenly said: "MKM and Clad-It failed in their duties to protect the roof workers and anyone working below them in the mill from a foreseeable risk of serious harm."