It is crucial that companies properly risk assess roof work, the Health and Safety Executive (HSE) has said.
The regulator issued the warning following the prosecution of a firm which allowed two of its staff to work unsafely on a seven-metre-high roof.
A court heard that the two workers were spotted by two passing HSE inspectors – investigating an unrelated case – carrying out repair work to a building.
The men's employers had not provided the necessary safeguards or equipment for the two workers. The risks, including the potentially fragile condition of the roof, had not been adequately considered.
Moreover, the company had failed to provide suitable equipment to prevent a fall from or through the roof, such as a mobile elevating platform or standard platform with handrails, the HSE said.
There was also nothing in place, such as netting or a boarded platform underneath the roof, which would have caught anyone falling.
The firm pleaded guilty to breaching Regulation 6(3) of the Work at Height Regulations 2005 and was fined £5,000 with £1,887 in costs.
After the case, HSE inspector Alan Sheldon said: "It was pure chance a passing HSE team spotted these two workers as both men were obviously in significant danger of falling off or through the roof. There was no safe system of work to prevent a fall and nothing in place to mitigate the effects of a fall, such as netting underneath the roof.
"Falls from height are still the biggest killer in the construction industry. In 2009/10, more than half of the deaths which occurred in the industry were due to falls. An estimated 4,000 or so workers were also badly injured last year after falling from heights."