The fire was reported at about 1:50 p.m. on June 20 at the two-story waterfront building at the intersection of The Embarcadero, Sansome and Chestnut streets.
The building was vacant but construction crews were on site to prepare it for next year's America's Cup sailing race. They apparently inadvertently started the blaze while doing some welding, fire department spokeswoman Mindy Talmadge said.
"The wood on the building underneath the concrete was really dry. It was like a tinderbox in there," Talmadge said.
Crews were welding a ladder to a wall near where there was a crack in the concrete and "a spark got in there," setting the building ablaze, she said.
Firefighters responded and spent more than two hours battling the flames before extinguishing the fire around 4 p.m.
The building is slated to house the operations staging area for the America's Cup regatta.