Six fire engines and around 40 firefighters were called to a fire at a house on Scholars Road in Balham.
Part of the first floor of mid-terraced house was damaged by fire and the roof of the building was destroyed by the blaze. There were no reports of any injuries.
The Brigade’s 999 Control Officers took seven calls to the fire.
The fire is believed to have been caused by the self-heating of film reels.
Station Officer Adrian Parker, who was at the scene said: "Up until the 1950s, cellulose nitrate was used as the plastic for photography and cinematic films.
"Whilst rare, these films can be dangerous when they reach high temperatures, as they can catch fire very easily and burn very quickly. In this case, the film was stored in the loft, which wasn't ventilated and in the extremely hot weather we have been having temperatures reached over 38 degrees, causing the film to self-heat and combust.
"If you do have old films, it's important to seek professional advice. Keep them in a cool (less than 20 degrees), dry place, away from doors and windows and away from any sources of heat or ignition (radiators, hotwater pipes, light fittings, fires etc).
“Thankfully, working smoke alarms fitted inside the property sounded and alerted a couple inside the house to the fire, who were then able to escape.
“It is vital that you have smoke alarms fitted in every room where a fire can start and a heat alarm in the kitchen. Ensure you test them regularly, as it gives you early warning in the event of a fire in your home.”
The Brigade was called at 0106 and the fire was under control by 0330. Fire crews from Tooting, Clapham, Battersea, Hammersmith and Brixton fire stations attended the scene.