Officials said the 15-year-old went out onto a second-floor roof shortly before 1 p.m. Wednesday to retrieve a baby gate that had fallen out of a window.
Assistant police chief Joe Hanna told The (Easton) Express-Times that she stepped on a piece of plywood that gave way, causing her to fall about 30 feet onto a cement stairwell.
Officials said the girl was able to walk back to her apartment, where emergency responders were called.
Hanna said she had bruises and a head laceration and didn't seem alert and was taken to Lehigh Valley Hospital in Salisbury Township.
He says she could have sustained serious injuries and was "very lucky." She could be released from the hospital as soon as Thursday.
Police plan to follow up with the victim once she is released. They have forwarded a copy of the case to Allentown building inspectors.
The incident remains under investigation.
Information from: The (Easton, Pa.) Express-Times