The Hamilton Fire Department says $140,000 in damage was done to a building under renovation on Main Street East at Houghton Avenue about 6:45 p.m. on Monday, July 2.
When firefighters arrived, there was smoke coming out of the second-floor windows of the two-storey building.
The flames were in the walls and joists between the first and second floors, and firefighters had to remove the wall and ceiling materials to knock down the fire.
With the high heat and humidity, relief crews were called in to spell off the first responders and one firefighter was treated in hospital for exhaustion and released.
The fire department lists the cause as electrical.