Four fire engines and around 25 firefighters were called to a fire at a flat on Rosecroft Avenue in Hampstead.
Almost half of the roof space was damaged by the fire. There were no reports of any injuries.
The Brigade was called at 1441 and the fire was under control by 1608. Fire crews from West Hampstead, Willesden and Stanmore fire stations attended the scene.
The fire is believed to be accidental and caused by an electrical fault with wiring.