At 16:28 Company 2 was dispatched for an automatic fire alarm at Big Lots in Kelly Township. Rescue 2, Truck 2, MICU 2, and Chief 201 responded and arrived to a single story commercial structure, nothing showing from side Alpha, Bravo, or Charlie. Capt. 207 was met at the front door by a staff member reporting a popping sound and an odor of smoke inside. Chief 201 then requested the Tower and Engine 15, and an Engine from Company 5. Rescue 2 laid from a hydrant roughly 800’ away, stretched a 2 1/2” attack line, and charged the sprinkler system to the building while Truck 2 investigated the cause of the smoke in the building. Tower 15 arrived and was sent to the roof to check the HVAC systems and Engine 15’s crew assisted inside with Truck 2. Tower 15 discovered an electrical fire occurred on one of the roof systems. The power was isolated and there was no extension. The building was then turned over to maintenance personnel and units picked up and cleared.