Sparks from a brazier caused a fire at a house in Lawkholme.
The Back Caledonia Road property was being renovated, and contractors were working on the building at the time.
A Keighley Fire Station spokesman said the blaze started at about 2pm on Saturday.
The workmen at the house were alerted to the threat by a passer-by.
The Keighley engine and the Bingley appliance both responded, and no one was hurt in the incident.
The spokesman added: “The heat from the fire fractured a gas main, so we remained there until 3.45pm so Northern Gas could switch off the supply. There was damage to window frames on the house, and a wooden door to an outbuilding was completely destroyed.”