Wates has been fined £640,000 for failing to stop deadly carbon monoxide (CO) gas from entering a property during construction work.
Cambridge Crown Court found Wates Construstion Ltd and R J Fitters LtD guilty of blocking an active flue pipe when conducting work to remove cowls from redundant flue pipes on the property in December 2014.
The court heard how a mistake over the correct floor levels on the building resulted in the live flue being blocked.
The error was only found when a CO monitor was activated and the homeowner and her son investigated the cause of the alarm.
A Health and Safety Executive investigation into the incident found that scaffolding erected outside a 13-storey block of flats without marked lift levels and the external wall of the building had no markings to identify floor levels or flat numbers.
The HSE investigation concluded that Wates Construction Ltd and R J Fitters Ltd failed to manage the risk involved with the project.
The investigation found that they could have marked the levels on the scaffold and the levels/flat numbers on the external wall of the building.
A supervisor could have marked the redundant flue pipes to ensure the correct cowls were removed and flue pipes blocked up, the HSE said.
Instead of blocking the redundant flue pipes, the companies could have put a cage around the cowls to ensure they did not fall.
Wates Construction Ltd pleaded guilty to breaching Section 3(1) of the Health and Safety at Work etc Act 1974, was fined £640,000 and ordered to pay costs of £20,862.52.
R J Fitters Ltd pleaded guilty breaching Section 3(1) of the Health and Safety at Work etc Act 1974, was fined £30,000 and ordered to pay costs of £10,431.28
Speaking after the hearing, HSE inspector Sandra Dias said: “It is the responsibility of both the principal contractor and subcontractor to ensure that safe systems of work have been identified and adopted.
“When there is risk of death to members of the public, the safe systems should be well thought through and robust.
“The risks associated with blocking a live flue could result in carbon monoxide entering properties and potentially killing all occupants.”