BC Wildfire Service's mapping of the fire on July 8 indicates the fire started when lighting struck a clearcut.
July 11 perimeter
July 14 perimeter
July 16 perimeter, approximately 14,000 hectares
July 18 perimeter, estimated size 19,000 hectares
July 22 perimeter, approximately 32,000 hectares
July 26 fire perimeter, estimated size 39,584 hectares
The image below is a composite of the Flat Lake Fire's final perimeter mapped on top of the ministry of forest's record of the logging that took place before the fire.