Six firefighters were injured when a chunk of the roof collapsed at 627 Felton St. just after 12:30 a.m., authorities said. The most badly injured was a firefighter paramedic, who was in "very, very serious condition" at San Francisco General Hospital after inhaling superheated gases when the roof to the two-story home gave way, Fire Chief Joanne Hayes-White said.

The injured firefighter paramedic, identified as Chris Posey, and the other firefighters, including a battalion chief, were trapped in the hallway and a front bedroom by the collapse and had to be pulled out by a rescue unit, authorities said.

Others injured

Besides Posey, two firefighters were admitted to San Francisco General, both suffering from second-degree burns, one to the neck and one to the face. Two other firefighters suffered smoke inhalation and one was treated for a twisted knee. The names of the other injured firefighters were not released.

Pat Gardner, deputy chief of operations who was in immediate charge of the fire, said the collapse acted "like a fan blowing 2,000-degree heated gases in on you."

Posey has more than 18 years' experience with the city, seven as a paramedic and 11 as a firefighter paramedic, officials said. He was wearing protective equipment, including a breathing mask, while fighting the three-alarm blaze, but it may have been knocked off before the roof fell in, authorities said.

Guided by strobe

Rescuers who charged into the house found him with the help of a strobe light attached to his breathing apparatus.

The fire had apparently been burning for a while inside the wood-frame building before the Fire Department was notified, Fire Lt. Mindy Talmadge said.

Arriving firefighters found heavy smoke coming from the building and decided to go through the front door, just as they normally would, said Lt. Ken Smith, a Fire Department spokesman.

"It's a typical fire for us. We have these fires just about every day in San Francisco," Smith said.

But firefighters then noticed that the roof was "possibly going to cave in," and a warning went out over the radio, Smith said. Within moments, the roof collapsed.

The fire was controlled by 1:45 a.m.

Public's prayers sought

Nearly 10 hours later, a somber Hayes-White and other department leaders stood in the street in front of the burned-out home and asked for the public's prayers for the injured firefighters.

"What occurred here was very unusual," the chief said. "It's a reminder how dangerous this profession is."

Authorities said the house had been vacant since a tenant moved out about two weeks ago.

Fire Department officials did not immediately know what started the fire. Hayes-White said the circumstances surrounding the blaze were suspicious, however, because the house was vacant.

The former tenant, Gerald Gropp, lived in the home for 14 years. He said he had moved out after settling a legal dispute with the home's new owners, who bought the house in July 2007 and had sought to evict him.

Electricity was off

Gropp said the electricity had been turned off when he left.

The home is owned by NALA Enterprise LLC, which consists of real estate agent Nancy Jen and her daughter, Laura Jen. Nancy Jen said she had no idea how the fire could have started. "I don't know anything about it," she said.

Chronicle staff writer Henry K. Lee contributed to this report. E-mail the writers at srubenstein@sfchronicle.com and jvanderbeken@sfchronicle.com.

This article appeared on page B - 1 of the San Francisco Chronicle