Firefighters have issued a reminder for residents to ensure that they test their smoke alarms to see if they are in good working order after a fire at a house on Hall Lane in Hendon.
Most of the roof a semi-detached house was damaged by fire. There were no reports of any injuries.
The fire was discovered by two tenants when they noticed materials falling from a light fitting and saw a fire in the space between the ceiling and the roof.
A London Fire Brigade spokesperson said: "There were no working smoke alarms in the property.
"Smoke alarms give the earliest possible warning when a fire starts and we would urge everyone to make sure that they have one fitted in every room where a fire can start, except in kitchens or bathrooms where heat alarms are more appropriate."
The Brigade was called at 1904 and the fire was under control by 1954. Six fire engines and around 40 firefighters from Hendon, Mill Hill, Finchley and West Hampstead fire stations attended the scene.
The cause of the fire is under investigation by the Brigade and the Metropolitan Police Service.