SAUGUS – Fire Capt. Chris Rizza and his crew knocked down a small fire Friday afternoon thwarting what Fire Chief James Blanchard said could have been a catastrophic event.Blanchard said the construction supervisor on the Hannaford Supermarket project at the site of the old Wild Oats was alone on the job when he thought he smelled something odd.”He was about to leave,” Blanchard said. “Then he was walking through the building and he smelled it again.”Following his nose through the largely unfinished building, Blanchard said the worker discovered a burning fire snaking up an exposed wooden roof deck. He believes it could have been started by a plumber’s soldering torch, but the cause is still under investigation.”It was nothing serious – it won’t set them back or probably even slow them down,” he said.However, Blanchard said if the supervisor had left, the fire could have burned unchecked for more than an hour.”That could have been catastrophic,” he said. “Construction is when a building is most vulnerable and there’s so much wood around. They were lucky he was there.”