City inspectors say illegal work with a blowtorch on the roof of a Prospect Lefferts Gardens apartment building caused a fire that displaced dozens of tenants Monday afternoon.

The Department of Buildings says the flames tore through the top floor of the six-story building on Hawthorne Avenue, ripping open parts of the roof and leaving the structure unsafe. A vacate order has now been issued for several apartments, and the building’s owner must hire an engineer to assess the stability of the building.

The Department of Buildings did not provide details on the illegal work and the building’s owner did not respond to a call for comment.

The American Red Cross says it’s helping at least 15 families — that’s 37 adults, six children, and even pets — with emergency housing and other assistance.

Longtime tenant Marjory Azor says the sudden displacement is unsettling.

“We are displaced for 2 days…I don’t like this. It makes me feel uncomfortable in spite of the fact that I don’t live too close to the fire. It makes me rethink my life,” Azor said.

FDNY says the fire call came in just before 1 p.m. Monday. Flames spread through three apartments and into the cockloft above. Ambulances lined the block, and one firefighter suffered minor injuries.

The department also posted a photo showing a van parked in front of a hydrant outside the building. Officials say two hydrants were blocked while crews worked to get water on the flames — a dangerous and illegal situation that can lead to fines up to $5,000.

The cause of the fire remains under investigation.