A fire investigation report released Tuesday gave more details about the cause of a five-alarm fire at an under-construction apartment building in San Francisco's Mission Bay neighborhood in March.
The March 11 fire was reported around 5 p.m. at a construction site at Fourth and China Basins streets, where a 172-unit building was being built as part of the MB360 development project by BRE Properties.
The unfinished building was destroyed and demolished in the days following the massive blaze.
In the report, written by San Francisco fire investigator Stephen Engler on April 15, the fire was deemed accidental after "an event involving a wire welding unit or grinder between the top floor and 'high roof' area."
The fire was sparked on the sixth-floor roof area in the southeast section of the building between the top floor and the roof, he wrote.
There was $40 million in property damage, along with $100,000 worth of contents destroyed in the building.
In Engler's report, he details arriving at the site and seeing a plume of vertical smoke. He determined all construction crews had left the site, where there had been three areas performing "hot work" earlier in the day.
One project was welding on the roof area, where the fire appears to have started.
Engler wrote that a sprinkler system had yet to be installed at the site.
The building contractor was Suffolk Construction Co., which had hired many subcontractors to complete the building in time to start leasing the building this summer.
The crew on the roof, which had been installing and welding a perimeter guardrail on the top of the building, had left the site by 3:25 p.m., according to the report.
The welding work had started at 7 a.m. that day and a crew manager told the investigator that the welding crewmembers had followed all safety precautions that day.
The crew was interviewed and it was determined they had unplugged all equipment before leaving for the day.
In Engler's report, he concluded "I am unable to eliminate an event involving either the by-products of welding coming into contact with the bare wood roof materials, or embers resulting from grinding the steel end-caps of the steel hand-rail coming into contact with the bare wood roofing materials as the ignition source."
Nearby buildings also sustained damage in the blaze, including an apartment building at 555 Mission Rock St. that sustained $500,000 worth of damage and an additional $300,000 worth of belongings. The future home of the Mercy Housing affordable housing project at 1180 Fourth St. sustained an estimated $5,000 in damage and there was $50,000 in damage to a roof at 535 China Basin St.
Residents in as many as 30 units in nearby apartment buildings were evacuated overnight.
Two firefighters were injured in the fire, according to Engler's report. According to Fire Chief Joanne Hayes-White, one firefighter suffered burns to his face. The injury was not serious.
The other injured firefighter, a battalion chief, suffered a minor ankle injury, according to a fire spokeswoman.
According to the report, the building was self-insured.
Further details into the cause of the blaze are unlikely, according to Engler, because of the demolition to the remains of the building that were threatening to collapse.