A fire that damaged two business suites Wednesday appeared to have spontaneously ignited with an oil-soaked rag, Santa Rosa fire officials said.

The blaze started in a design workshop and spread to an adjacent suite at the Piner Road business park just west of Range Avenue, Battalion Chief Michael Jones said. Fire officials estimated the damage at $100,000 to the building and another $100,000 to its contents.

Fire was coming from the roof when firefighters arrived at about 7:20 p.m. at the David Fowler Designs Workshop at 880 Piner Road, the center suite of a concrete tilt-up building, Jones said.

A metal roll-top door “was glowing red and disfigured,” Jones said.

Firefighters began an aggressive interior attack and found the fire had spread to an adjacent business, Gatley Gunsmithing, Jones said. Crews cut holes in the roof to let heat and smoke escape, aiding the firefighters within.

Crews had the fire under control within about 30 minutes and kept it from spreading to other business suites. They checked the gas cylinder storage behind the businesses at the Airgas Fill Plant and found the containers had not been affected by the nearby fire.

Fire Marshal Scott Moon said the fire originated with a rag soaked with linseed oil that had been used earlier that day on a project inside David Fowler Designs. Rags soaked with linseed, a naturally occurring oil, will generate heat when left out due to a chemical reaction with oxygen.

“As that rag continues to heat, it will ignite,” Moon said.

Fowler, a landscape designer and sculptor, said he had hung one rag up to dry before he left the business at about 3:30 p.m. to pick up one of his children.

“I followed the procedure I’d been taught and thought was safe,” Fowler said. “But I was told by a fire captain that if there’s a fold in the fabric, that can be enough to create that little bit of fire. I feel just awful it did damage to the building and the adjacent business as well.”

Moon said that oily rags should be soaked in water and placed in a metal container with a lid. Spring often brings an uptick in fires caused by oily rags as people prepare outdoor furniture, fences and decks for summer.

“Adhere to the warnings on labels,” Moon said. “All it takes is one rag.”

You can reach Staff Writer Julie Johnson at 521-5220 or julie.johnson@pressdemocrat.com. On Twitter @jjpressdem.