Fire crews responded around 10 p.m. to reports of smoke coming from the second floor of 20604 Valley Green Drive, Santa Clara County Fire Capt. Daron Pisciotta said.
Upon arrival they found light smoke coming from the roof of the west wing of the "Apple 6" building. Second and third alarms were requested with assistance coming from the San Jose Fire Department.
Crews fought until 12:30 a.m. today to contain the blaze. They remained on the scene with an Apple management liaison until about 7 a.m.
The two-story, 120,000-square-foot building was built about 40 years ago, Pisciotta said. It suffered significant smoke and water damage to the second floor, and the west wing is unusable for the foreseeable future, he said.
Fire officials said the blaze started under a large "heating ventilation air-conditioning unit" that was being suspended on the roof. Construction crews told investigators that the fire ignited from a welding operation to install the unit.
No injuries were reported and the building's east wing suffered only minimal damage.
