A member of public witnessed and photographed unsafe work at a construction site on March 17 2015, and contacted the HSE.
Manchester Magistrates Court heard that the photograph showed Jake Clarke, one of a pair of directors for Enviroply Roofing Limited.
His fellow director Aaron Hepworth was also spotted walking along a beam to pass something to Clarke, then walking back again.
There was nothing in place to prevent or mitigate a fall from this beam.
The HSE investigation discovered that there was a full time scaffolder on site who was employed to build any scaffolding required by contractors but on this occasion, Enviroply and its directors had chosen to rush the job in order to submit their invoice earlier.
Clarke and Hepworth put their lives at risk as the fall would have been one storey and onto timber beams.
All three defendants failed to take the proper precautions to manage the risk of a significant fall from height, despite there being adequate provision on site to provide them with scaffolding and safe working platforms.
Enviroply Roofing Limited of Chorley pleaded guilty to safety breaches and were fined £13,300 with £1160.50 costs.
Jake Joseph Clarke of Leyland, Preston pleaded guilty and was fined £1100 with £1160.50 costs.
Aaron Paul Hepworth of Radcliffe also pleaded guilty and was fined £2100 with £1160.50 costs.
HSE inspector Matt Greenly said after the case: “Enviroply Roofing Limited had a duty to protect any employees, even the directors, from a risk of a fall from height.
“Mr Clarke and Mr Hepworth in their position as directors recognised that their choices on that day placed themselves at a serious risk of death or life changing injury and admitted as much for themselves and their company.
‘They only realised afterwards that running those risks in order to submit an invoice early was a very unnecessary, considering how serious the consequences could have been.
“It is pure luck that no-one was injured or killed. I would like to thank the people who reported these concerns to us as they have been instrumental in saving the lives of these men.
“It is hoped that other construction workers will see these cases as a warning that HSE will act robustly to concerns received and perhaps they will take a little more time to protect themselves on their next site.”