It was a chain-reaction collision that triggered the metal roof's collapse over gas pumps at an Arco station near Ventura Canyon Ave. and Roscoe Blvd. collapsed at about 3:30 p.m.
Police say a silver Nissan Altima ran a red light and collided with a white van and another vehicle. The van sheared off the fire hydrant, sending a thick plume of water into the air and onto the roof over the gas pumps.
Crews rushed to shut off the water immediately.
"Probably between 2,000 and 3,000 gallons of water, and you've got to figure you've got a little bit of a breeze, northwest direction and gets on top of a roof like that," said LAPD Sgt. Robert White. "It's designed for rain, it's not designed for that much water."
The roof will likely need to be torn down and completely rebuilt.
No one was injured.