It was a busy night in San Francisco on June 24th: Pink Saturday was well underway and the Mission was packed with people. Some were listening to music at the Red Poppy Art House when firetrucks wailed south on Folsom Street, breaking the calm of the balmy evening.

A fire alarm inside 2675 Folsom St., a vacant building across the street, had been tripped.

The fire ladder was cranked up; firefighters from Fire Station # 7 pushed sycamore branches away as they climbed onto to the roof. Below them, squares of plywood, the standard feature of all semi-derelict buildings, were nailed to empty window frames.

Until last year when a fire broke out, the glass had been intact “They blew out,” a tall fireman said. According to him, an unattended propane cooking stove in use by a homeless encampment that had moved inside, lit the interior on fire.

He wasn’t surprised to be checking on the building again. It’s been empty for at least three years, and in a transition from the restaurant auction house it once housed to a 117-unit apartment building. “The back is totally unsecure,” he said briskly. “People get in and out of his building all the time.”

A neighbor, who lives two doors down, agreed. “I’ve seen a guy with dark hair and a bicycle enter the building at least on three occasions,” she wrote in an email. “He enters from the door that is close to the condo. He then walks upstairs from there.”

When asked if there had been a fire on Saturday night, the fireman shook his head. “Not tonight!” he said. But another is likely to happen. “We expect it,” he said.

The fireman said that before the previous owners moved out, they contacted the fire department and walked them through the building. “We have a plan for how we’re going to get inside in the case of a fire,” the fireman said. But that doesn’t let the new owners, Axis Development Group,  off the hook.

Another firefighter at the scene weighed in. “This place is a shooting gallery,” he said. “They need to secure this building.”

The Department of Building Inspections has 126 residential and commercial properties buildings listed as empty for the first quarter of this year. Most are in varying states of condition and use and many are registered, meaning they’re still problematic, but the owners have taken steps to secure entrances and prevent encampments from moving in.

Only four empty properties are listed in the Mission District. It’s likely the four got there because of complaints. “Neighbors call us,” said Mary Tse, a fire inspector with the fire prevention bureau of the San Francisco Fire Department. What happens then? “We can tell the owner to secure it, and if it’s empty, to keep it secure” said Tse. “We can order people to stop living in certain parts of the house. We can order it to be vacated. But that’s usually something that happens after the place burns down.”

Figuring out what will turn a small fire into a big one. Photo by Elizabeth Creely

Four empty buildings in the Mission District is low compared to the east side of the city where at least 19 are listed, but the low number of empty properties is small comfort. One building fully engulfed by fire can do a lot of damage. “These old buildings—they’re all built from thousand year old redwood trees,” Tse said.

She was looking at 3067 23rd St., one of the four listed, on Google maps. “That looks like an abandoned building to me. It’s too bad. It’s actually really nice.”

The next morning Station #7 was back in the neighborhood: firetrucks were stationed in front of 3067 23rd Street. This was not a coincidence. “This property was brought up to us by Mission Local,” Jack Cremins, Fire Battalion Chief told me.

Cremins thought that the Gaehwiler’s properties were an excellent place to kill two birds with one stone. “We want to be familiar with the neighborhood—the different buildings. We’re taking advantage of the opportunity in the vacant building here to practice. This is a good building for us to use.”

Fireman swarmed up two ladders positioned against both buildings, walking confidently on the aged roof, despite their probationary status. “We have firefighters that have never been on a roof at all,” Cremins remarked.

Vacant building are also mysteries, the scary kind: one of the biggest hazards of an empty, inflamed building is the lack of knowledge about what the fire feeding on. “It could be anything,” Cremins told me.

The men of Station #7 were doing their best to find what would feed a fire. One of them examined the cracks in the wood of the garage door of 3067 23rd. St. with a quizzical expression. “There’s a lot of stuff in there, but I can’t see what it is,” he said. When informed that it was old blacksmith equipment, his face lit up. “That’s cool! But kind of sad, too.”

When he was told that the building next door had an old woodworking studio with lots of sawdust in it, he looked stricken. “Sawdust?” he asked incredulously. “Good to know.”

Vacant buildings burn because they can. The first alert that something’s on fire often comes from a vigilant human on, in or near the premises. If no one’s around, fires can burn for a while before it becomes obvious that disaster has struck.

Disaster tends to announce itself: first as an empty building, then as dilapidation sets in and lastly as a fiery inferno. Fires seem sudden, but they are often years in the making. “We need to be prepared for when we have a fire inside these buildings, ” Cremins said, squinting up at the looming exterior of the building.

There was no fire inside the Gaehwiler property. But prevention depends on a not-if-but-when scenario: before the fire comes, Cremins told me. Acting as if the worst will happen is, perhaps, the best way to ensure that nothing does.