Firefighters responded Thursday to a fire burning on the campus of Jefferson Elementary School in North Park.
San Diego Fire-Rescue Department crews arrived just after 9 p.m. and saw smoke they thought was coming from the roof of the school building.
After investigating, firefighters determined the fire was caused by construction crews sweating copper pipes for air conditioning units.
The heat smoldered tar paper and other roofing material which then caught fire, SDFD officials said.
The fire then burned through the roof and dropped down onto the stage of the school's auditorium, burning some old Christmas ornaments.
Metro Arson Strike Team investigators were called to the scene.
School is not currently in session and there is a air conditioning project underway at the campus, according to SDFD.