A 31 year man died from carbon monoxide fumes because the tradesman rebuilding his chimney failed to check the flue was clear.
David Johnson, head of Merlin Building and Roof Contractors was found guilty of gross negligence manslaughter following a two-year investigation by the Health and Safety Executive and Cambridgeshire Police.
On March 2006, Johnson demolished and rebuilt a chimney for a customer who approached him about damp patches which had developed in his parents' house in Peterborough.
The morning after the chimney had been rebuilt, his customer Robert Schenker was found dead. He had died from fatal carbon monoxide poisoning, having 55% carboxyhaemoglobin in his blood.
On investigation of the boiler, a Glowworm 65-80 floor standing unit, was found to have high levels of carbon monoxide which were being produced as soon as it was switched on. A test showed most of the fumes entering the kitchen.
More tests showed that the boiler was producing fatal levels of CO after two minutes. After seven minutes, the CO had started to reach the upstairs bedroom where Mr Schenker was sleeping.
Further investigation found that the chimney flue was also almost completely blocked with debris and mortar, because of the work Johnson had carried out.
The chimney was also found to be unstable and had to be knocked down.
Paul Hoskins, the investigating inspector from the Health and Safety Executive said: 'Inspectors involved in this case were appalled by the poor workmanship involved in the demolition and rebuilding of the chimney.
'Following the work, the roofer failed to carry out even the most basic checks to ensure the correct operation of the flue'.
The law prohibits builders or anyone else from carrying out alterations that have an adverse effect on the safety of any gas installation, for example by restricting the amount of ventilation or enclosing/blocking a flue.
Anyone carrying out work on flues and chimneys should check that they are inspected and tested sufficiently to ensure that they are in a safe condition and working effectively.
The HSE strongly advises that gas appliances and/or flues are serviced regularly for safety by a CORGI-registered installer.
Johnson, who operated on his own, was also found guilty of breaching Section 3(2) of the Health and Safety at Work Act 1974. He will be sentenced at Norwich Crown Court on May 17.