About 25 staff were evacuated from a textile company when a section of the roof of a four-storey mill collapsed.
Firefighters were called to Dewhusrt Dent Textiles at Union Mill, Arrow Street, Halliwell, at 3pm yesterday.
An area of concrete supporting the 250 sq metre bitumen roof had given way, damaging pipes and triggering the sprinkler system and fire alarms.
Bolton Centre fire station officer Dave Pimblett said the roof was holding a six-inch deep reservoir of water, designed to prevent the bitumen from drying out. When firefighters arrived, the weight of the water was threatening to cause further collapse.
He said: "The structure was creaking and groaning with the weight. Staff didn't realise the extent of the immediate danger and it was very lucky no-one was hurt."
Mr Pimblett said fire officers helped staff to turn off the sprinklers and operate the roof's drainage system to prevent further damage.
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