Fire crews in Claymont called in backup and dozens of trucks from Pennsylvania and Delaware responded.
The chief says 54 firefighters dumped water on the flames; it took crews more than an hour to control the fire.
"The roof collapsed into the third floor, eventually through the third floor. All firefighters were outside by that time," said Harris.
Everyone who lives in the 18 apartments damaged made it out safely too, but some say they need help. The Red Cross has been called in to assist.
"Everything's gone," said resident Darlene Hull. She says she's counting her blessings.
"I thank God for everything. None of us got hurt. Everyone's fine. And that's the only thing I can look at right now, that we made it out of there," Hull said.