Rehoboth firefighters found heavy smoke coming from the roof and attic area of the unoccupied building when they arrived at 37285 Rehoboth Ave., in the Henlopen Junction shopping center.
The fire was reported at 1:21 a.m. Firefighters from Rehoboth Beach, Lewes, Indian River and Bethany Beach were on the scene for more than two hours.
No injuries were reported.
Investigators determined the blaze originated in the attic area and was caused by a malfunction in branch circuit wiring, said Harry R. Miller, chief deputy state fire marshal.