Crews were called out to a home on the 3300 block of Chatham Street just south of highway 169 around 4:30 Friday afternoon.
When crews arrived, they found smoke and had to cut a hole in the roof of the home to put out some smoldering that had come up a back wall.
"It started in the back," said SJFD inspector Robert Blizzard.
"They believe it was careless smoking. They had a smoker's station back there that melted the plastic down to the bucket."
None of the three people living at the home were there at the time of the fire.
Damage is estimated at about $25,000