Four fire engines and around 25 firefighters responded to a flat fire on the High Street, Northwood.
Part of a first floor flat above a shop was damaged by fire. There were no reports of any injuries.
A London Fire Brigade spokesperson said: “A member of the public called 999 after seeing smoke coming from the roof.
"Smoke alarms were present at the property but were not working.
"This incident shows just how important it is to have working smoke alarms fitted on every level of your home.
“Smoke alarms give the earliest possible warning when a fire starts and we would urge everyone to make sure they have one fitted in every room where a fire can start, except kitchens where heat alarms are more appropriate.
"Firefighters recommend that you have a look at our online Home Fire Safety Checker to make sure there are no hidden hazards in your home.
"The online tool will ask you a series of questions that will help us work out the level of risk in your home, or the home of someone you care for. It only takes a few minutes to get tailored advice to keep yourself and loved ones safe from fire."
The Brigade was called at 1459 and the fire was under control by 1610. Crews from Stanmore, Harrow and Northolt fire stations attended the scene.
The fire is believed to have been accidental and caused by damage to an electrical circuit.