PITTSBURG (KPIX 5) — A fire at a roofing supply company in Pittsburg Sunday afternoon sent a huge cloud of black smoke into the sky. And while the size of the fire may have been unusual, firefighters say its cause was not.

Investigators say the fire started at a small homeless camp at the far corner of the property. No one knows when someone took up residence there, but several scorched, portable barbecues offer evidence that fire was being used for cooking.

READ MORE: Stockton Man Gets 17 Year Prison Sentence For Child Sex Trafficking

At about 2:30 p.m. Sunday, with dry winds blowing off of Highway 4, flames erupted from that spot and tore through a storage yard of the Elite Roofing Supply Company. Damage was extensive to stacks of pipes and construction materials, and the flames came to within 50 yards of where Angela Villasenor was also camped.

She says it’s just one more danger of being homeless and living on the streets.

Villasenor believes the fire was started intentionally. “Intentionally, this happens all the time…all the time, constantly…constantly. To me, it’s covering up something they know they did wrong,” she said.

READ MORE: Petaluma Police Nab Man With 5 Arrest Warrants On Gun, Drug Charges

And when it comes to frequency, it turns out she’s right. Steve Hill, a Contra Costa County Fire spokesman, confirmed at the scene that they spend a lot of time chasing fires started by the homeless.

“The conditions are very dangerous and a lot of our fires…I don’t have the exact number, but I would estimate that more than 50 percent of these vegetation fires we fight end up starting in, around, adjacent to homeless camps. Sometimes they harmlessly burn grassy fields, sometimes they get into businesses and they can get into homes,” Hill said.

Angel Saucedo’s auto body shop backs up to the fire site and for a while, he thought his business could be destroyed. He says he feels sorry for the homeless campers, but admits he will be looking at them differently now.

“If they’re actually playing with fire, I mean that’s actually something that you have to be a little more aware of your surroundings…and see what they’re doing because that can actually affect everybody else. I mean fires are not a joke,” Saucedo said.

MORE NEWS: EDD Fraud: Fresno Man, Central Valley Inmate Plead Guilty To $103K Unemployment Benefit Scheme

The owner of the burned business says much of what was lost was inventory being stored there by a fellow contractor. About 100 firefighters fought the 3-alarm blaze and one was hospitalized briefly for smoke inhalation. Investigators know where the fire started but have not yet determined how it began.