SAN FRANCISCO (KRON) — The back deck of a 5-unit apartment building caught fire early Monday morning in San Francisco’s North Beach neighborhood, according to fire officials.

The 2-alarm fire happened on Union Street and appears to be accidental, fire officials said.

The 20 foot by 20 foot roof deck caught fire and damaged the upper units of the building.

Fire officials say the blaze has been contained, but remain on scene to investigate the possible cause of the fire.

The fire could have been caused by anything from a lit cigarette, to a grill that wasn’t turned off.

Fire officials warn that as the weather gets nicer, more people are spending time on their decks, and need to be vigilant of fire hazards.