Authorities say it happened around 12:30 p.m. at a building on the east side of the Excelsior apartment complex on Prospect Avenue in Hackensack.
Hacksensack firefighters responded to the scene and found the fifth-grader on the third-floor roof of the building next door. He had landed on the roof of a restaurant, which broke his fall.
Someone rushed into the restaurant to tell them that the child had fallen, and to call for help.
He was rushed to Hackensack University Medical Center in critical but stable condition. He suffered a broken pelvis and a broken nose.
His mother is at the hospital with her son. Their identities are not being released at this time.
There were no signs of a broken rail or any obvious indications of a problem with the balcony.
The building is located on the same block as Hackensack University Medical Center, so the response was rapid.
The reason for the fall is under investigation.
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