The 5:30 a.m. fire at McAsphalt Industries Ltd. affected more than 1,000 homes and businesses, the Toronto Star reported, but had caused no known injuries.
Firefighters thought they had the blaze under control by 9 a.m. only to have flames sprout in other areas, the newspaper said, and thick smoke was still visible in Pickering in mid-afternoon.
"It's gone through the roof and several areas of the building have collapsed," Oshawa Fire chief fire prevention officer Susan King said. "The whole building is compromised ... . It's for sure unsalvageable."
Oshawa residents were urge to remain indoors and keep their homes closed up tight for much of the day. The lockdown was lifted just before 6 p.m.
Environment Ministry officials conducted air sampling and "levels are within acceptable limits," the city website stated Saturday evening.