It happened at Fieldstone Lodge Nursing and Rehabilitation Center on Kappock Street between the Henry Hudson Parkway and Independence Avenue just before 10 a.m.
FDNY officials said a 20-foot-long portion of parapet from the roof collapsed onto the scaffold below.
The Department of Buildings said the debris from the collapse was caught by the scaffold netting and sidewalk shed.
No injuries were immediately reported.
Crews are assessing the damage and shoring up the structure in Spuyten Duyvil.
DOB inspectors did not find any evidence that the building is structurally unstable and no vacate orders have been issued.
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