Work carried out on a chimney stack by roofing contractor John Stanley, caused a blockage above a gas fire exposing an elderly couple to deadly fumes.

Mr Stanley, 47, trading as King Roofing and Durable Plastics, was given a four-month prison sentence suspended for two years, and ordered to pay £500 in compensation to the couple.

The court heard that Mr Stanley had been hired to fix a water leak between a double chimney stack and roof tiles by housholders at a property in Luton in August 2011. The repair work blocked the chimney above the gas fire, leading to a potentially deadly leak of combustion fumes within the property's loft space and above the couple's bedroom. The housholders only found out about the risk a year later.

Speaking after the hearing, HSE inspector Robert Meardon said: "John Stanley’s sub-standard work created an ongoing breach and a prolonged risk to the safety of a vulnerable and elderly couple. It cost them a huge amount of unnecessary anxiety and a considerable amount of money to rectify – on top of their savings they had used to pay Mr Stanley for the work.

"When a chimney is the flue for a gas fire, it is a vital part of the gas installation and should only be done by a competent gas engineer on the Gas Safe Register.

"Mr Stanley advertised himself as a specialist roofing contractor, but he was never on the Gas Safe Register and not qualified to carry out gas work. He should have never undertaken this job knowing that a gas fire was linked to the chimney.

"This incident could have resulted in fatalities as gas fire fumes can contain poisonous carbon monoxide, which can kill. It is vital that building contractors are aware of the risks they create and comply with the laws in place to control the risks."