Ten fire engines and 72 firefighters and officers were called to a fire at a terraced block of flats in Dartmouth Park Hill in Archway.
Two adjacent properties sustained the same damage: their roof was destroyed and 80 per cent of the second floors were damaged in the blaze.
One woman left the building before the arrival of the Brigade and was treatred at the scene by London Ambulance Service. Around 20 people were evacuated from adjoining flats as a precaution.
The Brigade was called at 1956 and the fire was under control at 0019, Fire crews from Holloway, Islington, Euston, Hornsey, and surrounding fire stations attended the scene.
The cause of the fire is under investigation.