A young construction worker whose safety harness was unclipped fell to his death from the roof of a Midtown office building yesterday.

Jose Luis Melendez Gutirez, 19, lost his balance and fell from the 12th floor of an 18-story building at 800 Second Ave. near East 42nd Street, authorities said. Police said the El Salvadorian immigrant landed on a sidewalk shed one story above the sidewalk.

Co-workers said Gutirez was taking equipment to the roof and removing debris when he slipped off a guardrail around 9 a.m. Witnesses said they heard the sound of his body landing and then saw his white hard hat tumble into the street.

“It was a thud,” a police officer stationed in a booth outside the building who requested anonymity said. “When I looked up, the other workers were looking down. But then blood was coming down. It was bad.”

Gutirez was employed by Skyline Reconstruction Corporation, a Long Island City-based general contractor.

According to a permit posted outside the building by the Department of Buildings, contractors were making façade repairs to the 18-story structure, which houses the consulates of Israel, Ecuador, and St. Lucia and is a few blocks from the United Nations.

A spokeswoman for the department, Jennifer Givens, said a citation was issued to the contractor on August 10 after inspectors determined a sidewalk shed was overloaded, but she said the citation was not related to the accident. “We were fortunate nothing happened there,” Ms. Givens said.

Yesterday’s accident was the third this week involving construction workers. On Monday, two pairs of workers were rescued from broken scaffoldings by firefighters.

Contractors in the area suggested yesterday that the accident could have been prevented, particularly if Gutirez’s harness had been secured. “They have to have somebody to watch workers so they are safe,” a maintenance worker at a building across the street who knew Gutirez, Pedro Gonzalez, said.