David Johnson, 56, of Ipswich, has been found guilty of the manslaughter of mechanic Robert Schenker, a jury ruled after a two-week trial at Norwich Crown Court.
Mr Schenker, from Peterborough, died of carbon monoxide poisoning after Johnson blocked the flue leading from his kitchen boiler.
Paul Hoskins, an inspector with the Health and Safety Executive (HSE), said: “Robert Schenker's tragic and wasteful death could and should have been avoided by Mr Johnson taking straightforward safety precautions.
”HSE inspectors involved in this case were appalled by the poor workmanship and failure to carry out even the most basic checks to ensure the correct operation of the flue, which directly led to the fatal incident.”
Johnson, who denied manslaughter, was prosecuted following a joint investigation by police and health and safety inspectors.
The court heard Mr Johnson had allowed mortar to fall into the chimney and this had solidified, preventing carbon monoxide from escaping into the atmosphere when the boiler was switched on.
Mr Schenker was found dead in his bed in March 2006 - three days after Johnson started work.
Detective Inspector Mick Birchall said: “This has been a very difficult, complex and unusual case.
”We hope this result will serve as a warning to other construction workers to make sure they comply with all health and safety regulations.”
Mr Johnson is now awaiting sentencing.