W Hughes and Son Ltd was prosecuted by the HSE after an investigation found the company had failed to prevent access to the scaffolding on the site.
Preston Magistrates’ Court heard that the firm had been hired to replace the flat roof on a single-storey section of the hospital.
It used scaffolding to reach the roof but failed to properly fence off the steps leading up the scaffolding tower.
The 17-year-old, who was staying in the hospital’s Mental Health Unit, was able to climb the scaffolding on 17 October 2013.
She fell from the roof in the gap between two buildings and the emergency services had to remove a hospital window to free her. She was in hospital for several weeks as a result of her injuries.
W Hughes and Son Ltd, of Preston, was fined £10,000 and ordered to pay £516 in prosecution costs after pleading guilty to a breach of the CDM Regulations.
Speaking after the hearing, HSE Inspector Chris Smith said: “A vulnerable teenager was badly injured because W Hughes and Son Ltd failed to make sure its scaffolding was properly fenced off.
“Construction firms have a legal duty to make sure construction sites are secure and clearly signed but that didn’t happen in this case.
“It’s vital that companies think carefully about how they plan projects in public places, such as hospitals, so that members of the public are not put at risk.”