A 5-year-old playing with a lighter started the blaze on Friday morning. The family of seven who lived there was left without a home.
According to Tucson Fire Capt. Barrett Baker, crews were dispatched to the fire, in the 6000 block of East 21st Street, at 8:51 a.m.
Crews found heavy smoke coming from a bedroom window. Firefighters cut a hole in the roof with a chainsaw and pulled a hose line through the window to put out the fire.
A man and women in their 40's, a man and woman in their 20's, a 5-year-old boy and two girls under the age of two were evacuated.
Investigators determined the 5-year-old started the blaze by lighting a mattress on fire.
The woman in her 20's was treated for smoke inhalation, said Baker.
Red Cross is assisting the family.