Kathleen Philipson of Allestree, Derby, was sitting at her desk at offices in Nottingham Road, Ripley, when the metre-long, 37kg roll fell through a roof light and came through the ceiling, hitting her on the shoulder.
She was taken to hospital with injuries to her head, shoulder and left arm and was off work for two weeks following the incident on 22 September 2010.
A safety investigation found contractor Jason Lunt allowed re-felting work to start on the flat roof before adequate protection had been installed.
It meant that as Gregory Wright, a self-employed roofer contracted by Lunt, moved one of around six rolls of roof felt that were stood up on the roof, another one toppled over and fell through an unguarded roof light.
After the hearing, HSE inspector Lee Greatorex said: “Mrs Philipson was extremely lucky not to have suffered more severe injuries. This was a frightening experience for her, but it could have been easily prevented had greater care been taken.
“Mr Lunt should have ensured an adequate system of work was in place and that roof lights were adequately protected before repair work began.
“As a roofer Mr Wright should have been aware the roof lights needed to be protected unless he had evidence they were capable of withstanding a substantial load.
“There were 22 sets of roof lights, only one of which had been covered or protected to prevent falls of people or materials through them during the re-felting job.”
Jason Lunt, 41, of Bloxwich, in the West Midlands, pleaded guilty to safety breaches as did Gregory Wright, also 41, of Bloxwich.
They both received sentences of 18 weeks, suspended for 12 months on the condition that they complete 280 hours of community service.
In addition they were both ordered to pay £2,114 costs each by Derby magistrates.