A Petaluma man fiddling with the backyard barbecue badly burned his arm when a leaking propane tank caught fire and a smaller one exploded on Wednesday afternoon, fire officials said.

The man had just refilled the 5-gallon tank and was lighting the barbecue when he heard a hissing sound shortly before 3 p.m., Petaluma Fire Battalion Chief Jeff Holden said.

As he bent over to investigate, the leaking gas flashed, burning the man's arm and spreading flames to an exterior wall of his McGregor Avenue house.

A smaller propane tank like those used for camping exploded under the heat, as well, fire officials said.

The fire from the burning barbecue spread up the wall of the single-story home and into the attic through a gable vent, catching its contents on fire, Holden said.

Petaluma and Rancho Adobe firefighters had to cut into the attic to vent the smoke and heat and to remove stored items from the attic to ensure the fire was out.

The injured man, meanwhile, suffered first- and second-degree burns on his arm, Holden said. Firefighters bandaged him, but he did not want to be transported to a hospital and said he would seek medical aid on his own, Holden said.

The structural damage from the fire was estimated at $15,000, Holden said. There also was smoke damage throughout, he said.