The roof of a semi-detached house in Harrow was largely gutted by fire at 1.30pm yesterday (Monday).
A watch manager at Harrow Fire Station, who asked not to be named, said: "There were two occupants and somebody, a passer-by, knocked on the door and told them their roof was on fire."
Four fire crews, and an aerial ladder platform, were sent to the property in Francis Road, where they evacuated those living in the neighbouring home before tackling the blaze.
The watch manager said: "It was in the roof of the house so we used breathing apparatus and went up into the loft to fight it. We were there until 5.30pm making sure it was all out; dampening down and cutting away all the burnt wood.
"The fire is not suspicious but the cause is still under investigation."
Nobody is believed to have been injured in the incident although the roof was almost entirely destroyed.