The owner at 13 College Ave. was using a chainsaw to remove part of the roof to start constructing a second floor above the office portion Around 5:40 p.m. Saturday.
When the chainsaw developed a fuel leak and the owner went to shut the saw off, it ignited the fuel on the roof.
The fire spread from the office building roof into the attached apartment building, causing extensive damage to the third floor of the apartment building.
The Waterville Fire Department extinguished the fire by 6:40 p.m. No one was injured and the building is insured.
One occupant is being put up in a hotel by the owner.