A Good Samaritan rescued a teenage boy yesterday from a Brooklyn rooftop as a fire tore through the youth’s home, killing three of his family members, police and fire officials said.

The fire ignited just before midnight Sunday evening in the attic of a two-story Queen Anne-style house in the Ditmas Park section of Brooklyn, witnesses and fire department officials said. The blaze took the lives of a 12-year-old boy, his 50-year-old aunt, and his 76-year-old grandmother, police officials and witnesses said.

A 16-year-old boy, the brother of the 12-year-old victim, suffered serious burns to his hands before being rescued by a man who discovered the teenager on his hands and knees, screaming for help from the roof of the building on East 19th Street as smoke and flames billowed from its crown, witnesses said.

“He ran up the stairs of the porch and coaxed the kid down,” a witness who lives in the home next door, Vincent Bly, 46, said. Fire marshals said they believe the cause of the blaze was electrical wiring inside the walls of the house, and that the only working smoke detector in the home was on the first floor. The owner of the home, who lives on the second floor, was served with three violations by the Department of Buildings yesterday, including one for illegally allowing occupancy in the attic of the home.

Family and witnesses said the attic was the rescued boy’s bedroom. When the fire was ignited, the 12-year-old victim was trapped on the second floor when he tried to go down stairs and wake his aunt and grandmother.

It is unclear how the young boy was stranded on the second floor.