Jack Sanderson and another builder were spotted carrying out work to the roof of a two-storey building on Burnley Road in Bacup by a passing inspector last January.
The inspector could see there were no handrails or toe boards around the platform at the top of the tower to protect the workers from falling seven metres to the ground.
He immediately issued a Prohibition Notice ordering the men to come down from the unsafe scaffolding.
But three hours later the inspector returned and found both men back at the top of the tower with no safety precautions in place.
Accrington Magistrates’ Court heard that Sanderson had been working on a renovation project on a terraced property, next to a row of shops and bus stop.
This meant that passers-by were also put at risk of being struck by falling building materials.
Sanderson, of Burnley, pleaded guilty to safety breaches and was fined £2,000 with no costs.
He is currently in prison for another unrelated offence,
Speaking after the hearing, HSE Inspector David Myrtle said: “Mr Sanderson was given a chance to put things right when he received a Prohibition Notice but he chose to ignore it. He found himself in court as a result.
“Several lives were put at risk because the scaffolding wasn’t safe to use, including the lives of another worker and members of the public doing their shopping on the street below.
“This case should act as a warning to those working in the construction industry that if they ignore formal enforcement notices issued by HSE then they are likely to face prosecution.”