The blaze was reported at a home in the 1700 block of Commodore Drive at 3:08 p.m., San Jose fire Capt. Mary Gutierrez said.
Responding crews found heavy smoke and fire in the attic of the 3,000 square-foot two-story house, she said.
Gutierrez said it appeared that no residents were in the home when firefighters responded.
At about 3:40 p.m., firefighters had to change tactics for fear that the structure's roof would collapse. Gutierrez said the fire was declared defensive and that all crews were battling it from outside the home.
One firefighter sustained an unspecified injury, she said.