A man died this morning after falling through the roof at a New York motorcycle shop owned by "American Chopper" star Paul Teutul Jr. ... law enforcement sources tell TMZ.

We're told the man -- a construction worker not employed by Paul Jr. Designs -- was working on the roof when the accident occurred.

The Orange County Sheriff's Office confirmed that cops are still on scene. We're told someone at the shop called 911 around 8:30 AM to report that someone "fell through the roof."

We're told none of the regular shop employees were inside the building at the time.

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