Mahwah firefighters carried the victim out after he was struck in the head and knocked out by a valve cap during a pressure test on an 8-inch water line on the second floor of the college library shortly after 8 a.m., they said.
A helicopter was cancelled because of the weather and he was taken by ambulance to Hackensack University Medical Center with severe head trauma, responders said.
They were doubtful about his chances for survival.
The library had been closed for a $40 million expansion at the Peter P. Mercer Learning Commons Library off Ramapo Valley Road.
Responders included township police and a Bergen County Hazardous Materials Unit. The Bergen County Prosecutor's Office and OSHA were notified.
Mutual aid responders either at the scene or in coverage included firefighters from Wyckoff, Hillburn and Suffern.
Click here to follow Daily Voice Ridgewood and receive free news updates.