It broke out at 4:30 p.m. in the roof of the Upper School Auditorium at 101 School Road in the Alapocas neighborhood of Wilmington.
It took crews about a half an hour to put the blaze under control.
No one was injured.
The fire marshal said construction crews were working on the building and sparks from their equipment ignited combustible materials on the roof.
The auditorium is currently being rebuilt after a 3-alarm fire heavily damaged the structure back on April 17th.
That fire, which left two firefighters injured and caused an estimated $1 million in damage, was later ruled a case of arson.