A 2ft firework burst through the roof of a family home in Eastbourne, setting the loft alight.
A teenage girl who was at home with two younger children raised the alarm after dashing upstairs when she heard the blast.
The 13-year-old opened the hatch door to the roof to discover the insulation had caught fire in the end-of-terrace house in Rockhurst Drive.
Fire brigade officers say she closed the hatch to contain the blaze and rushed next door to make a 999 call.
Firefighters arrived to douse the flames, which had caused a small amount of smoke damage.
They discovered a 2ft-long German-made firework called a Super Helios had penetrated through roof tiles and felt lining after being set off in a garden.
Leading Firefighter Andy Jackson said the girl had done the right thing.
He said: "She certainly prevented the fire from turning into something more serious. It was very brave of her."
One neighbour, Sarah Morgan, 17, said: "I'm not surprised to hear it was caused by fireworks because there were a lot being set off."
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