The firefighters suffered minor injuries. No major injuries were immediately reported among the 100 or so other firefighters.
The fire was first reported just before 10 p.m. in a swath of vegetation that quickly moved onto a two-story commercial building at 424 N. Belmont Ave. near Glendale Boulevard and next to the 101 Freeway.
A partial roof collapse forced firefighters to exit the building and roof, and move into defensive mode.
There was no information as to what may have caused the fire, but officials said the structure is believed to be an office building with heavy storage inside.
Traffic on the 101 Freeway came to a crawl at Alvarado Street.
Crews appeared to have gained the upper hand around 11:30 p.m.