Wayne Marshall, a builder from Seven Sisters, has been prosecuted by the Health and Safety Executive (HSE) for putting a family at risk due to improper chimney repairs.
Mr Marshall was appointed to fix a joint chimney in March 2010, but left an amount of rubble which blocked the flue of the neighbouring property, resulting in a potentially fatal carbon monoxide leak.
Neath Port Talbot Magistrates' Court also heard that Mr Marshall did not notify the neighbours to turn off their boiler at the bottom of the flue.
Speaking after the hearing, HSE inspector, Stephen Jones, said: "Anyone in control of rebuilding and renovation works, especially to chimneys and any flues, must take reasonable steps to establish what appliances, such as fires and boilers, are attached to them."
Earlier this month, a dry cleaners chain in Bedford was fined after the HSE found that it had endangered the lives of staff by utilising a poorly maintained gas boiler.