Phillip McGinn, 38, and two other workers were caught on camera retiling the roof of a property on Old Race Course Road on 21 August this year without any fall protection.
The Health & Safety Executive (HSE) sent an inspector to visit the site after receiving a complaint about the work from a member of the public. The inspector immediately issued a prohibition notice ordering the men to come down from the roof until it had been made safe.
South Sefton Magistrates' Court in Bootle was told that scaffolding had been erected at the front of the house, but there was nothing at the side or back of the roof to stop workers falling off.
Phillip McGinn, who trades at Stormguard Roofing, pleaded guilty to a breach of the Work at Height Regulations 2005. He was fined £3,000 and ordered to pay £1,668 in prosecution costs.
Speaking after the hearing, HSE inspector Jackie Western said: "There were several ways this work could have been carried out safely, including fitting hand rails and toe boards around the edge of the roof to prevent people falling.
"Mr McGinn put his own life and the lives of two other men at risk by allowing the work to go ahead without suitable safety measures in place. The scaffolding at the front of the building did nothing to prevent them falling off the side or back of the roof.
"I would like to thank the person who alerted us about the work as, if they hadn't, someone could have been seriously injured or even killed."
Phillip McGinn (right) and another worker on the roof of a semi-detached house in Maghull